Download MOLLUSC LABORATORY Class Scaphopoda Representative for

MOLLUSC LABORATORY
Class Scaphopoda
Representative for observation of scaphopod external characteristics
Class Polyplacophora (Amphineura)
1. Chiton for dissection
2. Chiton radula, prepared slide
3. demonstration specimens of various species
Class Gastropoda
Subclass Prosobranchia
Order Archaeogastropoda
1. Haliotis (abalone)
2. limpets
Order Mesogastropoda
1. Littorina irrorata (marsh periwinkle)
2. Crepidula fornicata (slipper shell)
3. Strombus gigas
4. Polinices duplicatus
Order Neogastropoda
1. Busycon sp.
2. Fasciolaria tulipa
Subclass Opisthobranchia
Order Nudibranchia
1. various demonstration specimens
Subclass Pulmonata
Order Basommatophora
1. Heliosoma – freshwater snail
Order Stylommatophora
1. Limax – terrestrial slug
2. Helix – terrestrial snail, demonstration specimen
3. Helix – specimen for dissection
Class Bivalvia (Pelecypoda)
Subclass Lamellibranchia
Order Anisomyaria
1. Crassostrea – oyster
2. Aequipecten – scallop
3. Mytilus edulis – blue mussel
Order Heterodonta (Eumellibranchia)
1. Mercenaria mercenaria – quahog
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2.
3.
4.
5.
6.
7.
8.
Mercenaria mercenaria/Mya arenaria – specimen for dissection
clam mantle slide
dried clam shell for external anatomy
Mya arenaria – gill material, microscopic anatomy
Mya arenaria – blood
clam veliger, trochophore larvae and blastula
Geukensia (Modiolus) demissa – demonstration specimen
Order Schizodonta
1. Representative freshwater bivalves
2. freshwater clam glochidia
Order Adapedonta
1. Ensis – razor clam
Class Cephalopoda
Subclass Nautiloidea
1. Nautilus – chambered nautilus shell
Subclass Coleoidea
Order Teuthida
1. Loligo – demonstration specimen
2. Loligo – specimen for dissection
Order Sepiida
1. representative demonstration specimen
Order Octopoda
1. Octopus – demonstration specimen
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The mollusks
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 specie. Early molluscs
were abundant in cambrian seas (these animals have an excellent fossil record due to the presence of a
calcified shell) 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 (no representatives in this lab),
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, bivalves, and cephalopods though we also have a representative from the scaphopods. For the
three numerically dominant classes you will perform dissections 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 SCAPHOPODA
Marine molluscs with a tapering, tubular, slightly curved tusk-like shell, that is open at both ends. The
large end remains anchored in the sand by a cone shaped foot. Incurrent and excurrent water flow occurs
at the narrow end.
The drawing of this thin shell, near bottom of page, shows its highly retractile, slender captacula. Like
sticky fingers, secretions on the tips of the captacula pick up food particles for transfer to the mouth,
which is deep inside the anterior end.
About 350 species are known, most found in 6 meters or more of water, with a few species, only, found
on shore. Cadula shells are typically 4 mm in length, whereas Dentalium shells up to 150 mm have been
found off shore of Japan. Fossil species of Dentalium up to 300 mm, with a 30 mm diameter, are known.
The shells are widely used for jewelry and are often massed in the beaded portions of American Indian
clothing decorations. According to those instructions, which genus are you looking at in lab?
Tusk shell molluscs feed on small organisms in the muddy sea bottom. They collect their food by means
by the ceptacula, then scrape off the food using a well developed radula that has wide flattened teeth. The
mouth is near the larger front end, at left in the diagrammatic picture above. Their rear end, at right in the
picture, is where the ciliary currents enter and leave. Its narrow opening has an entire smooth edge. In the
picture, one can also see the well-developed digestive system, which shows as an "x-ray" view (green)
through the mantle (pink). The fan shaped organ is a liver, not a gill, and the darker lobe to the right is the
sex organ. Larval forms are conveyed to the mantle cavity by a duct near the anal opening. The black dots
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represent nerve ganglia. This mollusc has no gills. Exchange of oxygen and carbon dioxide takes place
directly through the thin tubular mantle and the water. The mantle curves around the viscera, to almost
enclose the mollusc in a cylinder. Interestingly, the tubular mantle acts like a peristaltic pump. Water is
ingested at the wider anterior end, and waste is ejected with exhaled water at the rear end, in pulses every
10-12 minutes. The mollusc has no eyes, no osphradium (taste organ), and no sensory tentacles for touch
reception. It was formerly considered to be closely related to the bivalves based on a bi-lobed shell, bilobed mantle and reduced head --features which are more clearly shown in the embryonic form. However,
modern research suggests that it is more likely descended from a common ancestor of the cephalopod.
CLASS POLYPLACOPHORA
Chitons are the familiar group of organisms that have eight valves on their shells. A visit to any rocky
intertidal habitat around the world will introduce one to these beautiful molluscs. Though chitons are
important members of the molluscan clade, occupying a basal position in the phylogeny, and have
interesting diversity and life history traits, our knowledge of the group is spartan in comparison to other
mollusc clades.
All chitons are marine and the group has a worldwide distribution. Most live in the rocky intertidal zone
or shallow sublittoral (just below the low tide level), but some live in deep water to more than 7000 m. A
few species are associated with algae and marine plants, and in the deep sea, waterlogged wood will
serve as a common habitat.
Chitons are generally dioecious (have separate males and females), with sperm released by males into the
water. In most chitons, fertilized eggs are shed singly or in gelatinous strings, and once fertilized in the
water column, these develop into a trochophore larva (free-swimming and ciliated) that soon elongates
and then directly develops into a juvenile chiton; there is no veliger stage (having a velum, a lobed,
ciliate swimming organ). In brooding species, the eggs remain in the pallial cavity of the female where
they are fertilized by sperm moving through with the respiratory currents. Upon hatching from the
brooded eggs, the offspring may remain in the pallial cavity until they crawl away as young chitons or
exit the pallial cavity as trochophores for a short pelagic phase before settling.
Chitons have a unique morphology. Their calcareous shells consist of eight overlapping plates or
valves. These valves overlap posteriorly (the back ends overlap the one behind) and are encircled and
held in place by a muscular fold called the girdle. These overlapping valves allow the chiton to fit into
rock crevices and curl into a ball like an armadillo or pill bug if detached from the substrate. The valves
are made of two distinct layers. The first is the anterior (front) portion of each plate that is overlapped by
the plate in front of it. This layer is called the articulamentum. The second layer is the tegmentum; the
exposed layer not being overlapped. A third part of the plates are the parts concealed by the girdle. These
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sections are called insertion plates or teeth which can vary between different plates. The girdle itself can
vary. It can be covered with spines or scales.
The foot of chitons is usually flat and wide. It is this part of the chiton that is used for both locomotion
and holding onto rocks and other solid substrate. The foot works to keep the chiton attached to rocks
much like a suction cup. The mantle/girdle is a fold that hangs over the sides of the foot and encloses a
long trough along the body of the chiton at the back end. It is within this cavity that the gills, as well as
the genital and kidney pores are found. Chitons can have from 6 to 80 pairs of gills.
Chitons have rudimentary heads without tentacles but specialized sense organs are present. The most
important sense organs are the aesthetes and are found in groups in the shell. In some species, these
organs are modified to form eyes. Within this simple "head", the radula is found. The radula is a stiplike structure that is used for feeding. This organ scrapes algae, diatoms, and other plant food from rocks
and substratum. Most chitons are oval or elliptical, but some may be quite flat while others may have
highly arched back with a central "keel". They have been found to range anywhere in size from less than
5 cm to a maximum of 43 cm.
Chitons are found all over the world. They can be found in high intertidal zones or in depths over 12,000
feet and in any marine environment from the tropics to the polar seas, but are most abundant in temperate
rocky intertidal zones (Classification, 1997). The reason for such abundance in these areas is due to the
fact that these are the areas where chitons can find both solid substratum, such as rocks onto which they
attach, and an abundance of food.
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CLASS GASTROPODA
The gastropods are more similar to the ancestral molluscan form (HAM or hypothetical ancestral mollusk)
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 logarithmically 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 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 Strombus (Lobatus)
Procedure: Examine a cut shell of the queen crown conch Strombus gigas. Determine the three shell
layers. Now notice that Strombus actually does not have a true nacreous, instead the innermost shell layer
is a porcelain layer.
Very few conch shells produce pearls and even fewer produce pearls of commercial quality due to the
lack of a true nacreous (thus there is no creation of a nacreous pearl, as in the oyster). Instead of a
chemical composition of aragonite, chitin, lustrin and silk-like proteins, as in oysters the nacre of
Strombus is composed solely of aragonite and therefore lacks the iridescent quality.
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. 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
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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 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. he 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.
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. Leave only the
central column (columella) and be careful not to disturb the soft parts. 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
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pneumostome. Pin your specimen to a wax pan 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?
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.
Crepidula fornicata, however, are sequentially hermaphroditic prosobranchs. 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.
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CLASS BIVALVIA
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. 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. 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 filling 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. 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?
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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.
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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 that 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 water 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 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, carefully cut a couple small strips of
tissue from the leading edge of the ctenidium and mount them (using saltwater) on a slide and then
compare them to previously mounted slides. Describe what you see.
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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? If available, obtain a small (~lcm), live
mussel 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.
In addition, make a note that Geukensia demissa (Modiolus demissus) is generally found in sulfur rich,
oxygen poor habitats. How can an animal that requires oxygen for survival sustain itself in a sulfur-rich
environment? What are adaptations gained by Geukensia that allow these animals to survive an oxygen
poor environment?
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.
Loligo sp.
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, whereas the squid siphon, along with the arms and tentacles, is a
modification of the foot
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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.
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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 contraction 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
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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 havebeen
fertilized, a gelatinous matrix is secreted around them. The female holds this gelatinous mass within her
arms until she finds a rock or another the suitable object to attach it to.
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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.
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