LABORATORY 7 PHYLUM MOLLUSCA

							                     LABORATORY 7: PHYLUM MOLLUSCA

         The phylum mollusca is one of the largest of all phyla, both in the size of certain
species and the number of species which have been described. There are approximately
90,000 described species. Early molluscs were abundant in cambrian seas and the long
history of the group is reflected today in the variation among molluscan types. This
variation also attests to the success and plasticity of the basic molluscan body plan, which
is far from obvious in some modern members of the phylum.

       The basic molluscan body plan is bilaterally symmetrical, unsegmented,
protostomate, and coelomate and the body is divided into a ventral muscular foot, a
dorsal visceral mass, and a mantle (pallium) of epithelium and other tissue which
encloses the dorsal surface of the body. The cavity between the mantle and visceral mass
is termed the mantle cavity. The visceral mass is provided with a blood circulatory
system generally containing the oxygen carrying copper pigment haemocyanin, a
variably specialized and cephalized nervous system with ganglia and ventral nerve cords,
a well developed excretory system, and a distinct reproductive system. The mantle cavity
generally houses an efficient respiratory system. As will be seen in today's laboratory,
however, among the molluscan classes almost every one of the organ systems mentioned
above shows a wide spectrum of variation.

      Molluscs apparently arose as creeping types, probably living on hard surfaces and
scraping their food from the substrate by means of a unique organ, the radula, which is
found in all modern classes except the Bivalvia (Pelecypoda). Bivalves have extensively
modified their gills (ctenidia) for filtering particulate food from the water column. The
molluscs are closely related to the annelids. This affinity is seen in the similar
developmental patterns within the two groups, the trochophore larva, and the possible
vestiges of segmentation seen in some of the primitive molluscs.

      The molluscs are divided into six classes: Monoplacophora, Polyplacophora (=
Amphineura, chitons), Gastropoda (snails), Pelecypoda (bivalvia, clams), Cephalopoda
(squids and octopus), and Scaphopoda (tooth shells). In today's laboratory we will deal
primarily with gastropods, pelecypods, and cephalopods, the three numerically dominant
classes. You will perform dissections on members of each of these classes to get a
handle on their similarities and differences. We will also examine some aspects of
molluscan locomotion and feeding. You will have to carefully allocate portions of
today's work to members of your laboratory group to finish.

CLASS GASTROPODA

        The gastropods are more similar to the ancestral molluscan form than any of the
other molluscan classes we will be examining today. They differ from the primitive
ancestor in having an enlarged head and visceral mass, in most cases a logorithmically
spiraled shell, and a visceral mass that has undergone a 180° rotation during development
(torsion), so that the gills and anus are located on the anterior end of the snail. Before
proceeding with the dissection of a marine prosobranch and terrestrial pulmonate snail
examine the collection of shells available in the laboratory. Try to determine the possible
advantages and disadvantages of different shell types. How do the shells of rocky shore,
soft bottom marine, terrestrial, and freshwater snails differ?




Genus Busycon

       Procedure:
        Examine the external features of a preserved specimen of Busycon, which has
been removed from its shell. In life Busycon is a predator found in soft bottomed littoral
habitats which preys mostly on bivalves. On the foot locate the thin horny operculum,
which in life closes the aperture of the shell and protects the animal that is retracted
inside. Near the anterior end of the sole of the foot is the opening of the pedal gland,
which produces mucus which facilitates movement. The mucus also helps the snail
adhere to hard substrate by suction produced by contraction of the central portion of the
foot. The pedal gland is also important in forming egg cases which are attached by
females to hard substrate. The triangular mouth is located at the end of the proboscis
which extends from beneath the paired tentacles. Note the eyespots on the tentacles. In
male specimens, a large penis can be seen by the right tentacle. The coiled visceral mass
is covered by a thin mantle, which thickens to form a collar at the base of the viscera.
The shell is secreted at the edge of the collar. The collar is elongated posteriorly to form
an extensible siphon through which water is drawn into the mantle cavity. What is the
advantage of the siphon?

        Through the mantle at the apex, locate the right lobe of the brownish digestive
gland or liver on which lies the yellow or orange gonad and a straight (female) or coiled
(male) gonoduct. See pp 234-235 in S&S. These two organs fill the first and smallest
whorl of the shell. Examine the Busycon shells in the laboratory to get an orientation on
how your specimen would be situated in its shell. The next whorl is occupied by the left
liver lobe as well as the stomach and part of the intestine. The large brown kidney and
heart are located along the dorsal surface to the left of the base of the visceral mass.
Anterior to the heart and kidneys lies the oblong ctenidium and the sensory osphradium
and traces of the mucus-secreting hypobranchial gland. Examine the configuration of the
organs of Busycon. How would they be arranged in an untorted snail? List differences
between Busycon and an unsorted snail which have resulted from torsion.

       Examine a Busycon shell and notice that it is composed of several spirally arranged
whorls. The large terminal whorl is called the body whorl. The shell is actually a coiled
tube leading from the body whorl to the apex and wound around a central column or
columella. Examine a sectioned Busycon shell to see the three structural shell layers, the
outer periostracum, middle prismatic layer and inner nacreous layer. Busycon is one
of the few gastropod species which exhibits both right handed (dextral) and left handed
(sinistral) coiling. In Busycon this is a genetically determined trait. Most snails are
dextral. You can determine handedness in a snail by holding it with its apex up and
aperture pointed toward you. If the aperture opens to the right the shell is dextral; if it
opens to the left it is sinistral.

       Return to your preserved Busycon and open the mantle cavity by making a median
incision about a centimeter to the right of the middorsal line along the ctenidium until
you reach the pericardium. Avoid disturbing the heart. Observe the single attached
ctenidium composed of only one row of filaments. Anteriorly, on the inhalent side of the
ctenidium, is a brownish chemosensory osphradium composed of about 100 triangular
filaments covered with epithelium. The osphradium functions in monitoring the quality
of incoming water. Along the cut edge of the mantle lies the anus at the end of the
rectum, and to its left the mucus-secreting hypobranchial gland formed of heavy folds of
the mantle. The mucus secreted by this gland helps to consolidate particles rejected by
the gills before leaving the mantle cavity, preventing clogging.

        If the proboscis is extended, observe the position of the radula within the mouth at
the tip of the proboscis. Specimens in which the proboscis is not extended should be
dissected by making a cut between the tentacles to expose the proboscis. Slit the
proboscis along its length and examine the esophageal cavity, the radula, and the
odontophore (see p. 234, S&S). Examine the muscles that control the odontophore and
the radula. By cutting these muscles, free the radula and place it into 10% potassium
hydroxide for later examination. Next try to identify the components of the digestive
tract illustrated on p. 234 of S&S (Fig. 8.4B).

       Be sure you examine the one Busycon specimen in each laboratory which has been
injected with colored latex to show the details of the circulatory system. Try to determine
the flow of blood through Busycon.

      Before discarding your pickled Busycon be sure to take note of the esophageal
nerve ring. Also, trace the nerve cords from the nerve ring and verify that they do not
form a straight loop as in primitive molluscs (like chitons), but rather have a figure eight
configuration. Why is this so?

Genus Helix

      Procedure:
       Obtain a freshly killed specimen of the pulmonate snail Helix. Helix is a terrestrial
herbivore. Using bone cutters, carefully cut around the spirals of the shell and remove
the animal (See p. 236, Fig. 8.5C, S&S). Leave only the central column (columella)
and be careful not to disturb the soft parts. With the aid of the figures on p. 236 of S&S,
identify the external and obvious internal structures of Helix. What differences do you
see between Helix and Busycon? Pay particular attention to the mantle cavity, the roof of
which is richly supplied with blood vessels. The mantle cavity serves as a functional
lung. There are no ctenidia. Examine a living specimen of Helix under the dissecting
microscope and observe the rhythmic opening and closing of the pneumostome, a small
aperture on the right side of the body. This opening leads to the pulmonate lung.

       Returning to your shell-less Helix, sever the columellar muscle and, then by
inserting one blade of a scissors through the pneumostome, cut the mantle from the body
wall in both directions from the pneumostome. Pin your specimen to a wax pan as shown
in S&S, p. 236, Fig. 8.5D and try to identify the internal structures shown. Before
leaving your dissection, locate the buccal mass and cut out the radula and place it in 10%
potassium hydroxide.

       The radula is the characteristic feeding organ of all molluscan groups except
pelycepods. It is used as a scraper, a rasping tongue and as a drill. Its architecture differs
among species depending on how it is used. Obtain recently killed specimens of
Littorina and Urosalpinx, remove them from their shells and try to dissect out their
radulas. Place the radulas in 10% potassium hydroxide. You should now have the
radulas from two herbivorous snails Helix and Littorina and two carnivorous snails
Busycon and Urosalpinx. Boil each of these for 10 minutes in test tubes containing 10%
potassium hydroxide. This will dissolve unwanted tissue and make the teeth more clearly
visible. Examine each of the radulas under the microscope and describe and compare
their structure. Also, examine the prepared slides of the radula of a chiton. How does the
radula of each of these molluscs relate to its feeding habits?


Locomotion:
       Locomotion in most gastropods is accomplished by muscular contractions of the
foot aided by mucus secretion. Exceptions to this general pattern include swimming
gastropods and gastropods that use cilia to locomote. In gastropods that move by the
muscle/mucus method, there are two specific ways by which movement is achieved: 1)
direct muscular waves where the posterior edge of the foot is lifted, moved forward and
then this advancing wave is propagated forward and 2) retrograde muscular waves
where the anterior end of the foot is stretched and attached and the advancing wave is
propagated backwards. These two modes of locomotion are illustrated in Barnes, p. 341.
Attempt to characterize the type of locomotion mechanism found in Helix, Littorina, and
Urosalpinx. To do this have the snail settle on a plexiglass plate (in an aquarium with
Littorina and Urosalpinx) and then tilt the plate so that you can examine the movement of
the foot from beneath. Use a magnifying glass if they are available. Do all these snails
move in the same fashion? Does substrate angle influence whether a snail moves by
direct or retrograde waves? Which of these modes do you think is the most efficient?
       Torsion in gastropods probably evolved to increase the protective valve of a
gastropod's shell by allowing them to retract into their shells head first and cover the
aperture with a horny operculum. Torsion, however, created the sanitation problem of
putting the anus directly over the head. Gastropods have evolved a number of solutions
to this problem involving the direction of water currents in the sorted anterior mantle
cavity. To get an idea of how the most prevalent of these solutions works, place a
Littorina or Urosalpinx in a shallow finger bowl filled with saltwater and allow it to
settle. With a pipet gently place a drop of a carmine suspension in front of the snail and
observe the movement of the carmine. Try this also with Crepidula. What is the major
difference between the water currents of Crepidula and the other snail? Why do you
think this is so? Examine the ventral surface of Crepidula and describe how it differs
from the other snails you have examined. Why might Crepidula be described as
bivalve-like?

       While most prosobranch snails (torted, gill-bearing snails) are dioecious, some are
hermaphroditic. Pulmonate (lung bearing) and opistobranch (detorted, sometimes
shell-less) snails are hermaphroditic. In the aquaria, you will find a number of "stacks"
of Crepidula fornicata individuals, a sequentially hermaphroditic prosobranch snail.
Crepidula juveniles start out life as a males, generally settling on the shells of older
conspecifics. Later in life Crepidula individuals change sex to females. This change,
however, is mediated by the presence or absence of other females. In the presence of
other females, males stay males. Male Crepidula can be discerned by the presence of a
penis to the right side of their head. Examine the sexual composition of one of the
Crepidula communes in the aquaria. Do your observations confirm the above scenario?
Of what advantage do you think this tactic is to an individual snail?
CLASS PELECYPODA

      Bivalves do not initially appear to have much in common with snails or the
primitive molluscan form except for their protective shell. Bivalves are generally
sedentary. The foot, visceral mass, and mantle cavity dominate the body, and the head is
suppressed. Bivalves have developed from the primitive molluscan form by enlarging
the mantle and dividing it into symmetrical halves hanging down on both sides of the
body, enlarging the gills in the now huge mantle cavity, and extending the foot downward
between the mantle folds as a blade-like structure. Bivalves have lost the radula and the
majority are ciliary feeders with large, platelike food-gathering gills (ctenidia). The
extensive mantle encloses the entire body in two symmetrical flaps which secretes a
hinged, two-part shell.

Genus Mercenaria

      Procedure:
       Obtain a specimen of the clam, Mercenaria and examine its external features (see
p. 238 of S&S). Note the umbo of the shell and the growth lines. What does the umbo
represent? How do clams grow? Also identify the hinge ligament, the siphons, and
muscular foot. What is the function of each of these structures?

        Identify the right and left valves of your Mercenaria. Do this by orienting the
anterior end of the clam up and noting that the umbo is on the dorsal side of the body.
Take a sharp scalpel and carefully insert it between the valves and, moving the blade
along the ventral edge close to the left valve, cut the adductor muscles which effect shell
closure. You will probably need to insert a pair of scissors between the valves to hold
them open while cutting (see p. 238 of S&S). Once you have cut the muscles, remove the
left valve to examine internal anatomy. Underlying the shell is the fleshy mantle. Note
how it hangs like a sheet, attached dorsally and free ventrally. The dorsally located
pericardium which encloses the heart can be seen through the mantle. Posterior and
ventral to the pericardium is the brownish kidney. Note the anterior and posterior
adductor muscles which you cut to open your Mercenaria. Also identify the retractor
muscles which control extension and retraction of the foot. Water enters the mantle
cavity through the ventral inhalent siphon and exits via the dorsal exhalent siphon. The
water current is driven by the large, folded ctenidia which fill the bulk of the mantle
cavity. Keep your specimen under seawater when you are not looking at it so that it
doesn't dry out.
        Examine the shell you have removed from your specimen (see p. 238, S&S). On
the outside is the thin proteinaceous periostracum especially apparent at the hinge. By
breaking the shell you can be in middle, prismatic shell layer composed of calcium
carbonate plates and protein. The inside shell layer, the nacreous layer, is composed
mostly of calcium carbonate. Each of these shell layers is secreted by the mantle. The
outer lobe of the mantle secretes both the periostracum and the pismatic layer on the
leading edge of the mantle, while the nacreous layer is secreted by the entire mantle.
Compare the shell structure of Mercenaria with the ribbed mussel Geukensia. How do
they differ? Can you relate these differences to the habitats of these bivalves?

       The ctenidium of most bivalves serve both a respiratory and food gathering
function and are greatly enlarged when compared to the gills of other molluscs. For this
reason, filter feeding bivalves are termed lamellibranchs (plate gills). Most bivalves
possess the single pair of ctenidia found in the generalized mollusc, but each gill has been
expanded to form a large W-shaped structure. Try to verify this with your Mercenaria
specimen. In filter feeding, particles sieved by the gills are sorted to size by means of
ciliated grooves and moved to the labial palps which move food material into the mouth.
Locate the ciliated food groove between the labial palps and the slitlike mouth. Food is
trapped in a mucus strand secreted by the salivary glands and passed into the esophagus.
From the esophagus, food passes to the stomach which is surrounded by a large green
digestive gland. An outpocketing of the stomach called the style sac contains a
gelatinous rod called the crystalline style. The style is composed of enzymes and slowly
revolves by style sac cilia to wind the mucus string into the stomach while releasing its
enzymes which begin the digestive process. The style may not be present in your
specimen since bivalves resorb their styles under harsh conditions. Digestion occurs in
the stomach and in the digestive gland. The remainder of the digestive tract consists of a
long intestine and an anus which opens near the exhalent siphon.

        You should also be able to identify gonads, the heart, kidney, and cerebral ganglia
in your Mercenaria specimen.

        In all lamellibranch ("sheet gill") bivalves, the gill or ctenidium is typically
W-shaped. It is composed of numerous folded filaments which are connected to form
sheets or lamellae, each gill possessing four such lamellae. Each gill is positioned within
the mantle cavity so that one free arm of the W is connected to the mantle and the other
free arm is connected to the foot or visceral mass. Thus the gills effectively divide the
mantle cavity into several chambers. The large chamber below the gills is called the
inhalent chamber while the cavities above the gills are exhalent chambers.

        Gills are usually considered to have respiration as their primary function. In
lamellibranch bivalves, however, a much larger surface area of gills is present than is
actually needed for gas exchange, and the gills have assumed additional functions. In
freshwater bivalves, for example, the gills are used as brood chambers where glochidia
larvae are protected until they are mature enough to be released. Finally, in addition to
respiratory and reproductive functions, perhaps the most important function of
lamellibranch gills is in feeding.




         All lamellibranch bivalves are filter feeders. Special cilia located between the gill
filaments produce water currents which move water into the inhalent portion of the
mantle cavity and up through the gills into the exhalent chambers. Particles of food or
other suspended material which are above a certain size are filtered from the water by gill
cilia and accumulate on the inhalent faces of the gill lamellae. This material is then
moved by other cilia toward the ventral edges of the gills (the bottom points of the W)
where the food grooves are located. Once in the food grooves, the food moves anteriorly
until it reaches the palps, located on either side of the mouth. Here again sorting is
carried out on a size basis. Fine material is carried by cilia into the mouth. Coarser
particles accumulate at the edges of the palps and are periodically thrown off by muscular
twitches onto the mantle wall. This material which has never entered the gut is usually
called pseudofeces. The pseudofeces are eventually expelled from the mantle cavity by
spasmodic contractions of the adductor muscles which force wate and the accumulated
pseudofeces out through the normally inhalent opening or siphon.

        It should be noted that the anal opening (where true feces are released) and the
renal and genital openings are all located in the exhalent portion of the mantle cavity.
Thus, expulsion of wastes and of reproductive products is accomplished by the normal,
continuous flow of the feeding current, leaving the animal via the exhalent opening or
siphon.

       Obtain a fresh Mercenaria or Geukensia specimen and open it as described
previously being careful not to damage the gills. Place your clam on a halfshell in a dish
of seawater and carefully lift the free edge of the mantle to expose the gills and palps.
Examine the gills under a dissecting microscope and then by adding carmine particles
trace the movement of suspended material from the gills to the palps. Small pieces of
aluminum foil may be used to examine the rejection of larger particles. After you have
done this, carefully cut a couple small strips of tissue from the leading edge of the
ctenidium and mount them (using saltwater) on a slide and examine them under a
compound microscope. Describe what you see.

Locomotion:
      Bivalves locomote in a number of ways. The most common method is burrowing,
where the muscular foot is used to anchor and pull the clam into the substrate. Some
bivalves such as the razor clam, Ensis, do this quite well. If available, obtain a live Ensis
specimen and place it in a plexiglass burrowing chamber filled with soft sand. The
burrowing chamber should allow you to examine how burrowing is accomplished.
Describe a typical burrowing movement sequence in Ensis. Do the same with
Mercenaria and compare your observations.

       A number of bivalve species attach and move on hard substrates using byssal
threads which are secreted by pedal glands and attached by a small modified foot.
Mytilus and Geukensia are examples of bivalves with this life style. Examine the byssal
thread attachment of these species in the aquaria. Do these mussels respond to stimuli or
are they entirely sessile? Obtain a small (~lcm) mussel from the aquaria and place it in a
fingerbowl of seawater. Examine it under a dissecting microscope. Can you identify the
foot? Set the mussel aside for a while and then reexamine it. Is the foot extended? Has
the foot begun to secrete byssal threads? Make sure that the edge of the shell is in close
contact with the surface so that byssal attachment is possible.

       Some bivalves such as scallops, Pectin, swim by rapidly opening and closing their
valves creating a water jet. Observe this in the aquaria by locating an Aequipectin
specimen and draping it through the water. What structures are responsible for opening
and closing the valves in swimming? If starfish are available in the aquaria, see if they
stimulate escape swimming in Aequipectin. To do this properly you will want to
discriminate between a chemosensory and tactile escape response.


CLASS CEPHALOPODA

        Cephalopods are easily the most advanced molluscs, or invertebrates for that
matter, and their relationship to other molluscs is not immediately obvious. In contrast to
other molluscs the head and foot of cephalopods has become fused to form the cephalized
anterior end, and there has been a tendency towards reduction and loss of the shell. The
adaptive radiation of cephalopods can be viewed as a response to their taking up an
active, pelagic, predatory life style.

        Since cephalopods are rather expensive (live or dead) and are not readily available
in the local area, we will have to restrict our examination of cephalopod in the laboratory
to a dissection of the squid, Loligo. In performing the dissection, you will want to refer
to pages 240-241 of S&S for diagrams.

       Procedure:
        Obtain a pickled specimen of Loligo and place it in a wax bottomed dissecting
pan. Notice the streamlined shape of the squid and the presence of lateral fins. While
carrying out your dissection, keep in mind that Loligo is an active, free-swimming
predator and relate this life style to the design of the animal. The viscera of the squid are
completely enveloped by a thick mantle, the free edge of which forms a collar about the
neck. The head bears a pair of complex eyes. The head is drawn out into 10
appendages--four pairs of arms, each with two rows of stalked suckers, and one pair of
long retractile tentacles, with stalked suckers only at the ends. The tentacles shoot out to
catch the prey. The arms hold the prey while it is eaten. Examine the structure of the
suckers under a lens. In the mature male the left ventral arm (hectocotylus) is modified
for transferring spermatophores to the female. On this arm the distal suckers are
replaced by long papillae.

        The mouth lies within the circle of arms. It is surrounded by a peristomial
membrane, around which is a buccal membrane with seven projections, each with suckers
on the inner surface. In the mature female there is a small pouch or sperm receptacle on
the buccal membrane in the median ventral line, one of the places where the male may
place the spermatophore. The female uses one of her arms to pick up strings of eggs as
they come from her siphon, fertilizes them with spermatozoa from the pouch, and then
attaches the strings to some object in the sea. Probe in the mouth to find two horny
beaklike jaws.

       A muscular siphon (funnel) usually projects under the collar on the ventral side,
but it may be partially withdrawn. Water forced through the siphon by muscular
contraction of the mantle furnishes the power for the "jet propulsion" locomotion that
carries the squid backward through the water. Wastes, sexual products, and ink are
carried out by the current of water than enters through the collar and leaves through the
siphon. The siphon of the squid is not homologous to the siphon of the clam; the clam
siphon is a modification of the mantle, wherea the squid siphon, along with the arms and
tentacles, is a modification of the foot

      The mottled appearance of the skin is due to chromatophores—irregularly shaped
pigment cells, to which radiating muscle fibers are attached. The spreading of the
pigment throughout the cells causes darkening of the skin; the concentration of the
pigment lightens the skin color. The squid can change from almost white through shades
of purple to almost black. Of what adaptive advantage is this to the squid?

      Beginning near the siphon, make a longitudinal incision through the mantle from
the collar to the tip. Pin out the mantle and cover with water. The space between the
mantle and the visceral mass is the mantle cavity. Find a cartilaginous structure on each
side of the siphon and similar structures on the inside of the mantle. These interlocking
pieces of cartilage help support the siphon and close the space between the neck and the
mantle during jet propulsion. There are other cartilages in the head, fins, etc.

       Lateral to the siphon, find large saclike valves that prevent outflow of water by
way of the collar. Slit open the siphon to see the muscular tonguelike valve that prevents
inflow of water through the siphon. Note the large pair of retractor muscles of the
siphon and beneath them the large retractor muscles of the head. Locate the free end of
the rectum with its anus near the inner opening of the siphon. Between it and the visceral
mass is the ink sac. Do not puncture it. When endangered, the squid sends out a cloud
of black ink through the siphon as it darts off in the opposite direction.

       A pair of long gills (ctenidia) are attached at one end to the visceral mass and at the
other to the mantle. A thin skin covers the organs of the visceral mass and encloses the
coelom. Remove this membrane carefully as you expose the visceral organs. If the
specimen if a female, a pair of large whitish nidamental glands (which secrete the outer
capsules of the egg masses) should be carefully removed. Note their location and lay
them aside for later study.

Respiratory and circulatory systems:
       At the base of each gill is a small whitish bulblike branchial heart (gill heart).
Blood from the branchial heart is carried to the gill by an afferent branchial vein and
returned by an efferent branchial vein to the systemic heart, a larger whitish organ lying
between the branchial hearts. Each of the branchial hearts receives the blood from a large
conical posterior vena cave as well as from a fork of the anterior vena cave (cephalic
vein). The systemic heart pumps oxygenated blood through the cephalic aorta (anterior)
and the short posterior aorta, which branches to form medial and lateral mantle arteries.

Excretory system:
        A pair of kidneys, somewhat triangular in shape and usually white or pale in
uninjected specimens, lie between and slightly anterior to the branchial hearts. The
kidneys will take up the color of an injection fluid, if used. A renal papilla lies at the
anterior tip of each kidney.

Digestive system:
         Remove the siphon by first cutting the siphon retractor muscles and then the
lateral siphon valves and the two small protractor muscles. Cut between the two ventral
arms to expose the pharynx (buccal bulb). Cut away the buccal and peristomial
membranes to expose the chitinous jaws. Dissect away the overlapping lower jaw and
bend back the tonguelike ligula. Note the radula with its rows of minute teeth. Remove
the radula and examine under a microscope, sketching the arrangement of the teeth.

       The esophagus leads down through the liver, a soft pale organ lying between the
head retractor muscles. It emerges from the posterior end of the liver, passes through the
pancreas, and leads to the thick-walled muscular stomach, lying back somewhat posterior
to the visceral heart. The stomach communicates directly with the cecum, a thin-walled
sac that may, when filled with partly digested food, be quite large. The intestine leaves
the stomach near the entrance of the esophagus and passes anteriorly to the rectum and
anus. Open and rinse out the cecum and examine on its ventral surface the fan-shaped
spiral valve, a complex device for sorting food particles.

       The ink sac is a diverticulum of the intestine located back of the rectum and anus.
It secretes a dark fluid of melanin pigment that is carried to the rectum by a short duct.

Nervous system:
        Push the head to one side to see a pair of large stellate ganglia on the inner surface
of the mantle close to the neck. These ganglia function in the movement of the mantle.
From each ganglion several large nerve radiate out over the inner mantle surface. Each
nerve contains, along with smaller fibers, one of the giant fibers which are used in rapid
maximal contracl of the mantle. Directions will not be given here for dissection of the
brain, which is composed of ganglia lying partly above and partly below the esophagus.
Sense organs:
        Sense organs of cephalopods are highly developed. The eyes are capable of
forming an image. Remove the thin outer transparent integument (false cornea) to
uncover the true cornea. Cut away the cornea to observe the circular iris diaphragm.
Behind the iris is the almost spherical lens, suspended by a ciliary muscle. Remove the
lens to see the darkly pigmented sensory lining (retina) of the optic cavity. Sensory cells
are numerous in the skin, particularly in the rims of the suckers. Statocysts are found
embedded in the cartilages on each side of the brain.

Reproductive System:
        In the male, the testis is an elongated light-colored organ in the posterior end of
the coelomic cavity. It may be concealed by the cecum. Spermatozoa are shed into the
coelom from an opening in the testis. They then travel up the vas deferens. The vas
deferens connects to the spermatophori gland, which produces substances which
"package" the sperm into spermatophores. These spermatophores are stored in the
spermatophoric sac. During copulation the hectocotylized arm (left ventral) takes the
spermatophores from the genital opening at the tip of the penis and transfers them to the
female.

       In the female the nidamental glands are conspicuous white organs filling most of
the lower part of the mantle cavity. You have probably already removed them. The
ovary lies posterior and sheds eggs into the coelomic cavity. Push the ovary to one side
and try to locate the oviduct (it may be covered by the cecum). Near the left branchial
heart the oviduct enlarges into the oviducal gland, which secretes the individual egg
cases. The oviduct continues anterior! beside the nidamental glands to its flared opening,
the ostium, in the mantle cavity. In the process of mating the male may thrust the
spermatophores inside the female's mantle cavity, or into the sperm receptacle near her
mouth. When the eggs have been fertilized, a gelatinous matrix is secreted around them.
The female holds this gelatinous mass within her arms until she finds a rock or othe
suitable object to attach it to. The young squid hatch in 2-3 weeks.

Skeletal system:
        Dissect out the chitinous pen that lies dorsal to the visceral organs and extends
from the free edge of the collar to the apex of the mantle. There are also a number of
cartilages in the head, near the siphon, and in the mantle.