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Invertebrate Anatomy OnLine
Callinectes sapidus ©
Blue Crab
Copyright 2001 by
Richard Fox - Lander University
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.
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 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.
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but this has been replaced by head, thorax, and abdomen or cephalothorax and abdomen in many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.
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).
External Anatomy
Examine the crab and note that, unlike more primitive decapods such as shrimps and crayfish,
the body is short, very wide, and dorsoventrally depressed (Fig 1, 19-31). The ancestral
crustacean tagmata of head, thorax, and abdomen have been reorganized into cephalothorax,
pereon, and abdomen but the abdomen is reduced and almost vestigial. The malacostracan head
has five segments, the thorax eight, and the abdomen six. The head and first three thoracic segments
are fused to form a new tagma, the cephalothorax leaving five free thorax segments as the pereon.
The reduced abdomen is folded beneath the thorax and is not visible dorsally. Most of the crab
body is cephalothorax and pereon which are covered dorsally by a large, hard, shield-like carapace.
The carapace is an outgrowth of the most posterior head segment.
Head and Cephalothorax
In decapods, the first three thoracic segments are fused with the five head segments and their
appendages are maxillipeds. Together the head and first three thoracic segments form the
cephalothorax. Each thoracic segment of the cephalothorax bears a pair of maxillipeds but there
are few external indications of the original segmentation. Dorsally and laterally the head and entire
thorax are covered by the carapace.
Thorax
The thorax consists of eight segments, or thoracomeres, of which the first three are part of the
cephalothorax. The five posterior thoracomeres are not fused with the head although dorsally they
appear to be because all are covered by the carapace. Ventrally, however, the five segments can be
seen to be independent of the cephalothorax (Fig 2, 19-31). The five independent thoracic
segments are known collectively as the pereon and their appendages are pereopods. Each of
these segments is a pereomere. The pereopods are the five pairs of large, obvious walking legs
(Fig 1, 19-31). The name Decapoda (“ten feet”) alludes to these ten appendages.
Ventrally, where they are not covered by the carapace, the thoracomeres are easy to
distinguish (Fig 2, 19-31). The first three, which bear maxillipeds, are fused to each other and to the
head but the posterior five, numbered 4-8 in Figure 2, are clearly independent of each other. The
ventral surface of a typical arthropod segment is covered by an exoskeletal plate known as a sternite
and those of the thoracomeres form the ventral surface of the thorax (Fig 2, 19-31).
Figure 1. Dorsal view of the blue crab, Callinectes sapidus.
Carapace
The carapace has two long lateral spines and several strong teeth on each anterolateral
margin (Fig 1). The posterior margin of the carapace is smooth or minutely beaded. The lateral
extensions of the carapace, known as branchiostegites, enclose large branchial chambers which
house the gills (Fig 19-36, 19-3A).
On the anterior edge of the carapace, on each side of the midline are two shallow, notched
excavations. These are the orbits, from which the eyestalks protrude (Fig 1). Each has a
compound eye at its distal end. Anteriorly the cephalothorax bears a small anterior, median
process, the rostrum (Fig 1).
Figure 2. Ventral view of a male blue crab. The sternites of the thoracomeres are numbered
1-8. The left coxae of the pereopods are numbered 1-5. The abdomen is extended to reveal
the abdominal appendages and penis. Crab21La.gif
Abdomen
On the ventral surface locate the abdomen (Fig 2) flexed beneath the thorax (Fig 19-31).
The abdomen is also called the pleon, its segments are pleomeres, and its appendages are
pleopods. In true crabs (i.e. Brachyura, such as Callinectes and Cancer) the abdomen is a small
segmented structure whose shape varies with sex and maturity. In mature females it is broad with
convex sides and covers most of the posterior ventral surface of the thorax. In immature females the
abdomen is a nearly equilateral triangle whereas the abdomen of males is very narrow although it has
a broad base. Determine the sex of your specimen from the shape of the abdomen.
Extend the abdomen so it is no longer flexed but points posteriorly from the thorax as it would
in a crayfish or shrimp. In dorsal view most of its segments are easily seen and can be counted,
especially in females. The small, triangular, terminal portion is the telson, which is not a true
segment. Most malacostracans have six abdominal segments plus the terminal telson. In female
blue crabs the six segments are independent of each other and five of them are visible, the first being
hidden by the carapace. In males, segment 1 is hidden under edge of the carapace, segment 2 is
visible and wide, and 3, 4, and 5 are visible but fused together and narrowed posteriorly (Fig 2).
Segment 6 is separate, slender, and has the telson attached to its end. (The segments differs in
other genera).
The transparent, membranous intestine runs along the ventral midline of the abdomen, under
the thin membranous ventral exoskeleton, and terminates at the anus on the telson (Fig 2). It may
be filled with dark feces in which case it is easier to see. Press its posterior end with a probe to
extrude feces from the anus, thereby confirming its position.
On the ventral surface of the thorax is a median, longitudinal groove hidden by the abdomen.
The abdomen of the male occupies this groove and in females the gonopores are in its walls. The
female gonopores are large triangular openings in the sternites of the sixth thoracic segment, in line
with the third pair of legs. Male gonopores are located at the tip of the inconspicuous penis on the
last leg and will be seen later.
Appendages
Study the appendages without removing them from the animal. Each section of an arthropod
limb is known as an article. The term segment is reserved for the modular divisions of the body.
The basic crustacean appendage is biramous. The proximal article, the one that articulates
with the body, is the protopod. In many instances the protopod is divided into two articles, the coxa
and basis. From the protopod (or basis if the protopod is divided) arise two branches, or rami. The
lateral ramus is the exopod and the medial is the endopod. If one ramus is absent, the appendage
is uniramous. If both are present, it is biramous. Any additional structures on the lateral side the
limb are exites, on the medial side they are endites. Finally, an exite on the base of the appendage
is an epipod.
Although appendages are numbered from anterior to posterior it is easier to study them in
reverse order, from posterior to anterior. Begin with the pleopods, or abdominal appendages, and
work your way forward through the pereopods, maxillipeds, and mouthparts, to end with the
antennae. Brachyuran crabs have no uropods.
Extend the abdomen again, look at its ventral surface, and find the abdominal appendages
(pleopods). Be sure you see and study both sexes.
Pleopods
Like the abdomen itself, the pleopods are sexually dimorphic.
Male
Males have only two pairs of pleopods and they are located anteriorly on the abdomen, on
segments 1 and 2 (Fig 2). Both function in the transfer of sperm to the female during copulation.
They are hidden under the flexed abdomen, which must be extended to reveal them. The long,
curved, tubular first pleopod is the gonopod. It, not the penis, is the intromittent organ used to
deliver spermatophores to the female gonopore. The second pleopod is much smaller and
functions as a piston to push spermatophores through the hollow core of the gonopod.
Female
Females have paired biramous pleopods on abdominal segments 2-5 and, as in the male,
they are hidden under the flexed abdomen which must be extended to reveal them. The first article,
or coxa (Fig 3), of a female pleopod is attached to the body by a soft and flexible articulating
membrane. The coxa is small and poorly calcified but the next article, the basis, is large and
conspicuous. Two rami, the exopod and endopod, arise from the basis. After release from the
gonopores, the eggs attach to the long setae of the pleopods where they are ventilated by
movements of the abdomen and the pleopods.
Figure 3. The pleopod of abdominal segment 5 of a female crab. Crab22L.gif
Pereopods
The five pairs of pereopods, or walking legs, of the posterior thorax lack exopods and thus are
uniramous. They are slender stenopods consisting solely of the endopod and protopod. They have
no exopod. In almost all swimming crabs (Portunidae) pereopod 5 is a broad, oarlike swimming leg
(Fig 1, 19-31). (The fifth pereopod of Carcinus maenas, although a portunid, is not modified for
swimming.)
Find the characteristic seven articles of a typical thoracic malacostracan endopod (Fig 1).
The short, proximal coxa articulates with the body (Fig 2). The next two articles, the basis and
ischium, are fused together to form the basischium. The suture marking the line of fusion is visible.
The basischium is followed successively by the merus, carpus, propodus, and dactyl. The dactyl
and propodus of pereopod 5 are both flattened to form the paddle.
In males the gonopore is located at the tip of a long, thin, limp, transparent, colorless penis
on the proximal edge of the coxa of the fifth pereopod (Fig 2). (The penis of Cancer is a short, blunt
papilla). The penis fits into the groove of the gonopod to which it delivers sperm. The gonopods,
not the penis, are the intromittent organs.
Pereopods can be voluntarily autotomized (= self cut) to escape predation, reduce blood
loss from a distal wound, or in response to physiological stress. A special fracture plane is located
approximately at the line of fusion of the basis and ischium, where each leg can be shed with a
minimum of bleeding and trauma.
Pereopods 2-4 resemble pereopod 5 except that their distal articles are not oarlike (Fig 1).
Pereopod 1 is the cheliped and the pincer at its distal end is the chela. The cheliped is larger and
more robust than the other pereopods and is constructed so that the dactyl is a movable finger that
opposes an immovable finger protruding from the propodus. This arrangement creates a
prehensile chela. Note the teeth on the fingers.
Note the slight asymmetry of the two chelipeds. The left, or cutter cheliped, is smaller and its
teeth are a little smaller and sharper. The right, or crusher cheliped, is a bit larger and has larger
and slightly more rounded teeth. This dimorphism may be reversed in some individuals and it is
more pronounced in many other crab species. (There is little difference in the two chelipeds of
Cancer irroratus)
The large opening in the carapace dorsal to the coxa of the cheliped is the inhalant aperture
leading into the branchial chamber (Fig 2) where the gills are located.
If your crab is missing any of its legs, they were probably deliberately autotomized by the crab.
If so, the end of the stump of the leg will be cleanly sheared at the fracture plane and covered by a
membrane. The membrane is penetrated by a small hole through which pass an artery and nerve
(before autotomy). A one-way valve over this opening prevents loss of blood after autotomy.
Autotomy avoids irregularly broken exoskeleton, and torn muscles. Regeneration of an autotomized
leg begins before the next molt.
Maxillipeds
The three pairs of maxillipeds are the appendages of the first three thoracomeres. Unlike
other thoracopods they are biramous. Their endopods are homologous to the endopods of the
pereopods and are composed of the same seven articles.
The maxillipeds and other mouthparts overlie each other so only maxilliped 3 can be seen at
present. After you study each appendage it should be moved aside to reveal the one beneath it
(anterior to it). Do not detach the maxillipeds from the body.
Third Maxilliped
Look at the crab en face, with magnification as needed, and find the quadrate mouth field
including the mouth, the area around it, and the mouthparts. The third maxillipeds cover the mouth
field (Fig 2). They are the appendages of the third thoracomere and together they resemble a pair of
doors protecting the mouth field and hiding the other mouthparts.
Move one of the third maxillipeds and note its mobility. It is attached to the body by its
protopod (Fig 4). The flabellum is used to clean the gills.
Figure 4. The third maxilliped. Crab23L.gif
Second and First Maxillipeds
The second and first maxillipeds are similar to the third but are smaller (Figs 5, 6). They
are the appendages of the second and first thoracomeres respectively. Like the third, they bear
endopods, exopods, and lateral flabellae that extend into the branchial chamber.
Figure 5. The second maxilliped. Crab24L.gif
The two exhalant apertures, through which water exits the branchial chambers, are not so
obvious as the inhalant aperture but can be seen lateral to the first and second maxillipeds. Water
can exit these openings even when the third maxillipeds are closed over the mouth field.
Figure 6. The first maxilliped. Crab25L.gif
Mouthparts
The remaining five pairs of appendages belong to the head segments and comprise three
pairs of mouthparts and two pairs of antennae. Move the maxillipeds aside to reveal the mouthparts.
Second Maxilla
The posteriormost head appendage is the second maxilla (Fig 7) lying immediately anterior
to the first maxilliped. It is small, complex, and more delicate than the maxillipeds. It has several
parts, one of which is a large, lateral, flat, rectangular gill bailer, or scaphognathite, which projects
through the exhalent aperture into the branchial chamber. The bailer, which generates the
respiratory current through the branchial chamber, is the exopod of this appendage (Fig 19-38C).
The protopod has two flat, setose, bilobed endites and a vestigial endopod between the bailer and the
endites.
Figure 7. The second maxilla. Crab26L.gif
First Maxilla
The first maxilla (Fig 8) is even smaller and more delicate than the second. It has two
endites and an endopod but no exopod.
Figure 8. The first maxilla. Crab27L.gif
Mandible
Anterior to the first maxillae are the large, hard mandibles (Fig 9). Each mandible consists of
a heavily calcified protopod from which arises a small palp. The protopod is divided into a medial
cutting surface and a large basal region for the insertion of the muscles that operate the mandible.
Only the smooth, white cutting surfaces can be seen externally. Push the mandibles back and forth
and watch their motion. They rotate on two movable articulations, or condyles, with the head
skeleton and are adapted for cutting, rather than grinding, food.
Figure 9. The mandible. Crab28L.gif
Push the two cutting surfaces apart and notice the soft protuberant upper lip; a transverse
fold of the body wall. Ventral to the lip and between the mandibles, is the mouth. Gently insert your
blunt probe into the mouth to confirm its location.
Antennae
Both pairs of antennae are small and may go unnoticed if they are folded out of sight under
the anterior edge of the carapace (Fig 1).
Antenna 2
The lateral pair is the second antennae. In crabs they lack exopods and are uniramous (Fig
10). Each consists of a basal peduncle, of three articles, whose proximal article is fused with the
carapace and bears a small, heavy, transverse ridge. Distally the peduncle bears a slender
flagellum of many articles.
Figure 10. The first (right) and second (left) antenna of Callinectes similis. The peduncular
articles of each are numbered. The inset is an enlargement of the base of antenna 2 showing
the nephridiopore and its operculum. Crab29L.gif
Find the short pronounced ridge running transversely across the first peduncle article (Fig 10).
Below the ridge is a second, longer, ridge forming the dorsal border of the mouth field. The external
opening of the nephridium, the nephridiopore, is located in the depression between these two
ridges. The pore is covered by a movable calcified operculum. (In Cancer there are no transverse
ridges but there is a tiny oval operculum covering the nephridiopore.) Insert the tip of a needle under
the ventral edge of the operculum and lift it to demonstrate its mobility. In living animals urine may
escape when the operculum is lifted. The operculum is best observed with magnification.
Antennule
The first antenna (Fig 10) consists of a triarticulate peduncle from which arise two short
inconspicuous flagella. The basal article of the peduncle is broad and swollen and fits in a socket in
the anterior body wall. It contains a statocyst for the detection of gravity (Fig 19-7B). The statocyst
is an invagination of the exoskeleton containing a statolith resting on sensory setae. The basal
peduncular article is not fused to the carapace. The first antennae fold on themselves and fit neatly
into a recess in the carapace.
A small, pointed, median rostrum extends anteriorly between the bases of the two first
antennae (Fig 1).
Eyestalks
The two short, thick eyestalks, which are not segmental appendages, are located on the
anterior edge of the head in the orbits (Fig 1). A large compound eye, located at the end of each
eyestalk is composed of hundreds of independent photoreceptive units, or ommatidia. The external
manifestation of each ommatidium is its cuticular lens.
Internal Anatomy

Turn the crab so its dorsal side is up. Insert the tip of a heavy scissors beneath the lateral,
posterior edge of the carapace, dorsal to the coxa of the fifth leg, and make a cut around the
periphery of the carapace on its dorsal surface (Fig 11). Be careful that you cut only the heavy calcified exoskeleton and not the organs beneath it. Keep your scissors about 5 mm from the edge of the
carapace and cut completely around it. Use a scalpel to separate it (by scraping, not cutting) from
the underlying tissues. Carefully remove the carapace, in pieces if necessary, with minimal
disturbance to the underlying tissues.
The thin, dark body wall, which is little more than the epidermis, lies immediately beneath the
carapace and as much of it as possible should be removed with the carapace. The exoskeleton and
epidermis are the body wall, as there is no musculature, connective tissue, or peritoneum in it.
Notice two small, digitiform, calcareous processes on the inner surface of the carapace almost
exactly in its center. These are apodemes for the origin of muscles running to the gut. These
muscles must be disconnected to remove the carapace.
The hemocoel is the large space in which the viscera lie. The coelom is present only as
small spaces associated with the gonads and nephridia.
Carefully remove most of the remaining epidermis (i.e. body wall) without damaging the
underlying tissues. It may help to flood its surface with water to facilitate its removal. This is a
tedious task but must be done with care as some of the crab's organs adhere tightly to it.
>1a. Make a wholemount of a small piece of epidermis being sure the tissue is flat and not
folded. Examine the preparation with the compound microscope. The pigment in the body wall is
contained in conspicuous chromatophores (Fig 19-44A,B). These are irregular, star-shaped
multicellular organs containing red, blue, or white pigment granules. Red and black can be found
easily but the white chromatophores are harder to see because the pigment is not so vivid.
The pigment can be dispersed into the lobes of the chromatophore to increase its visual effect
or it can be concentrated in the center to minimize it. <
Preview
Make a preliminary examination of the hemocoel and its viscera to locate major structures for
use as landmarks. If your specimen is a mature female, the orange ovaries may cover and obscure
other structures. The smaller, white testes of the mature male do not obscure other structures. It
may be necessary to remove the ovary (but nothing else) from one side in order to see the stomach
and digestive ceca beneath.
The stomach (= proventriculus) is a large, bulging, transparent, thin-walled sac lying dorsally
on the midline in the anterior thorax (Fig 11, 19-35). The digestive ceca are large, soft, amorphous,
yellow or greenish organs occupying the periphery of the dorsal thorax. They may be completely
obscured by the ovary in mature females. (The digestive ceca of Cancer are gray-brown and consist
of abundant small, fingerlike papillae.)
Figure 11. Dorsal dissection of a mature male crab. The digestive cecum, gonads, and gills
have been removed from the left side. Crab30La.gif
The large, triangular, firm, beige or greyish mass of gills occupies the branchial chamber in
the space medial to the lateral spine (mostly posterior to the lateral spine in Cancer). The gills are
sometimes called "dead man's fingers".
The triangular mass of gills is covered by a very thin, transparent membrane which you should
avoid damaging. Posterior to each gill chamber is a heavy endoskeletal plate called the flanc that
covers the powerful swimming muscles of pereopod 5. These muscles are the "backfin" crabmeat of
the seafood industry. (In Cancer the skeletal plate enclosing the muscles of the last leg is not
markedly larger than those of the other legs). The soft, white or gray heart lies on the midline
posterior to the stomach and between the flancs.
A more careful study of the organ systems can now be undertaken. Systems are considered
in the order in which they are exposed by the dissection. The internal organs of crabs have little
connective tissue and are very delicate so that some organs appear shapeless and without definite
structure.
Hemal System
The opaque white or gray heart (Figs 11, 12, 19-39) lies in a hemocoelic space known as the
pericardial sinus.
Contractions of the heart pump blood anteriorly, ventrally, and posteriorly via a set of seven
arteries (Fig 12, 19-39). The arteries are difficult to observe in fresh specimens but some, or all, of
them are usually visible in preserved animals. The arteries branch repeatedly until, by the time they
reach the tissues, their diameter is that of capillaries.
The blood leaves the capillaries and bathes the tissues. It then flows into the hemocoel,
passes through the gills, and then drains back into the cardiac hemocoel. When the heart relaxes,
the valves of the ostia open and admit blood to the heart lumen. Upon contraction the valves close
and blood enters the arteries.
The blood, or hemolymph, contains the respiratory pigment hemocyanin, which is colorless
when deoxygenated and pale blue when oxygenated. The pigment is in solution in the hemolymph.
Digestive System
The arthropod gut consists of an anterior foregut, middle midgut, and posterior hindgut. The
foregut and hindgut are derived from ectoderm and are lined with exoskeleton that is shed with each
molt. The midgut is an endodermal derivative and has no cuticular lining. The foregut comprises
the mouth, esophagus, and stomach. The crab midgut consists of the tubular midgut itself and
several diverticula including the digestive ceca and three midgut ceca. All digestion and absorption
occur in the midgut and its derivatives. The hindgut consists of the tubular intestine, rectum, and
anus.
The large yellowish digestive ceca (= midgut glands, digestive glands, or hepatopancreas)
are diverticula from the midgut (Fig 11). They connect with the midgut near its junction with the
stomach but the connections are difficult to observe. The digestive ceca extend along the anterior
edge of the carapace, over the branchial chambers, and in the spaces around the heart and
gonoducts. The ceca secrete hydrolytic enzymes into the stomach, absorb the products of
hydrolysis, and store food reserves.
The stomach is the largest and most conspicuous part of the gut (Fig 11, 19-34, 19-35). It is
an exceptionally complex structure whose walls bear some 40 calcareous ossicles and 80 muscles.
It is divided into a large, dorsal cardiac stomach (or anterior chamber) and a smaller, ventral pyloric
stomach (or posterior chamber).
The cardiac stomach is the large balloon-like structure in the anterior thorax. It lies dorsal to
the mouth to which it is connected by the short esophagus (Fig 19-34). Its walls contain the
ossicles of the gastric mill, used for grinding food. The ossicles are part of the exoskeleton and,
being exoskeletal, are shed and replaced with each molt. The mandibles cut food into small pieces
which are passed to the gastric mill for trituration and mixing with hydrolytic enzymes from the
digestive ceca.
The pyloric stomach is the much smaller ventral region of the stomach. It lies posterior and
ventral to the cardiac stomach and is hidden by it.
Look on either side of the mouth and esophagus to find the white calcareous internal part of
the mandible extending into the anterior body cavity (Fig 9). The four powerful muscles that operate
the mandible insert here, three of them by calcareous white tendons. Gently push the mandible
back and forth and watch the response of the cutting edge. Two muscles, the lateral adductor and
posterior adductor, move the cutting surfaces together (Fig 9, 11). Two other muscles, the abductor
minor and abductor major, move the surfaces apart (Fig 9).

Postpone examination of the remainder of the digestive system and then return to this
part of the exercise AFTER study of the reproductive system. At that time, remove the
reproductive organs in the vicinity of the heart if you have not already done so. GENTLY lift the
posterior end of the cardiac stomach and look beneath it for the small pouchlike pyloric stomach
(14, 19-35, 19-34). Do not damage the muscles and other tissues in this area. The walls of the
pyloric stomach bear the abundant ossicles and fine setae of the filter press that is used to sieve
particles from the liquid in the stomach. The liquid, which contains the products of hydrolysis, is sent
to the digestive ceca for absorption.
While the proventriculus is elevated, find the delicate, transparent midgut running posteriorly
from the posterior chamber under the heart to join the hindgut in the anterior abdomen (Fig 11).
The hindgut, or intestine, runs along the ventral midline of the abdomen to empty through the
anus located on the ventral surface of the telson (Fig 2).

With your scissors, make a transverse cut across the top of the stomach and look inside.
Locate the ossicles of the gastric mill, find the pyloric stomach on the floor of the posterior half of the
cardiac stomach. Find the filter press in the pyloric stomach. Insert the tip of the probe into the
mouth and use it to locate the opening of the esophagus into the stomach.
Respiratory System
The respiratory system consists of the gills located in the two lateral branchial chambers
(Figs 1, 11, 19-36). There are two layers of the carapace. Of its two layers, the outer is heavy and
calcified and part of it has been removed. The inner layer is thin, uncalcified and unsclerotized and
is still intact, covering the gills. This transparent body wall lying over the gills is no more than a thin
chitinous membrane investing the dorsal surface of the branchial chamber. It is almost invisible but it
is all that separates the branchial chamber (which is filled with seawater) from the hemocoel (which is
filled with blood). Find and remove this thin, transparent sheet from the surface of the gills. The
branchial chamber is now open and the gills are exposed for study.
There are eight gills on each side of the body but two of them are small and easily overlooked.
The gills are exites of thoracopods. Each of the eight gills consists of a long central axis to which
are attached, on opposite sides, two rows of very closely spaced flat branchial lamellae (Fig 1937E,F). Use a microneedle to separate two adjacent lamellae from each other and look at them with
magnification. Together the branchial lamellae provide an immense surface area for gas exchange.
The lamellae are covered by cuticle which is molted periodically along with the rest of the
exoskeleton.

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.
The gills projecting into the branchial chamber divide it into dorsal and ventral regions (Fig 1936). Water flows in the inhalant aperture to the ventral inhalant chamber (= hypobranchial
chamber), then across the gill filaments into the dorsal exhalant chamber (= epibranchial chamber).
It then exits via the exhalant aperture.
Insert a blunt probe into the inhalant aperture at the base of the cheliped and observe that it
enters the ventral inhalant chamber, below the gills. The probe can be pushed gently upward
through the curtain of gills into the dorsal exhalant chamber above the gills thus tracing the route
taken by the respiratory water current through the gill chamber (Fig 19-38). Now insert your probe
into the exhalant aperture and note that it enters dorsally, above the gills, in the exhalant chamber.
Lift the gills of your dissected specimen and look at the floor of the branchial chamber. Here
you will see the narrow, setose flabella of the second and third maxillipeds in the inhalant chamber
(Fig 11, 19-36). The flabellum of the third maxilliped is the longer of the two. With forceps wiggle
the two maxillipeds in turn and watch their flabella move. These flabella are moved by the activity of
their respective maxillipeds.
Look in the exhalant chamber along the anterior edge of the base of the gills to find the
flabellum of the first maxilliped. This flabellum has its own muscles and operates independently of
its maxilliped. Wiggle the first maxilliped and see that its flabellum does not respond as did those of
the other two maxillipeds. The flabella sweep over the surface of the gills and keep them clean.
The second and third maxillipedal flabella clean the inhalant side of the gills whereas the first
maxillipedal flabellum cleans the exhalant side.
The flabella also participate in circulating water through the branchial chamber. The gill bailer
of maxilla 2 can be seen beside the flabellum of maxilliped 1. The gill bailer beats to move water out
the exhalant apertures and has the chief responsibility for generating the respiratory current through
the branchial chamber.
In spite of the activity of the flabella the gills are sometimes fouled. The tiny stalked barnacle,
Octolasmis (2mm) lives attached to the gill lamella of several species of crabs. In this position these
filter feeders take advantage of a protected habitat in the branchial chamber through which there is a
constant flow of food and oxygen rich water. You may see some of these on your crab’s gills. A
small, orange, nemertean worm, Carcinonemertes carcinophila, also inhabits the branchial chamber
of blue crabs.
Reproductive System
The morphology and appearance of the reproductive system vary markedly depending on sex
and degree of maturity of the specimen. The following accounts are of mature individuals and if your
specimen is immature you may not find all of the structures mentioned.
Male
The two long, paired, indistinct, white or grayish, testes lie dorsally in the anterior body where
they may be difficult to distinguish from the digestive ceca beneath them (Fig 11). Each testis is a
translucent convoluted tube that begins laterally near the base of the lateral spine and extends
anteriorly and medially. It lies on top of the digestive ceca and parallels the anterolateral border of
the carapace. Near the stomach it turns posteriorly and parallels the border of the stomach and
approaches the midline.
>1e. Remove a few spermatophores from the proximal vas deferens and place them on a
slide with seawater or isotonic saline but do not add a coverslip yet. Look at the spermatophores
with the dissecting microscope. They are ovoid in shape and are packed with spermatozoa. Affix a
coverslip and withdraw enough of the water from beneath it so that some of the spermatophores
rupture from the pressure of the coverslip. Examine the contents of a ruptured spermatophore with
high power of the compound microscope (400X). The spermatozoa are small, irregular, unflagellated
cells with short cytoplasmic processes. <
Female
In the mature female, the orange ovaries may be small or so large they obscure other organs
in the hemocoel (Fig 12, 19-35). In immature individuals, the ovary is beige or white and much less
conspicuous. The right and left ovaries are connected across the midline by a narrow isthmus and
each consists of anterior and posterior horns. Together the ovaries and the connecting isthmus form
an "H" (Fig 12).
The oviducts exit the ovary and connect with the female gonopores on the sternite of
thoracomere 6. The distal region of the oviduct is the seminal receptacle, part of which has a hard
chitinous wall (Fig 12). Each of the two receptacles is located a little lateral to the midline between
the stomach and heart along the posterolateral border of the stomach next to the gill mass. In the
male the middle vas deferens occupies the equivalent position.
The size and color of the receptacle varies and may be quite large, hard, and pink for a brief
period following copulation when the sperm mass and its large, pink, jelly plug are present. Later the
receptacles shrink again and turn white as the jelly is absorbed. Sperm are retained in the seminal
receptacle where they will later be used to fertilize the eggs. Copulation occurs while the ovary is still
white and immature. Later it will turn orange and expand greatly in size as orange yolk accumulates.
Nervous System
The nervous system of brachyuran crabs is highly cephalized into a brain and a large thoracic
ganglion (Fig 13). The brain is located dorsally in the head immediately posterior to the rostrum,
between the two eyestalks, and on the midline. It lies under a layer of muscle and connective tissue
that must be removed before it can be seen. Emerging from the brain are optic nerves to the
compound eye, oculomotor nerves to the eyestalk muscles, antenna 1 and antenna 2 nerves to the
two pairs of antennae, and a tegumentary nerve to the anterior integument (Fig 13).
Two long circumesophageal connectives leave the brain to run posteriorly and ventrally
around the sides of the esophagus. Well posterior to the esophagus they join the thoracic ganglion
which is formed of the coalesced paired ganglia of all thoracic and abdominal segments. Paired
nerves radiate from this ganglion to each thoracic appendage and a single abdominal nerve extends
to the reduced abdomen and its appendages (Fig 13). The thoracic ganglion is obscured by
connective tissue and muscle which must be removed to reveal it. It lies at the intersection of the
midline and a line connecting the seventh anterolateral teeth of the two sides of the carapace (Fig 1).
Segmental nerves to the thoracic segments exit this ganglion as does a single, median nerve to the