LABORATORY 7 PHYLUM MOLLUSCA
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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.