Invertebrate Anatomy OnLine
Homarus americanus
©
American Lobster
with notes on crayfish
20jun2006
Copyright 2001 by
Richard Fox
Lander University
Preface
This
is one of many exercises available from
Invertebrate Anatomy OnLine
,
an Internet laboratory manual for courses in Invertebrate
Zoology. Additional exercises can be accessed by clicking on the links to the left. A
glossary and chapters on supplies and laboratory techniques are also available. Terminology
and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by
Ruppert, Fox, and Barnes (2004). Hyphenated figure callouts refer to figures in the
textbook. Callouts that are not hyphenated refer to figures embedded in the exercise. The
glossary includes terms from this textbook as well as the laboratory exercises.
Systematics
Arthropoda
P, Mandibulata, Crustacea
sP, Eucrustacea, Thoracopoda, Phyllopodomorpha, Ostraca, Malacostraca
C, Eumalacostraca, Caridoida, Decapoda
O, Dendrobranchiata
sO, Astacidea
iO, Nephropoidea
SF, Nephropidae
F, (Fig 16-15, 19-67, 19-90)
Arthropoda
P
Arthropoda, by far the largest
and most diverse animal taxon, includes chelicerates, insects, myriapods, and crustaceans as well
as many extinct taxa such as Trilobitomorpha. The segmented body primitively bears a pair of
jointed appendages on each segment. The epidermis secretes a complex cuticular exoskeleton
which must be molted to permit increase in size. Extant arthropods exhibit regional
specialization in the structure and function of segments and appendages but the ancestor probably
had similar appendages on all segments. The body is typically divided into a head and trunk, of
which the trunk is often further divided into thorax and abdomen.
The gut consists of foregut,
midgut, and hindgut and extends the length of the body from anterior mouth to posterior
anus. Foregut and hindgut are epidermal invaginations, being derived from the embryonic
stomodeum and proctodeum respectively, and are lined by cuticle, as are all epidermal surfaces of
arthropods. The midgut is endodermal and is responsible for most enzyme secretion, hydrolysis,
and absorption.
The coelom is reduced to small
spaces associated with the gonads and kidney. The functional body cavity is a spacious
hemocoel divided by a horizontal diaphragm into a dorsal pericardial sinus and a much larger
perivisceral sinus. Sometimes there is a small ventral perineural sinus surrounding the
ventral nerve cord.
The hemal system includes a
dorsal, contractile, tubular, ostiate heart that pumps blood to the hemocoel. Excretory organs
vary with taxon and include Malpighian tubules, saccate nephridia, and
nephrocytes. Respiratory organs also vary with taxon and include many types of gills, book
lungs, and tracheae.
The nervous system consists of a
dorsal, anterior brain of two or three pairs of ganglia, circumenteric connectives, and a paired
ventral nerve cord with segmental ganglia and segmental peripheral nerves. Various degrees of
condensation and cephalization are found in different taxa.
Development is derived with
centrolecithal eggs and superficial cleavage. There is frequently a larva although development
is direct in many. Juveniles pass through a series of instars separated by molts until reaching the
adult size and reproductive condition. At this time molting and growth may cease or continue,
depending on taxon.
Mandibulata
Mandibulata is the sister
taxon of Chelicerata and in contrast has antennae on the first head segment, mandibles on the
third, and maxillae on the fourth. The brain is a syncerebrum with three pairs of ganglia
rather than the two of chelicerates. The ancestral mandibulate probably had biramous appendages and
a J-shaped gut, posterior-facing mouth, and a ventral food groove. The two highest level
mandibulate taxa are Crustacea and Tracheata.
Crustacea
sP
Crustacea is the sister
taxon of Tracheata and is different in having antennae on the second head segment resulting in a
total of 2 pairs, which is unique. The original crustacean appendages were biramous but
uniramous limbs are common in derived taxa. The original tagmata were head but this has been
replaced by head, thorax, and abdomen or cephalothorax and abdomen in many taxa. Excretion is via
one, sometimes two, pairs of saccate nephridia and respiration is accomplished by a wide variety of
gills, sometimes by the body surface. The nauplius is the earliest hatching stage and the naupliar
eye consists of three or four median ocelli.
Eucrustacea
Eucrustacea includes all
Recent crustaceans except the remipedes. The taxon is characterized by a primary tagmosis
consisting of heat, thorax, and abdomen although the derived condition of cephalothorax and abdomen
is more common. Eight is the maximum number of thoracic segments.
Thoracopoda
In the ancestral
thoracopod the thoracic appendages were turgor appendages used for suspension feeding in
conjunction with a ventral food groove. Such appendages and feeding persist in several Recent taxa
but have been modified in many others.
Phyllopodomorpha
The compound eyes are
stalked primitively although derived sessile eyes occur in many taxa.
Malacostraca
C
Malacostraca includes most
of the large and familiar crustaceans such as crabs, shrimps, lobsters, crayfish, isopods, and
amphipods. Primitively the trunk consists of 15 segments, eight in the thorax and seven in the
abdomen but in most Recent species the abdomen has only six segments (Fig 19-19). The female
gonopore is on the eighth thoracic segment and the male on the sixth.
Decapoda
O
The largest and most
familiar crustaceans belong to Decapoda. The 10,000 species of crabs, shrimps, crayfishes,
lobsters, and their relatives are decapods. The first three segments of the decapod thorax are
fused with the head to form a cephalothorax and their appendages are maxillipeds. The remaining
five pairs of thoracic appendages bear simple or chelate walking legs. The resulting ten legs
accounts for the name “decapod”. A large carapace extends posteriorly from the head and is
fused dorsally with all eight thoracic segments. Laterally the overhang of the carapace
encloses the branchial chamber with the gills. The most primitive decapods (shrimps, lobsters,
and crayfishes) have well developed abdomens whereas the most derived taxa (true crabs in
Brachyura) have reduced, almost vestigial, abdomens (Fig 19-24).
Laboratory Specimens
The crustaceans in Astacidea
iO are excellent examples of the primitive crustacean condition. In this group are
the clawed lobsters in Nephropoidea
SF and the freshwater crayfishes in Astacoidea
SF, either of which can be used for this study.
This exercise is written
specifically for the American lobster,
Homarus americanus
, but can also be used with any genus of freshwater crayfish or any other clawed lobster
such as
Nephrops. These animals differ chiefly in minor anatomical details such as the number
of abdominal appendages and gills which should cause no confusion. The obvious advantage of
the lobster for this study is its large size. Its availability alive from local supermarkets
is also an advantage. The disadvantage is the expense.
External Anatomy
Place a living or preserved
lobster or crayfish on a dissecting pan of appropriate size and take it to your bench for
study. The claws of active lobsters must be restrained with strong elastic bands.
Lobsters can be relaxed with
magnesium chloride. A 20 cm (body length) lobster will succumb to magnesium chloride in about
2 hours. It will be quiescent in about 1 hour. After one hour it can be removed from the
relaxant and handled. If returned to seawater it may recover and become active. If kept
out of seawater during the exercise it will soon die.
Lobsters and crayfish have
bodies similar to the ancestral "caridoid facies" of the presumed ancestral crustacean. Such a
body is essentially shrimp-like in that it is elongate and nearly cylindrical in cross
section. The abdomen is well developed and its segments and appendages are readily
apparent. Note the
bilateral symmetry.
Body Wall
The decapod body wall is reduced
to just two layers, the exoskeleton and the epidermis that secretes it. No layers of muscle are
present because the circular and longitudinal body wall muscles have become specialized individual
muscles and no longer form continuous unspecialized layers as they do in the worm-like ancestors of
the arthropods. Continuous layers of circular and longitudinal muscles would be useless under
a solid and immovable exoskeleton. Connective tissue is absent because the exoskeleton itself
provides strength, structural support, and protection. No peritoneum is present because the
body cavity is a hemocoel, not a coelom. The exoskeleton is secreted by the epidermis and is
molted periodically to allow the animal to increase in size.
A complicated musculature,
derived from the circular and longitudinal muscles of the ancestors, originates and inserts on the
inner surface of the exoskeleton and moves its many parts. The exoskeleton between adjacent
regions is thin and flexible to permit motion. There is also a complicated "endoskeleton"
composed of internal processes, the apodemes, from the inner surface of the exoskeleton. The
gills and the anterior and posterior regions of the gut are covered with a very thin
exoskeleton.
Many parts of the exoskeleton
bear small, articulated, movable bristles, or setae. These can be seen at many places on the
body, as for example on the cutting edge of the smaller claw. Thicker articulated processes
from the exoskeleton are spines. Processes that are simply outgrowths of the exoskeleton and
are not articulated are usually called teeth.
The connective tissue
compartment is well developed and occupies most of the interior of the animal. Most of it is a
blood space, the hemocoel.
Tagmata
The
body is composed of a linear series of
segments. In malacostracans there are 19 segments but you cannot see or count
them all. Each one bears a pair of jointed appendages, which can be seen and
counted.
In the ancestral crustacean all
segments were identical (homonomous), or nearly so, as were their appendages. In derived
crustaceans the segments and their appendages are specialized for various purposes and to a large
extent no longer resemble each other closely (heteronomous).
Groups of adjacent segments and
their appendages tend to function similarly and together accomplish certain specialized
tasks. This results in a regionalization of the body into
tagmata. In crustaceans there are typically three tagmata; the head, thorax,
and abdomen but secondary tagmosis frequently modifies this plan.
The crustacean head is always
composed of five segments but the thorax and abdomen are variable. In Malacostraca, however,
there are always eight segments in the thorax and almost always six in the abdomen. The
anteriormost tagma is the
head (Fig 1). The middle region is the
thorax and the posterior region is the
abdomen. In decapods, as in many crustaceans, the head and anterior part of
the thorax are combined to form a new tagma, the
cephalothorax leaving the remaining five thorax segments to form another new
tagma, the
pleon. It is customary to refer to the combined head and
all thoracic segments as the cephalothorax when discussing decapods but that practice is
not applied uniformly to other crustacean taxa and is not followed here.
Figure 1. A dorsal view of an American lobster,
Homarus americanus. Adapted from Herrick, 1909. crab32L.gif
Carapace
The cephalothorax is covered
dorsally and laterally, but not ventrally, by a double sheet of exoskeleton called the
carapace (Fig 1, 14, 19-1). Although you cannot tell it by looking at the
animal, the carapace is an outgrowth of the exoskeleton of the most posterior head segment. It
grows posteriorly to cover and protect the thoracic segments. It is a fold of the body wall
and as such consists of two complete layers of the wall. The outer wall of the carapace is a
thick sclerotized exoskeleton and is hard and strong but the inner wall has only a thin exoskeleton
and is transparent and flexible (Fig 14). The carapace forms a shell over the back and sides
of the thorax with which it is attached dorsally. The lateral extensions of the carapace, known as
branchiostegites, enclose large lateral branchial chambers which house the gills
(Fig 14, 19-36, 19-3A).
Head and Cephalothorax
The five segments of the head
and first three thoracic segments are fused together and cannot be distinguished. Together these
eight segments are the
cephalothorax. Each cephalothoracic segment bears a pair of
appendages. No sign of segmentation can be seen on any surface of the head. The carapace
covers more than just the cephalothorax and the two are not synonymous. Dorsally and laterally
the entire thorax is covered by the carapace.
A conspicuous transverse
cervical groove divides the carapace into an anterior third, which is roughly the
cephalothorax, and posterior two thirds, which is the rest of the thorax (pereon) (Fig
1).
Anteriorly the head bears a
conspicuous anterior, median process, the
rostrum. The
orbits are a pair of semicircular notches, or sinuses, in the carapace lateral to
the base of the rostrum. Each orbit contains an
eyestalk with a
compound eye at its distal end. The black, multifaceted
cornea of the eye covers the entire circumference of the end of the stalk except
medially. The anterior end of the head is the acron but it is fused with the head and cannot
be distinguished from it.
Thorax
The thorax is composed of eight
segments, called
thoracomeres, and as we have seen, all eight are hidden beneath the carapace, when
viewed dorsally or laterally. Each thoracic segment bears a pair of appendages known by the
general term
thoracopod.
The anterior thorax, consisting
of three segments, is fused with the head to form the cephalothorax, as described above. The
posterior five segments remain independent of each other and of the head. The posterior
thorax, composed of these five segments, is the
pereon and its segments are known specifically as
pereomeres (= pereonites), which are a type of thoracomere. The pereon is not
part of the cephalothorax even though it is covered by the carapace.
Under the carapace, the
posterior thorax, or pereon, is segmented. This segmentation is apparent ventrally where he
pereon is not covered by carapace.
Abdomen
The
abdomen (Fig 1, 19-1) of primitive decapods is well developed with clearly visible
segments and powerful longitudinal muscles. It is this abdominal musculature that is largely
responsible for the great popularity of lobsters and crayfish at the table.
The abdomen is also called the
pleon and its segments are
pleomeres (= pleonites). Count the abdominal segments. All six are
clearly visible and none is fused with each other or with the thorax, nor are they covered by the
carapace.
The posterior end of the body is
the
telson, which is not a segment (Fig 1, 19-1). The
anus is located on the ventral side of the telson.
Abdominal Exoskeleton
The exoskeleton of the abdominal
segments of the lobster approximates the typical ancestral condition. Primitively, each body
segment is enclosed in skeletal ring of four articulated exoskeletal plates, or
sclerites, that form a complete circle around the segment (Fig 16-1B). The
dorsal sclerite is the
tergite, the ventral one the
sternite, and on the sides are two lateral
pleurites.
In astacideans (lobsters and
crayfish) the tergites and pleurites are fused together to form a hard arch of exoskeleton covering
the dorsal and lateral aspects of the segment. The pleurites extend ventrally past the body as
side plates, or
epimera, which together form a shallow ventral space below the
abdomen.
The sternites cover most of the
ventral surface of the abdomen and the pleurites cover the lateral parts of it. Most of a
sternite is thinner and more flexible than the tergites and pleura. It is transparent and the
abdominal musculature and ventral nerve cord can be seen through it. The posterior margin of
each sternite however is very thick and heavy and forms a reinforcing arch across the venter from
one pleurite to the other. The appendages articulate with the pleurites at the ends of this
sternal arch.
Appendages
General
Begin the study of appendages by
reviewing the morphology of a typical crustacean limb. It is jointed, or composed of
articulated sections called
articles (not segments), and is primitively biramous. A
biramous appendage has a basal article, the
protopod that attaches by its proximal end to the body (Fig 1, 2,
19-1). Sometimes the protopod is divided into two articles, the coxa and basis. From its
distal end arise two
rami, or branches. The rami are an outer, or lateral,
exopod and an inner, or medial,
endopod (Fig 1, 2).
The two rami may be composed of
any number of articles depending on their function. They may be similar to each other or
different. Sometimes only one ramus is present and the appendage is
uniramous. Sometimes there are additional branches of the protopod or of the
rami. Any additional branch on the lateral side is an
exite and any extra medial branch is an
endite. Finally, an exite on the base of the appendage is given the special
name of
epipod.
Decapod appendages are easiest
to study by beginning at the posterior end and working forward. As you do this, keep in mind
that they are named and numbered in the opposite direction, from anterior to posterior.
Abdominal Appendages
Each of the six abdominal
segments bears a pair of appendages. Most of these are
biramous and, more than any other lobster appendage, resemble the ancestral
crustacean appendage.
The last (posteriormost) pair of
abdominal appendages, located on abdominal segment 6, are
uropods (Figs 1, 2, 19-1). The uropods have a relatively small protopod and
two very large, flat rami. The exopod is biarticulate (composed of two articles). The
distal border of each ramus bears a fringe of setae. Spread the rami of the two uropods apart
and array them beside the telson. The four rami plus the telson make up the
tail fan, which functions as a large paddle (Fig 1). With the fan deployed,
flexure of the abdominal muscles moves the fan rapidly forward under the body and results in the
generation of a powerful forward jet of water that propels the animal backwards in a characteristic
escape response.
The first five pairs of
abdominal appendages (counting from anterior to posterior) are
pleopods (= swimmerets)1-5. Pleopods 2-5 are biramous and are similar to each
other. Both rami are flat, leaflike phyllopods whose rhythmic movements generate a water
current. Female pleopods are better developed than those of males and are used to carry the
eggs, which are attached to the fringe of setae around the rami. (The rami of crayfish pleopods are
not broad and leaflike. Instead they are narrow and whiplike.)
Figure 2. A lobster uropod. Redrawn from Herrick (1909). Crab33L.gif
Figure 3. Pleopod 3. Redrawn from Herrick (1909). Crab34L.gif
The second pleopod of male
lobsters bears a small endite. The first pleopods are uniramous (Fig 4). In females the
first pleopods are small and vestigial but in males they are modified and heavily sclerotized to
serve as an intromittent organ to transfer sperm to the female. In males they are referred to
as
gonopods.
Figure 4. Pleopod 1 of female. Modified from Herrick (1909). Crab35L.gif
Branchial Chamber
The lateral extensions of
the carapace, known as
branchiostegites (Fig 14, 19-36, 19-3A), enclose the lateral branchial chambers
where the gills are located.
" Lift the ventral edge of the carapace and
note that it is attached dorsally to the body but is free laterally. With strong scissors cut
away the unattached lateral portion on the left side of the carapace without cutting into the
attached portion. Be careful that you do not cut into the body and do not damage the numerous
structures in the space below the carapace. You have removed the left branchiostegite.
The space thus uncovered is the
branchial chamber and it contains the
gills. The gills, which are epipods of the thoracic appendages or adjacent
pleurites, will be studied later. They are feathery, white, filamentous processes. Keep
them moist so they do not dry out. Removal of the branchiostegite exposes the entire length of
the thoracic appendages and makes their study easier.
Thoracic Appendages
Each thoracic segment bears a
pair of appendages but as there are two distinctly different regions of the thorax, two distinctly
different types of thoracic appendages characterize those regions. The appendages of the
anterior three thoracic segments are
maxillipeds and function as auxiliary mouthparts. The appendages of the
posterior five thoracic segments (pereomeres) are
pereopods and function as walking legs or pincers. Maxillipeds and pereopods
are the two types of thoracopods.
The naming and numbering of
thoracic segments and appendages is potentially confusing and requires explanation. The first
thoracic segment is thoracomere 1 and it bears maxilliped 1 (which is also thoracopod
1). Thoracomeres 2 and 3 bear maxillipeds 2 and 3 respectively (= thoracopods 2 and
3). The fourth thoracomere is also the first pereomere and its appendage is pereopod 1 (=
thoracopod 4) and so forth.
The five segments of the pereon
bear a total of 10 appendages which accounts for the name Decapoda (= ten feet). The ten
appendages are pereopods but are usually referred to loosely as "walking legs", whether or not they
are used for walking. All pereopods lack the exopod and are uniramous. The endopod is
long and narrow. This shape of ramus is referred to as stenopodous in contrast to a broad,
flat, leaflike phyllopod.
Pereopods
Look at the posteriormost
thoracic appendage. It is pereopod 5. Which thoracopod is it? The typical malacostracan
thoracopod (including pereopods and maxillipeds alike) is composed of seven articles. The two
proximal articles represent the subdivided protopod and the distal five are the five articles of
the endopod.
Find the seven articles of
pereopod 5 (Fig 5, 19-1, 19-20). The proximal article is the
coxa. It is wide and short and articulates with the sternite of the last
pereomere. Distally it articulates with a short, narrow
basis. The basis joins with the
ischium along an oblique articulation.
Notice that the ischium appears
to be composed of two articles in that it has an oblique groove encircling it near its articulation
with the basis. This groove marks the location of the
fracture plane where the lobster can deliberately autotomize (auto=self, tome=cut)
its limb (Fig 5, 19-36). This plane is specialized for this function and the animal can loose
its limb, at this plane only, with minimal trauma or blood loss.
The ischium articulates with a long narrow
merus. Next there is a short
carpus followed by a long
propodus. The final article is a sharp, pointed
dactyl, or nail.
A large, white, feathery
gill is attached to the pleurite immediately dorsal to the coxa. Gills are
associated with all thoracopods except maxilliped 1. Most appendages have more than one gill
and they may be attached to the pleurite, coxa, or the articulating membrane between the pleurite
and coxa (Fig 14).
Figure 5. Pereopod 3. Adapted from Herrick, 1909. crab36L.gif
Pereopods 4 and 5 are
almost identical. Pereopod 4 has a large membranous, leaflike epipod on its coxa that is
absent from 5. This epipod extends vertically between the gills. Similar epipods are
present on the remaining pereopods and on maxilliped 3.
Pereopods 1-3 resemble each
other in that the propodus and dactyl form a prehensile, or grasping, pincer. The propodus
bears a long, fingerlike, distal process against which the dactyl opens and closes (Fig 5,
19-2B). The dactyl is a
movable finger and the propodal process is an
immovable finger. Such a pincer is known as a
chela and appendages bearing them are
chelate. Pereopods 4 and five do not have chelae and are "
simple". The small chelae of pereopods 2 and 3 are used to transfer food to
the mouth (Fig 5).
Chelipeds
The first pair of pereopods is
much larger than any other appendage (Fig 1, 19-2B). They are chelate and, because of the
striking size of their chelae, are referred to as
chelipeds, and rarely as walking legs. The usual seven articles are present
in the chelipeds. The chelae, as expected, are formed of the opposing propodus and
dactyl.
Notice the strong dimorphism in
the two chelipeds. One, the
crusher claw, is heavy and massive with low rounded teeth. The other, the
cutter claw, is slender and bears sharp, pointed teeth.
The crusher has rounded,
molarlike teeth and closes relatively slowly but with force sufficient to break the shells of
oysters and mussels. The muscle that closes the claw is very large, to the delight of
gourmets, and occupies almost the entire interior of the propodus. It is composed entirely of
slow fibers.
The cutter claw is slimmer and
bears sharper teeth. It closes much faster than that of the crusher and can do so many times
faster than the human reflex that might try to remove fingers from harm's way. The fibers of
its closing muscle are almost entirely fast fibers. Each propodus contains a small opener
muscle, in both cases composed of slow fibers, that opens the claw.
In about 50% of the population
the left claw is the cutter and in the other half it is the crusher. Which side becomes which
claw is determined simply by which one happens to get the most use during a critical period early
in the life of the individual.
Notice the variety of
articulations in the joints of the chelipeds. Flex and extend each joint to see what kinds of
motion its articulation allows. Each joint has an axis on which its two articles rotate with
respect to each other. Determine the axis of rotation for each of the six articulations of the
cheliped.
>1a. Insert a #1 insect pin along the axis of rotation of each joint, or as
close to it as the exoskeleton will permit. Are any of the pins parallel to each other, or
close to it? How many different planes of motion can you distinguish? Notice how the six
different axes together allow the chela to move to almost any position. Might this be a good
model for an articulated arm on a Martian rover? Determine the contribution of each articulation to
the range of motion of the entire chela by immobilizing each of the articulations. Do any seem
to be more important than others?
<
Gonopores
The
female gonopores are the external openings of the oviducts. They are located
on the medial side of the coxa of pereopod 3 (thoracopod 6). The
male gonopores are the external openings of the vasa differentia from the testes
and are found at the tip of the two short
genital papillae on the medial surface of the coxa of pereopod 5 (thoracopod
8). The position of the male and female gonopores is constant throughout
Malacostraca.
A large, conspicuous, blue,
so-called
seminal receptacle is located on the ventral surface of the female between the
coxae of pereopods 4 and 5. It bears a deep median longitudinal cleft that is slightly
expanded anteriorly. The walls of this anterior expansion are flexible and, if pushed with a
fine forceps, will open to disclose a recess. The male gonopod (which is the first pleopod and
not the genital papilla) is inserted into this flexible pocket where it deposits spermatozoa into
the recess. (In crayfish the female receptacle is called the
annulus ventralis).
Maxillipeds
The anterior three pairs of
thoracic appendages are maxillipeds (Fig 19-2A). These three pairs of thoracopods are
biramous, unlike the pereopods, but their endopods still have the usual seven articles.
The
third maxillipeds are on the third thoracomere immediately anterior to the
chelipeds. Each is large and intermediate in shape and size between the pereopods and the
mouthparts. Each has a large, stenopodous
endopod and a small filamentous
exopod (Fig 6).
The
protopod is divided into a
coxa and a
basis, as it is in all thoracopods (eg see Fig 5). The
endopod arises from the distal end of the basis and consists of the usual five
articles of a thoracic endopod (ischium, merus, carpus, propodus, and
dactyl). The long heavy ischium bears an impressive row of medial teeth that,
along with the mandibles, are the chief means of reducing the size of food particles before
ingestion. The small
exopod arises from the distolateral corner of the basis. One function of the
third maxilliped is to protect the more delicate appendages anterior to it.
Figure 6. Maxilliped 3. Redrawn from Herrick (1909). Crab37La.gif
Hold the third maxillipeds aside
and look at the next appendage. It is the
second maxilliped (Fig 7). It too is biramous but is much smaller that the
third. Its exopod is longer than its endopod.
Figure 7. Maxilliped 2. Modified from Herrick (1909). Crab38La.gif
The
first maxilliped is the appendage of the first thoracomere (Fig 8). Its
exopod resembles those of the other maxillipeds and is long and narrow. Its endopod has two
articles and is also long and narrow looking superficially like the exopod (the crayfish endopod is
short and inconspicuous). The exopod lies in a groove on its lateral border. Two large
wide, thin endites curve over the bulge of the mandible. A long posterior
epipod extends posteriorly into the branchial chamber.
Head Appendages
The remaining five pairs of
appendages are those of the five head segments. The posterior three are mouthparts whereas the
anterior two are antennae and have a sensory function (Fig 19-2A).
Figure 8. Maxilliped 1. After Herrick (1909). Crab39La.gif
Figure 9. Maxilla 2. After Herrick (1909). Crab40La.gif
The
second maxilla is the appendage of the fifth head segment and it lies immediately
anterior to the first maxilliped (Fig 9). It generates a water current that pumps water out of
the anterior end of the branchial chamber. Its basal portion bears four flat, narrow
endites, a slender
endopod, an
epipod, and an
exopod. The long flat exopod and epipod form the all-important
gill bailer, or scaphognathite (Fig 9, 19-38B), whose motion generates the
respiratory current through the branchial chamber.
The gill bailer lies beside the
carapace and extends anterior to and posterior to the basal part of the second maxilla. The
large thin trough-shaped epipod of the first maxilliped extends back toward the branchial chamber
(Fig 8). It functions in concert with the gill bailer of the second maxilla. The bailer
lies in the trough formed by the epipod of the first maxilliped where it beats to create a
current.
The
first maxillae (Fig 10) are small and more delicate than the second. The
smallest of the mouthparts, they lie curved tightly against the smooth, hard surface of the
mandible. Each has two broad endites and a narrow, larger endopod. The exopod is
absent.
Figure 10. Maxilla 1. After Herrick (1909). Crab41L.gif
Figure 11. Mandible. Redrawn from Herrick (1909). Crab42L.gif
The
mandibles are the most anterior of the mouthparts. Each is heavily calcified
and equipped with powerful muscles (Fig 11). The large basal portion bears a cutting edge on a
medial lobe. A three-articled
palp arches over the cutting edge. The mandible has partial responsibility
for shearing small pieces of food from larger ones. It can rotate only slightly on its
axis.
A single, large, fleshy
labrum, or upper lip, attaches to the anterior body wall just dorsal to the
mandibles and fills much of the space behind the cutting lobes. The labrum is a fold of the
body wall and is not an appendage.
The remaining two pairs of
appendages are the sensory antennae. The biramous
second antennae are by far the larger of the two pairs (Fig 12, 19-2). Each
arises by a biarticulate
protopod consisting of a proximal coxa and a distal basis. The short, wide,
phyllopod-like exopod, which is called the
antennal scale, arises from the basis. The endopod, which also arises from
the basis, has a short thick basal
peduncle of three articles and a very long narrow, whiplike
flagellum of many articles.
Figure 12. Antenna 2. The ventral surface and nephridiopore are shown in the
inset. Redrawn from Herrick (1909). Crab43L.gif
The lower surface of the coxa
bears a small circular tubercle with an opening, the
nephridiopore, in its center. The
nephridiopore is the external opening of the nephridium.
The
first antennae (= antennules), are situated below the eyestalks (Fig 1, 19-2A,B)
and are much smaller than the second antennae. Each has a triarticulate basal "protopod" from
which arise two slender multiarticulate
flagella of nearly equal length (the medial flagellum of crayfish is shorter than
the lateral). A statocyst is present in the basal article of each first antenna (Fig
19-7B).
For several reasons,
embryologic, morphological, and phylogenetic, the first antennae are not considered to be truly
biramous even though they have two branches. Do you see anything about their anatomy that is
at odds with the basic structure of the other biramous appendages you have studied? (Count the
number of articles in the "protopod").
Figure 13. Antenna 1. Redrawn from Herrick (1909). Crab44L.gif
Respiratory System
The gas exchange surfaces of
decapod crustaceans consists of numerous gills, a branchial chamber to house them, and a pump to
generate the respiratory current over them. The gills are associated with the proximal ends of
most thoracopods.
Homarus has 20 pairs of gills (Nephrops has 19 and in crayfishes the number varies).
The pale, feathery
gills are housed in a branchial chamber between the lateral carapace and the body
(Fig 14, 19-3). You opened the left branchial chamber when you removed the left side of the
carapace. The gills on that side are exposed to view and readily found. The right chamber
should still be intact and covered by the right branchiostegite. The gill bailer of the second
maxilla is the pump. It is assisted by the epipodite of the first maxilliped.
" Snip the end from one of the gills, place
it in a small dish (6-cm culture dish) of water and examine it with the dissecting
microscope.
Lobsters and crayfish have
filamentous (= trichobranchiate) gills in which the respiratory surface consists of numerous long
filaments radiating from a central axis, rather like a bottlebrush (Fig 19-37C,D).
Look at the cut surface of the
gill axis. Here you will see two blood channels, cut in cross-section, that extend the length
of the gill (Fig 19-37C). One is the afferent channel that takes blood into the gill and the
other is the efferent vessel that drains oxygenated blood away from the gill. Similarly, each
filament is partitioned into two channels by a longitudinal septum. One channel is afferent,
the other efferent. The lamellar gills of crabs have flat plates instead of filaments (Fig
19-37E,F).
The gills extend vertically into
the branchial chamber from their attachments on or near the coxae of the thoracopods. Look
closely and see that the gills of successive appendages are separated from each other by the long,
membranous
epipods of those appendages. The epipods form the boundaries of water
channels that extend vertically from the free, unattached ventral edge of the carapace upwards to
the attached dorsal edge.
Notice that the coxae of each
pair of adjacent pereopods are shaped so that together they form a V-shaped inhalant channel that
leads into one of the vertical channels in the branchial chamber (Fig 19-38B). There are five
such
inhalant channels.
Dorsally the several vertical
channels converge on an oblique,
exhalant channel that runs anteriorly along the dorsal margin of the branchial
chamber (Fig 19-38B). The floor of the anterior half of the exhalant channel is the epipod of
maxilliped 1 and it separates the channel from the gills. The roof and walls of the channel
are formed by the carapace and body.
Figure 14. Cross section of the branchial chamber and gills of a generalized decapod.
Shrimp20L.gif
The gill bailer of maxilla 2
lies in the anterior end of the exhalant canal. Undulations of the bailer generate the water
current that enters the inhalant canals, passes vertically over the gills, and then exits
anteriorly, lateral to the mouthparts. This strong current is detectable for a distance of
three animal lengths in front of the lobster.
A reverse current can be
generated by the three maxillipedal endopods. Lobsters may reverse the current when they wish
to draw water from in front of themselves over the chemosensory antennae.
All decapod gills are associated
with thoracopods but differ in the exact location of their attachment. Podobranchs (podo=foot,
branch=gill) arise on the lateral surface of the coxa of the thoracopod (Fig
14). Arthrobranchs (arthro=joint) arise on the thin articulating membrane between the coxa and
the pleurite of the body wall. Pleurobranchs (pleuro=side) are attached to the pleurite dorsal
to the limb articulation.
Look at pereopod 4 to see
examples of all three types. It has one pleurobranch, one podobranch, and two arthrobranchs
(one anterior and one posterior). Pereopods 2-4 each have one podobranch, one pleurobranch,
and two arthrobranchs. Pereopod 5 has one pleurobranch. The cheliped (pereopod 1) and the
third maxilliped each have one podobranch and two arthrobranchs. The second maxilliped has one
small podobranch (plus an arthrobranch in crayfishes). Maxilliped 1 has no gills but has the
important epipod that encloses the exhalant water channel and the gill bailer. There is a
total of 20 pairs of gills in
Homarus. (The number varies in crayfishes where the pleurobranchs are usually
absent. Some have a pleurobranch on pereopod 5 and some do not.)
Internal
Anatomy
During this dissection be sure
to keep the tissues moist as you work. It is best to conduct the dissection under water but
that is difficult to do with an animal as large as a lobster. Open the body cavity carefully
so you do not damage the organs within. Many of these organs are soft and lack substantial
connective tissue support. Both the cephalothorax and abdomen should be opened.
" Open the cephalothorax as follows. Start
on the left side where you have already removed the unattached carapace. Using a blunt probe,
supplemented by strong scissors where necessary, free the exoskeleton of the carapace from the
underlying body wall. You are not to cut through the thin body wall, rather are to separate
the carapace from it. Do this all along the cut edge of the carapace on the left
side. Use scissors to free the posterior margins of the carapace.
Insert the tip of your strong
scissors under the anteroventral corner of the carapace on the left side and cut dorsally through
the exoskeleton (but nothing else) toward the right side. Extend this cut all the way over the
carapace and end at the anteroventral corner of the carapace on the right side. The cut will
describe a transverse arch over the carapace just posterior to the base of the rostrum and the
orbits.
Lift the cut edge of the
carapace and use your blunt probe again to free the body wall from the inner surface of the
exoskeleton. Free the entire inner surface and lift the carapace off the
cephalothorax. Note the transverse ridge and two small apodemes near the midline of the inside
surface of the carapace under the cervical groove. Set the carapace aside.
The body cavity (hemocoel) is
not yet exposed and for it is still covered by the thin pigmented body wall. Leave the body
wall intact for the time being.
Now remove the dorsal
exoskeleton from the abdomen. With scissors cut the tough connective tissue transversely along
the anterior edge of the tergite of abdominal segment 1. Then insert the sharp tip of the
scissors under the anterior edge of the tergite at the base of the epimeron (side plate) on the
left side. Make a longitudinal cut posteriorly through the exoskeleton of the first
tergite. Do the same on the other (right) side. Now lift the anterior edge of the first
tergite and free it from the underlying body muscles and body wall using the blunt
probe. Leave it attached to the second tergite.
Make cuts through the second
tergite and free it from the underlying tissues as you did the first. Proceed to tergite
three, then 4, 5, 6, and the telson. Remove and discard the chain of tergites. When you
finish, the dorsal surface of the abdomen will be exposed. The epimera will remain in place on
either side but the middle of all the tergites will be gone and the underlying muscle
exposed.
The body wall of the
cephalothorax is, once the exoskeleton has been removed, almost entirely
epidermis. Lift the epidermis with fine forceps and cut a hole in it with
scissors. Tug on the edges of the hole to pull the epidermis away from the underlying
tissue. Use a scraping (not cutting) motion of the scalpel to accomplish this
separation. All you are removing is the very thin, pigmented epidermis. Nothing
else! In many places you will have to separate muscles from the epidermis. These muscles
insert on the inner surface of the exoskeleton but do so through the mediation of microtubules in
the epidermal cells. The muscles attach to the epidermis and the epidermis to the
exoskeleton.
>1b. Make a wetmount of a small piece of the epidermis and examine it with the
compound microscope. Find the irregular, stellate (starlike)
chromatophores (Fig 19-44A,B). What colors of chromatophores do you find?
<
The space disclosed by the above
procedure is the
hemocoel. The body cavity of arthropods is part of the hemal system of the
connective tissue compartment. It is not a coelomic space and is not lined by
peritoneum. The visceral organs which you will now study lie in the hemocoel and are bathed in
blood (= hemolymph).
Preview
A brief preview of the major
structures in the body cavity will help you find your way around the hemocoel (Fig 15,
19-2B). The nearly shapeless, very soft, yellow-green
digestive cecum fills most of the space in the cavity. The digestive cecum is
called "tomale" by lobstermen. The
stomach occupies most of the anterior end of the cephalothorax. It is a
large, thin-walled, translucent sac. The
heart is a narrow, triangular, white organ lying dorsally on the midline in the
posterior cephalothorax. The heart is easily dislodged and lost during removal of the
carapace. Look for it on the bottom of the dissecting pan if you cannot find it in the
hemocoel.
Gonads may be obvious or inconspicuous depending on age and season. Mature
ovaries are dark green and testes are white. Immature ovaries are various shades of pink or
red. The ovary is known as "coral" to lobstermen.
Hemal System
Much of the hemal system is
exposed by the removal of the dorsal body wall. Since it is delicate, it is best to study it
first before it is destroyed. The system consists of a heart, arteries, and the open blood
sinuses of the hemocoel. The blood contains hemocyanin.
" Remove the shallow arch of muscle,
connective tissue, and apodeme running transversely across the posterior dorsal margins of the
cephalothorax, if it is still present.
The
heart is a large, white, triangular organ lying dorsally in the posterior half of
the cephalothorax (The heart of crayfishes is an irregular rectangle). It is dorsal to the
coxae of pereopods 3 and 4 (Fig 15). The sharp point of the triangle points
anteriorly. Large white muscle masses lie beside, behind, and below it. Three pairs of
elastic alary ligaments and muscles run from the walls of the heart to the surrounding
tissues. These are stretched during systole. Their contraction, due to elastic recoil,
restores the heart to its original shape during diastole.
Figure 15. Sagittal section of a lobster. Adapted from Herrick (1909).
Crab45La.gif
The heart is located in the
pericardial sinus, which is a region of the hemocoel (Fig 19-3A). There is no pericardium and
the heart is immersed in the blood it pumps. Blood from the hemocoel enters the heart via
ostia which will be discussed later.
Five arteries leave the anterior
heart. Four of them are paired and one is unpaired. The large, unpaired
ophthalmic artery (= anterior aorta) (Fig 15) exits the acute anterior end of the
heart and extends anteriorly on the dorsal midline, dorsal to everything else in the body
cavity. It is a slender, transparent, colorless tube running to the head.
Near the anterior end of the
cephalothorax the ophthalmic artery expands to form an accessory heart, the
cor frontale. This can be seen on the dorsal surface of the membranous dorsal
wall of the stomach. It will probably be at about the level of the cut edge of the
carapace. The ophthalmic artery supplies the brain and eyestalks with blood.
A pair of antennal arteries (=
anterolateral arteries) exits anterolaterally from the anterior end of the heart or from the base
of the ophthalmic artery. Each extends diagonally anterolaterally across the surface of the
digestive cecum and eventually ends up laterally in the head. They send branches to the
stomach and stomach muscles, branchial chamber, nephridia, antennae, eyestalks, and other
structures.
The two hepatic arteries also
exit the anterior end of the heart and extend anterolaterally from it. They leave the heart
posterior to the point of exit of the antennal arteries and are farther ventral. It will be
necessary to push the digestive cecum, which they supply, aside to see
them.
Two unpaired arteries exit the
posterior end of the heart. There are no paired posterior arteries. One of the unpaired
arteries, the
dorsal abdominal artery (= posterior aorta) (Fig 15) leaves from a posteroventral
protuberance of the heart and extends posteriorly along the dorsal midline into the abdominal
musculature. There is a swelling at its base. It gives off lateral segmental arteries to
the gonads, pleopods, posterior digestive ceca, and muscles of the abdomen.
"
With scissors make a middorsal incision along the length
of the abdomen. Do this under magnification and cut only deep enough to expose the dorsal
abdominal artery and its segmental branches. Do not cut or damage any tissue in the abdomen
except the muscles and body wall. The intestine lies immediately ventral to the
artery. You will also see the digestive ceca and gonad in the space between the muscle masses
in the anterior abdomen.
The
sternal artery is the second of the two unpaired posterior arteries (Fig
15). Find the origin of the sternal artery but do not attempt to trace it now. It runs
ventrally from the posterior end of the heart (Fig 15,19-3A). In doing so it passes to either
the right or left of the intestine and then penetrates the nerve cord by passing between the right
and left connectives between the ganglia of thoracomeres 7 and 8. Upon arrival at the ventral part
of the body dorsal to the sternites it bifurcates into a ventral thoracic artery to thoracopods
1-6, the mouthparts and anterior nerve cord and a ventral abdominal artery to thoracopods 7 and 8
and the posterior nerve cord (Fig 15, 19-2B).
Three pairs of
ostia penetrate the walls of the heart (Fig 15, 19-2B). Ostia are pores
through the wall of the heart through which blood enters the heart during diastole. Each is
equipped with a no-return valve that prevents escape of blood during systole. The heart of
Homarus has dorsal, lateral, and ventral pairs of ostia. The dorsal pair is not
immediately apparent in dorsal view because it is overhung by a thin shelf of tissue.
" Remove the heart and place it in a 6-cm
dish of water and look for its six ostia with the dissecting microscope. Lateral and ventral
pairs are easily found but the dorsal pair is hidden by a small sheet of tissue.
Reproductive System
The reproductive system consists
of a pair of gonads which together form a single H-shaped organ connected to the exterior by a pair
of gonoducts. (The gonad is Y-shaped in crayfish with the stem of the Y pointed posteriorly
and the two arms anteriorly.) The size of the gonad depends on the season and state of
maturity of the specimen. They may be very small, very large, or anything in between. The
gonad occupies a coelomic remnant, one of the few remaining in arthropods, and the gonoducts are
ancient coelomoducts. The gonad lies on the floor of the pericardial sinus immediately ventral
to the heart (Fig 15, 19-2B). Its arms may extend far anteriorly and posteriorly.
" Remove the heart if you have not already
done so. Remove as much digestive cecum as necessary to expose the gonad. If you are
dissecting out of water it will help to rinse the body cavity with a gentle stream of water
occasionally.
Female
The
ovary (Fig 15) has the shape of a very long H with its vertical arms oriented
longitudinally and the crossbar transversely. The crossbar of the H is on the floor of the
pericardial sinus at the approximate level of pereopod 3. The two longitudinal lobes lie
laterally and may extend from far back in the abdomen to the anterior end of the stomach. In
juveniles they are white but as they mature they successively become yellow, salmon, pale green and
finally dark green. After oviposition the ovary is temporarily gray. Eggs can be seen
within the transparent walls of mature and maturing ovaries. A straight
oviduct exits each lateral arm just a little posterior to the crossbar and extends
ventrally to the female gonopore on the coxae of the third pereopods (thoracomere 6).
Male
The
testis (Fig 19-2B) is usually H-shaped (Y-shaped in crayfish) although the
crossbar may sometimes be absent. It occupies the same position on the floor of the
pericardial sinus as the ovary. A coiled vas deferens (Fig 19-2B) exits each side of the
testis and extends to a male gonopore on the coxa of pereopod 5 (thoracomere 8). The testis is
white and may be confused with the digestive ceca.
Copulation
During copulation the male and
female oppose their ventral surfaces (face each other) with the male holding the female in position
(Fig 19-47A). The male gonopods (pleopod 1) are held together and inserted into the seminal
receptacle of the female. The genital papilla of the male delivers sperm to the base of the
gonopods. The sperm travel through grooves in the gonopods to reach the seminal
receptacle.
The sperm remain up to two years
in the receptacle. When the female deposits eggs, the sperm leave the receptacle and fertilize
them. The eggs are covered by a gluelike secretion that sticks them to the setae of the female
pleopods. The eggs are brooded on the pleopods until they hatch into planktonic
larvae. (The eggs of crayfish hatch into miniature crayfish which remain with the mother for a
time.)
Digestive System
The digestive system consists of
an ectodermally lined foregut, endodermal midgut, and ectodermal hindgut. The ectodermal
portions are lined by epidermis which secretes an exoskeleton, or cuticle that is molted with the
rest of the exoskeleton.
The
mouth is located on the ventral surface of the head between the two mandibles (Fig
15).
Insert a blunt probe into the
space between the cutting edges of the mandible and slip it gently into the mouth. Push it
vertically (gently) until you can see it pushing into the roof of the stomach.
The
stomach (= proventriculus) is located directly over the mouth, as you have just
demonstrated, and the short esophagus connecting the two is a vertical tube (Fig 15,
19-2B). You will see the esophagus later. The stomach and esophagus make up the
foregut. The stomach consists of two chambers. Anteriorly is the large, wide, hard
cardiac stomach and posteriorly is the smaller, narrower, softer pyloric stomach(Fig 16,
19-34). The intestine, or midgut, exits the posterior ventral end of the pyloric
stomach.
The two
digestive ceca (right and left), also known as the digestive glands, liver or
hepatopancreas, are large, soft, lobulated organs filling most of the space beside the stomach and
heart (Fig 15). Their true shape can be seen only when immersed in water. Each connects
by a duct to the anteriormost end of the midgut in the area between the pyloric stomach and the
intestine (Fig 16). Try to find this connection.
"
After you have studied the digestive ceca, remove
them. This is easiest to do by immersing the animal in water or by running a gentle stream of
water over the body cavity to support the ceca which can then be removed in pieces with
forceps.
With the digestive ceca out of
the way many features of the body cavity are easier to see. You may want to take another look
at the gonads and gonoducts. If your specimen is reproductive and has very large gonads you
should remove them at this time also.
Look at the stomach
again. The
cardiac stomach contains the
gastric mill which is a series of calcareous plates and teeth that grind, mix, and
regrind the food (Fig 16). Preliminary trituration of the food takes place at the third
maxillipeds and mandibles but the chief trituration occurs in the cardiac stomach. Ungrindable
or indigestible materials are regurgitated.
The gastric mill grinds the food
exceedingly fine and it is then filtered by a sieve, the
filter press (Fig 16, 19-34), of setae in the pyloric stomach. Solutes and
ultrafine particles then enter the digestive ceca. Large particles must be either reground or
regurgitated. Most hydrolysis and absorption takes place in the digestive ceca.
" Open the cardiac and pyloric stomachs with
a median, dorsal longitudinal incision.
The complex structure of the
gastric mill of the cardiac stomach and the sorting system of the pyloric stomach will be
immediately apparent (Fig 16). The stomach has over 30 hard exoskeletal elements and 14
muscles to operate them. The ossicles of the gastric mill are part of the exoskeleton and are
lost and replaced with each molt. The cuticular lining breaks into pieces during ecdysis and
the pieces are voided through the intestine. Find some of the larger ossicles in the cardiac
stomach. Some of them look very much like mammalian molars (Fig 16). Find the setal
filter press in the pyloric stomach.
The
gastroliths are two large, oval calcified areas, one on each anterolateral wall of
the cardiac stomach (Fig 16). Prior to each molt calcium carbonate is secreted here to form large,
hard, white pads. They are areas of calcium storage protected by the cuticular lining of the
stomach from dissolution by stomach acid. The loss of the cuticle during the molt removes this
protection and the calcium dissolves in stomach fluid. It is then absorbed into the blood and
becomes available for deposition in the newly formed exoskeleton.
Fin the short
esophagus connecting the cardiac chamber with the mouth (Fig 15, 16, 19-2B).
The anterior end of the midgut,
or
intestine, is swollen and resembles the pyloric stomach which it exits (Figs 15,
16). The ducts of the digestive ceca open into this swollen region.
Figure 16. Sagittal section of the stomach of the lobster,
Homarus. After Herrick (1909). Crab46La.gif
The intestine extends
posteriorly as a straight tube and enters the abdomen where it passes through a median space
between muscle masses. It can be seen ventral to the dorsal abdominal artery by pushing the
muscle masses apart. The intestine extends posteriorly to abdominal segment 6 where it ends at
a
sphincter.
Beyond the sphincter the gut is
wider and is the
rectum (Fig 15). The rectum is the hindgut. Immediately anterior to the
sphincter the large, simple, tubular
posterior midgut cecum arises from the dorsal wall of the intestine and extends
posteriorly to the end of abdominal segment 6 (Fig 15). It lies atop the rectum and is about equal
to it in diameter,
The rectum continues on to open
at the
anus on the ventral surface of the base of the telson. (In crayfish the
situation of the mid- and hindguts is different. The midgut is very, very short. It
receives the two ducts of the digestive ceca, gives rise to an anterior midgut cecum that arches
over the pyloric stomach and lies on its surface. Posterior to the point of exit of the
anterior midgut cecum the gut is rectum (hindgut.)
Excretory System
" Cut the esophagus and intestine and remove
the stomach.
The excretory organs of decapods
are a pair of highly modified metanephridia located in the segment of the second antenna. They
are saccate nephridia variously known as kidneys, nephridia, green glands, antennal glands, or
coxal glands. Each consists of a glandular region which contains a remnant of the coelomic
space called the end sac, a convoluted tubule, and a thin-walled bladder that empties to the
exterior via the nephridiopore on the coxa of antenna 2 (Fig 19-6B).
The
antennal gland is a large pale greenish organ closely adhering to the
anterolateral wall of the head immediately posterior to the base of the second antenna (Fig
15). The
bladder is a very large, thin-walled, transparent sac lying atop the
gland. It is not readily apparent that it is a sac and usually looks like a simple transparent
membrane.
Nervous System
The
brain, or supraesophageal ganglion, is a white mass located on the midline of the
anterior wall of the head between the bases of the two eyestalks (Figs 15, 16, 19-2B). Four
major pairs of sensory nerves enter it. In addition, a pair of large circumesophageal
connectives leave it posteriorly. Three pairs of the sensory nerves are easily seen but the
fourth exits the ventral surface of the brain and requires some additional dissection to reveal
it.
Carefully remove any connective
tissue remaining over the surface of the brain and find the three dorsal pairs of nerves. The
anteriormost is the short
optic nervethat run posteriorly from the eyestalks (Fig 16-10A). The middle
pair consists of the longer
tegmental nerves from the epidermal sense organs of the dorsal head. The
third and posteriormost pair is the
antennal nerve from antenna 2. The fourth pair, entering the ventral surface
of the brain, comes from the chemoreceptors of the first antenna, or antennula. These are the
antennulary nerves. Each exits the base of a first antenna and enters the
ventral surface of the posterior brain, almost as far posterior as the antennal nerves.
Two large
circumesophageal connectives leave the posterior border of the brain and pass
posteriorly, one on either side of the esophagus (Fig 16). Posterior to the esophagus they
join the median subesophageal ganglion, which is not yet visible. A small tritocerebral
commissure extends transversely from one connective to the other immediately posterior to the
esophagus. Its functional significance is slight and it is of more interest
phylogenetically. Its presence is evidence that the currently preoral tritocerebrum was once
postoral.
Posterior to the tritocerebral
commissure, the thoracic portion of the nerve cord, including the subesophageal ganglion, lies
deep below the internal skeletal supports of the thorax. It is difficult to uncover
the subesophageal ganglion and nerve cord and the remainder of the nervous system dissection may be
omitted if time is short.
" To reveal the ventral portions of
the nervous system use strong scissors to cut posteriorly through the skeletal
apodemes (= endophragmal shelf) in the floor of the cephalothorax in order to
trace the circumesophageal connectives posteriorly beyond the esophagus. Upon reaching the
abdomen, use a scalpel to cut the transverse connections between right and left ventral muscle
masses and remove those longitudinal columns of muscle. The nerve cord will then be seen lying
on the inner surface of the abdominal sternites.
The
subesophageal ganglion (Fig 16) innervates the mandibles, maxillae, and
maxillipeds and is formed of the fused segmental ganglia of their segments. The double
ventral nerve cord proceeds posteriorly from the subesophageal ganglion and bears
paired
segmental ganglia in the floor of each segment beginning with thoracomere 4, which
is the segment of the chelipeds (Fig 15). The double nerve cord runs posteriorly to the sixth
abdominal segment with a segmental ganglion in each segment.
References
Bullough WS
. 1958. Practical Invertebrate Anatomy 2
nd ed. MacMillan, New York. 483p.
Govind CK. 1989. Asymmetry in lobster claws. Am. Sci.
77:468-474.
Herrick FH. 1909. Natural History of the American Lobster. Bull.
Bur. Fish. 26:150-408, pls 33-47.
Huxley TH. 1880. TheCrayfish, An Introduction to the Study of
Zoology. Appleton, New York. 371p. (Reprinted 1973, M.I.T. Press, Cambridge.)
Lochhead JH. 1950. Crayfishes (and
Homarus) in F. A. Brown (ed) Selected Invertebrate Types. Wiley, New York. Pp
422-447.
Ruppert EE, Fox RS, Barnes RB.
2004. Invertebrate Zoology, A functional evolutionary approach, 7
th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
Snodgrass RE
. 1952. A Textbook of Arthropod Anatomy. Cornell Univ.
Press, Ithaca. 363 p. (reprinted 1971 by Hafer Publishing, New York) (crayfish on pp
142-179).
Supplies
Dissecting microscope
Compound microscope
Lobster or crayfish, living or preserved
Large dissecting pans
#1 insect pins
8-cm culture dishes
Isotonic magnesium chloride for living
specimens
Dissecting set