Dissection guide for Mytilus californianus and Venus merceanaria
Each student (along with their partner) should accomplish the following during the lab:
1. Sketch the external anatomy of your animal (shell, ligament etc.)
2. Sketch the generalized internal anatomy of your animal (making sure to label all
relevant features)
3. Sketch and compare the relative anatomy of the ctenidia
4. Expose and sketch the circulatory system in the region of the heart
5. Time permitting dissect and sketch the digestive system (stomach, style, intestine)
Remember... Annotate, annotate, annotate!
Introduction
Your first reaction to this lab might be: "Ugh--bivalves. All they do is filter feed
seawater". That's quite true--that's pretty much all they do most of the time. But the
extreme adaptations to filter - feeding one sees in bivalves are part of what makes them
such fascinating objects of study. And even though all bivalves utilize basically the same
food source, they inhabit a variety of habitats. The forms you will study today--the
mussels and the quahog clam -- inhabit places as different as night and day: the mussel,
the wave swept mid - tidal zone: the quahog, the sand or mud of quiet bays. As you go
through this lab, keep comparing/ contrasting the two animals in the light of their
different habitats and lifestyles, but don't lose sight of the fact that they are built on the
same basic plan. Once you identify the elements in this plan, it will be easier for you to
get a handle in the modifications each form has undergone.
A Little Background
In bivalves, or pelecypods ("knife-foot" in Latin) the mantle has become greatly enlarged,
and covers the head. The gills are highly modified for filter feeding; because the food
particles they eat are so small, bivalves have no need for the scraping radula and
muscular buccal apparatus seen in most other mollusks.
Today you will be looking at typical representatives of the two largest bivalve groups: the
Pteriomorpha, which have adapted themselves primarily to living attached to a substrate,
and the Heterodonta, which are mainly burrowers in mud or sand. Like many of its
pteriomorph relatives, the mussel (Mytilus californianus) has the comparatively primitive
"lamellibranch" or fillibranch ctenidia, in which the individual filaments are not bound
together. The quahog or little neck clam (Venus mercenaria), a typical heterodont, has the
more complex eulamellibranch ctenidia, with cross linked filaments. In addition to this,
pteriomorphs have a comparatively weak hinge area, without true teeth, and they draw
sea water in and out through a simple opening in a fused region of the mantle rather than
trough true inhalant and exhalant siphons, as heterodonts do.
Life Histories
Unlike its relatives, the oysters and rock scallops, which attach their lower shell valve to
the substrate, Mytilus attaches itself with elastic byssal threads. You can see these threads
protruding from the valves of your specimens.
Like most bivalves, Mytilus feeds by drawing water, by means of ciliary currents over the
ctenidia. Food particles are captured in a mucus sheet overlying the ctenidia, and beating
cilia propel the mucus sheet to the mouth. Mytilus does not have much of a head -- at
least you will find no tentacles or eyes around the mouth. In many bivalves, the mantle
edge had taken over the main sensory functions; the mantle edge of Mytilus is extremely
sensitive to touch, and can also detect changes in light intensity. (Some bivalves, notably
scallops, even have eyes or long tentacles on the mantle edge.)
Venus , in contrast to Mytilus , never settles down. It's foot remains relatively large and
powerful throughout its life, and the clam uses it to burrow through sand or mud. It
natural home is along the Atlantic coast, but small colonies have become established in
California.
One of Venus's most notable adaptations to its burrowing lifestyle is its siphons, which
can protrude slightly from the shell. Since, unlike Mytilus, it is largely sheltered from
food - bearing currents or wave action, its water - pumping mechanism should be much
more powerful than the mussel's. (How might you test this?) But once it takes water into
the mantle cavity, it feeds in a manner quite similar to the mussel.
Getting started
First of all, get yourself a partner. One should dissect a clam while the other does a
mussel. At each stage, look at each other's work and note the similarities/ differences
between the two forms you are revealing as a team. Once you've gotten your clam or
mussel, get oriented: with the beak of the clam pointing left, the clam's left end is the
anterior end (where the mouth is); the right end is posterior (siphons and anus); the top
(hinge area) is dorsal; the opposite edge (bottom) is ventral. For the mussel, with the
hinge pointing right, and the beak up, the hinge area (upper right) is dorsal; the lower left
edge is ventral; the upper left (to the left of the beak) is anterior, and the lower right is
posterior. Confused? Look at the drawings (Figure 1).
External Anatomy: The Shell
The molluscan shell, as you already know, is secreted by the mantle, which in bivalves
has three marginal folds. The thin middle fold secretes the innermost (nacreous) layer of
the shell, and the middle (prismatic) layer. The outer fold secretes the periostracum, the
cuticle-like outer shell covering. (the inner fold is not involved with shell secretion.) The
periostracum of Venus is quite thin, and its wears away as the animal grows. In Venus,
the porcelain- like nacreous layer is by far the thickest (Fig 2A). In Mytilus , the
brownish-black periostracum covers a large portion of the valves; the middle prismatic
layer thickens the shell, and the nacreous layer forms an iridescent coating on the inside
of the shell. While Mytilus 's shell is much thinner than Venus's, it's structure and
composition make it resilient. One might think of it as a lacquer bowl. What factors do
you think could account for the different shell thickness and structure of the two forms?
Environmental forces? Lifestyles? Predators? Which of these factors do you think would
have the most influence in each form? To answer this question completely, however, one
other thing must be taken into account. The shell is an external skeleton. Muscles are
attached to it. Depending on muscle size, configuration, and attachment points, as well as
shell configuration, very different results may be achieved. "Results" can mean many
things: the forces the animal can exert, both to its environment and to its own shell, and
how much force it can withstand (for instance, wave shock, or the force of something
trying to open the shell). In Venus, the powerful adductor muscles, which close the
valves, may place a limit on how thin the shell can be. If you barely crack Venus's shell,
these muscles exert enough force to cave in the shell completely. The shell could not be
much thinner than it is, or else it would collapse under the force of the animal's muscles.
The position the animal assumes in its environment, and where it lives in the intertidal
zone, are useful information not only in comparing shell thickness, but also shell shape.
The mussel attaches itself to the substrate with its anterior end usually facing the ocean).
In this position a streamlined surface faces the oncoming wave: what does the shape
remind you of? A sail? A dolphin?
The clam, which lives in comparatively calm, low-intertidal areas, buries its anterior end
in the sand or mud so that water only passes over the extreme posterior end, where the
siphons are located. As it buries itself, the narrow ventral edge of the shell encounters the
sand first. In cross-section, its shell is shaped like a wedge, an efficient shape for
burrowing.
Notice the concentric grooves, or growth lines on the shell. They are much more distinct
in the clam than in the mussel, giving the clam's shell a rigid surface. The mussel’s shell
is quite smooth in comparison. The growth lines originate from the beak, or umbo, which
curls anteriorly. In the clam you will see a pair of deep grooves just anterior to the beak,
in the shape of the heart; this structure is called the lunule (Fig 2B) What do you think the
lunule is doing there? (Hint: look at the inside of the shell where the lunule is.) The
mussel’s beak lacks a pronounced curl and accompanying lunule, and it is often worn
away to reveal a mother-of-pearl surface underneath.
Posterior to the beak, the clam has a bulbous external ligament and a flattened area on
either side of it, the escutcheon. The mussel doesn't have an external ligament; however,
its internal ligament, located close to the hinge, looks very similar to the clam's. Keep in
mind that the ligament is secreted by the mantle, and is just as much part of the shell as
the valves are. (The main difference lies in the relative proportion of calcium and
concholin, the elastic material which also makes up the periostracum.) The hinge
ligament is constructed so that when the valves are closed, the outer part is released,
causing the valves to open.
Figure 1 A: General orientation of Mytilus and Venus
Figure 1 B External and internal shell anatomy of Venus
Figure 1 C External and internal shell anatomy of Mytilus
Once you have removed the animal's soft parts, you can examine the internal hinge area.
(You might want to come back to this, after preparing the animal for dissection--see
Preparation) Notice that the hinge ligament in both animals is outlined by a calcareous
border, and that both have a triangular piece of periostracum at the posterior end of the
hinge. In the mussels, the periostracum covers the inner margin of the valves, including
the beak area. At the inner edge of the beak, underneath a thin layer of periostracum, the
mussel also has minute "denticles", which aid in fitting the valves together accurately.
The clam"s "interlocking mechanism" is much more sophisticated than the mussel's.
Anterior to the ligament are three pairs of large interlocking teeth. (See Figure V1) The
entire ventral edge of the shell is crenulated; thus the shell valves mesh tightly. Why do
you think Venusand its burrowing heterodont relatives need such an efficient locking
mechanism, while Mytilus and most of its pteriomorph relatives do not?
Inside a cleaned-out shell, you can see the chalky-white pallial line running parallel to the
shell margin; this represents the connection between the mantle and the shell. Tiny
muscles originating from the inner marginal fold of the mantle attach here. Notice the
embayment, or sinus, in Venus's pallial line; this corresponds to where the siphon
retractor muscles attach. Also noticeable in both forms are adductor muscle scars: two
roughly equal ones in Venus, one anterior and one posterior; one large and one very small
on (near the beak) in Mytilus . An elongate scar anterior to the large posterior adductor
scar in Mytilus is where the pedal retractor muscles attach. In Venus, you can see a very
small scar, just under the hinge teeth, made by its anterior pedal retractor.
Internal Anatomy
Opening up your specimen
Your instructor will demonstrate how to open both Mytilus and Venus. Basically in order
to open the shell, the adductor muscles must be severed to allow the shell valves to hinge
open.
The shell edge of Venus must first be chipped to allow for a scalpel to be inserted into the
mantle cavity for the purpose of severing the adductor muscles. Once the shell has been
broken, lift any broken pieces off with forceps. Then insert a small knife or a scalpel
under the left shell's left-hand side until you find where the posterior adductor muscle
attaches to the shell. (See Figure 1 B & C) Keep the knife flush with the shell as you go,
and sever the posterior adductor (located posterodorsally), where it meets the shell. Do
the same for he anterior adductor muscle, which is located anteroventrally. Pull away
what remains of the left valve, and sever the small pedal retractor muscle (just to the left
of the beak) where it connects to the left valve. Now you can remove the rest of the left
valve, so you're left with a clam on the half-shell ready for dissection.
Mussels are easier to get into. The large posterior adductor muscle can be severed with
the knife and then the valves can be gradually pulled open and the the posterior and
anterior foot retractors can be severed as well. Finally, pull the valves completely open
and sever the tiny anterior adductor muscle (just to the left of the beak.) Now you are
ready to begin examining the internal anatomy.
Musculature
Examining and comparing the shells of Mytilus and Venus should have started you
thinking about the environmental factors these two forms have to deal with, and how
shell and muscles form an interacting system. As you examine the muscles, think about
the forces they exert on the shell. Where are the animal's strong and weak points? And
what forces can the animal transmit to its environment via the combination of shell shape
and construction, and the nature and placement of the muscles?
In removing the left valve from your animal, you notice several muscle scars on its inner
surface. Now turn back the mantle--the thin "blanket" of tissue covering the animal"s
body--and carefully cut it away from the adductor muscles, from the dorsal portion of the
visceral mass (just above the ctenidia), and in Venus from around the pink siphonretractor muscle. (Leave the siphon apparatus intact for now.) In Mytilus it will also be
necessary to fold back the left ctenidium to examine the musculature (See Figure 2 A&B)
Now look at the muscles corresponding to the muscle scars. In Venus you see two large
pink muscles of roughly equal size: the anterior and posterior adductors. Together, these
two muscles closed the shell valves so tightly that you were hard-pressed to open them.
On the outer edge of these muscles is a crescent of paler muscle. (Can you detect any
difference in structure between the two parts of these muscles?) Between the adductors
Fig. 2A: Internal anatomy of Mytilus
Fig 2B Internal anatomy of Venus
lies the large, muscular, hatchet-shape foot. The lowest part of the foot is fibrous and
yellowish, and the foot has a bladelike edge. You know that Venus burrows in sand; how
do you think it uses its foot to locomote? And how is the foot's shape particularly suited
to moving through sand? Two small muscles connect the foot to the shell dorsally and
serve to draw the foot inside the shell: the anterior and posterior foot-retractors. (you had
to scrape the anterior retractor from its attachment point under the hinge when you
removed the left valve.) Why are these muscles so small--or, instead, why don't they have
to be larger?
Now turn to Mytilus . The basic pattern is the same as in Venus, but the sizes and shapes
of the muscles and foot are quite different. The posterior adductor dwarfs the anterior
adductor, which lies just left of the beak. (Some pteriomorphs have no anterior adductor
at all: scallops, for instance.) And yet the more advanced heterodonts, of which Venus is
one, have retained the apparently ancestral condition of having two roughly equal
adductors. Below Mytilus 's anterior adductor lies the brown, finger-like foot. The
mussel's byssal threads emerge just below the foot. Embedded in the tissue at the base of
the foot is the opaque white thread-producing byssal gland. Notice also the groove
running down its upper surface. The byssal gland invests the foot tissue just below the
grove. Consider the arrangement of byssal threads, glands, and foot: how does the animal
secrete the threads and attach them to the substrate?
Now trace Mytilus 's other (shiny) muscles back from where they attach to the foot and
byssal area. You will find a single pair of long, cord-like anterior byssus retractors
running anteriorly from the byssal gland and attaching to a small shelf just to the right of
the beak. Besides pulling on the byssal threads, these muscle's contractions cause the foot
to move forward. Running posteriorly from the foot is a pair of foot retractors; running
posteriorly from the byssal gland are two pairs of posterior byssus retractors. These three
pairs of muscles attach to the shell in a line just anterior to the posterior adductor, and
correspond to the single, small posterior foot retractor in Venus. This arrangement of
several "lines" (muscles) to a single point (base of byssal threads and foot) distributes the
waves' force into many lines of tension, as a sail's rigging distributes the wind's force.
Mantle
The bivalve mantle is a large bilobed structure which covers the animal's body and also
secretes the shell. The mantle edge has three folds. The outermost fold, the least
conspicuous of the three, actually secretes the shell. Its inner surface secretes the
periostracum; its outer surface secretes the calcareous prismatic layer of the shell. The
entire outer surface of the mantle secretes the nacreous layer which lines the shell. The
middle fold has a sensory function. (In Mytilus it appears that this (the middle) fold
actually secretes the periostracum, rather than the outer fold; the middle fold is
continuous with the periostracum and ensheaths, the thin outer fold.) The outer fold--by
far the largest--is supplied with muscles which attach the mantle to the shell. The outer
fold of Venus's mantle is ruffled and thickened but not otherwise distinctive, whereas in
Mytilus it is relatively muscular and edged with branching, purplish-black tentacles.
These tentacles retract in response to touch, and can detect changes in light intensity. And
unlike Venus, Mytilus can extend its mantle just beyond the shell's edges. (Why do you
think the sessile Mytilus might be more sensitive to its environment than the mobile
Venus?) Notice also that while in both forms the mantle edges are fused dorsally,
relatively smaller portions of the mantle's edges are fused in Mytilus .
The space enclosed by the mantle lobes is called the mantle cavity. Cilia lining the mantle
and covering the ctenidia (which hang down in the mantle cavity) set up a current that
draws water into the mantle cavity. In Mytilus the cavity's exhalant orifice is a simple
opening in the fused portion of the mantle just behind the posterior adductor muscle (See
Figure 3A). You can best observe it in a living, undissected specimen which has been
undisturbed long enough to open up and begin circulating in water. Notice that behind the
exhalant opening are a pair of membranes which can open and close the opening, wiping
over the posterior adductor as they do so. They resemble the nictating membrane of a
bird's eye. What function do you think these membranes serve? (Hint: As you may see
later, the rectum wraps around the posterior adductor muscle.)
In Venus and its heterodont relatives, the posterior edge of the mantle is modified into a
pair of true siphons (See Figure 3B). Water enters the mantle cavity through the ventral
siphon and exits through the dorsal siphon. The two siphons are fused; but this is not the
case in many heterodonts. Unlike many heterodonts, Venus has relatively small,
immobile siphons, made almost entirely of tough, cartilage-like material. They are
however, supplied with siphon retractor muscles at their bases. When these muscles are
relaxed, the siphons extend just beyond the shell edge; contraction pulls them inside. The
siphon tips are supplied with tiny sensitive tentacles; when they are touched, the siphon
muscles retract immediately and the clam closes-up--it literally "clams up". Note that
one of the siphons has more developed tentacles – why?
Ctenidia
Besides the enclosing shell and mantle, the major novel feature of bivavled mollusks is
the the modification of the gills--the ctenidia --for filter-feeding. Bivalves are deemed
"primitive" or "advanced" primarily on how much their ctenidia are modified over
supposed ancestral forms, whose ctenidia are reckoned to be almost snail-like.
Examine the curtain-like ctenidia of your and your partner's animals, which you exposed
when you lifted off the left mantle lobes and cut them away. Their central axes are
attached along the dorsal wall of the mantle cavity. Each ctenidium consists of two lobes,
the inner and outer demibranchs. Each demibranch is formed by a single line of hair-pin
shaped filaments.
Mytilus's ctenidia are more "primitive" than Venus's although they are certainly much
more advanced than those of ark shells or other protobranch bivalves. In Mytilus the
filaments are greatly elongated and folded back on themselves, so that the filament-rows
Fig 3a: Inhalent (left) and exhalant (right) openings of Mytilus.
Fig. 3B Inahalant and exhalant siphons of Venus.
form two double layers--a "W" hanging from the axis by its central peak. This type of
ctenidium is called lamellibranch or filibranch. In Venus which has the more advanced
eulamellibranch ctenidium, the general structure is the same, except that bridges of tissue
link adjacent filiments together. You can observe this difference for yourself--as you lift
up a lobe of the Mytilus ctenidium, the demibranchs' fibers separate easily. They are
crosslinked only by tufts of cilia. Venus's ctenidia, on the other hand, hold together as a
spongy mass when you lift them up.
In Venus the outer demibranch of each ctenidium is attached to the mantle near where it
meets the visceral mass; the inner demibranch is attached to the visceral mass itself. The
inner portions of the two demibranches are also connected to each other, thus creating a
small space inside the ctenidium where water can flow: the suprabranchial chamber. The
suprabranchial chamber connects with the excurrent siphon, whereas the incurrent siphon
leads into the branchial chamber (the space enclosed by the mantle and surrounding the
ctenidial complex).
Notice that the ascending lamellae of Mytilus 's demibranchs do not connect with the
mantle lobes and visceral cavity; thus, Mytilus does not have a separate suprabranchial
chamber. A suprabranchial chamber is present only when true siphons are present. Why
do you think this is so? (We'll come back to the suprabranchial chamber in the Excretory
and Reproductive System sections.)
In Venus the linkage of adjacent demibranches by bands of connecting tissue, creates
numerous inhalant ostia on the surface of the ctenidium. Ciliary beating propels water
through the ostia, into the water tubes within each demibranch. These tubes in turn
connect dorsally with the suprabranchial chamber. Gas-exchange occurs in the tissue
surrounding the water tubes, which is invested with afferent and efferent blood vessels
going to and from the heart.
The outer surfaces of the ctenidium are abundantly ciliated. Besides transporting water
through the ctenidia for gas-exchange, ciliary beating also brings food particles--diatoms,
protozoa, organic detritus--to the mouth. Cilia pass food particles to the edge of the gills,
where they become entangled in a mucous string moving like a conveyor-belt towards the
mouth. Once the mucous string nears the mouth, cilia on the labial palps (See Digestive
System) take over the job of sorting and transporting food particles.
If you have time, observe the ciliary beating in a fresh specimen (with the left shell
removed) under the high power of your dissecting scope. Light should hit the ctenidium
at an oblique angle for the beating cilia to be visible. Try cutting a small square from the
edge of a ctenidium an putting it under the compound microscope. Are the cilia
differentiated into types? How are they arranged on the filaments?
Circulatory System: Mytilus
As we mentioned, it is easiest to examine the heart of Mytilus by removing the animal
completely from its shell but this dissection can also be done with the animal still on the
half shell. To remove the animal from the shell, cut the muscles away from the right shell
valve where they attach to the shell and lay the animal on the dissection pan with the
ctenidia edges facing down. Submerge the animal. You will see the heart just posterior to
the large, dark digestive gland. The mantle lobes are fused over the heart region, but they
are extremely thin there. Slip tiny scissors under it with the points pointing anteriorly and
cut open the pericardium to reveal the heart even better. The transparent, tubular
ventricle, which surround the intestine, and the pale-greenish-brown Keber's organs
flanking the ventricle are the most apparent organs (See Figure 4A). To find the auricles,
pull gently on the Keber's organs with forceps. The transparent triangular extensions of
the ventricle are the auricles. The afferent oblique vein of each mantle lobe enters each
auricle anteriorly. The renopericardial duct empties out of the pericardium into the
kidneys at an anteriolateral angle. Just anterior to the ventricle is a slight swelling which
may appear reddish: this is the aortic bulb, where the anterior and posterior aortae arise.
Circulatory System: Venus
The heart lies dorsally, just below and slightly posterior to the hinge ligament. It is
covered by a thin membrane, the pericardium, which you may want to remove to see the
heart more clearly. The large single ventricle surrounds the intestine just where it leaves
the visceral mass and runs towards the posterior adductor (see Figures 5 A&B). Have you
seen this sort of arrangement before? What function do you think it serves? Note also in
both forms the small (roughly 5 mm long) paired Keber's organ lying on either side of the
ventricle. They are pale greenish-brown--and easy to miss in the clam, where you might
think they are merely discolored portions of the mantle. In fact, most dissection guides
fail to note the presence of Keber's organs in Venus. But a key feature of Keber's organ is
that they connect to the pericardium--and by tugging gently on the clam's "Keber's
organ," you can show this connection. What does the Keber's organ do?--Nobody knows.
If your animal was in good condition to begin with, you will probably be able to see the
ventricles contracting even two hours into your dissection. Compare the heart rates of
Venus and Mytilus . Do the heartbeats seem regular or arrhythmic? Which of the two
bivalves seems to have stronger ventricular contractions?
The ventricle and paired auricles of Mytilus are simple thin-walled sacs; the clam's
ventricle is much more muscular than the mussel's, although its auricles are also thinwalled. It looks much more like a "heart," in other words. When the triangular, fleshcolored ventricle contracts, the two auricles connected to it dorsally expand like tiny
balloons, and blood from the efferent vessels of the ctenidia fill them . The ventricular
contraction sends blood through the posterior aorta and into the bulbous arteriosus, a pale
yellow triangular organ just posterior to the ventricle, and from there to the hemocoel (the
sinuses surrounding the viscera). Anterior to the ventricle and dorsal to the intestine is the
anterior aorta; the ventricular contraction also sends blood through it, to the hemocoel.
The circulatory system of bivalves and other mollusks is an open one; arteries and veins
do not connect by capillaries. (The pattern of blood circulation in Mytilus is the same as
in Venus) The blood sinuses are
Fig. 4A Circulatory System of Mytilus
Fig. 4B: Circulatory System of Venus
quite large, and one might confuse them with the coelom, which in mollusk is represented
only by three small cavities: pericardial, gonadial and nephridial cavities.
Excretory System: Mytilus
Locate the mussel's kidneys by pulling back the inner lobe of the ctenidia. The kidneys
are not well-defined organs, but rather strips of reddish-brown or brown dendritic tissue
at the base of each ctenidium. They run the length of the visceral mass and taper at both
ends. At the midpoint of each kidney is a tiny white urogenital papilla which bears both
the renal pore and the gonopore (the opening of the gonadal duct). The kidneys also
connect to the pericardium via (of course) the renopericardial duct (See Circulatory
System for directions).
Excretory System: Venus
The brownish to grayish, dark-speckled kidneys lie ventral to the pericardium. They are
larger and better differentiated than the Mytilus 's kidneys: a pair of thick U-shaped tubes.
They have a glandular appearance dorsally, but become thin-walled sacs ventrally. The
saccular portions of the kidneys function as a bladder. Each kidney communicates with
the pericardium via a minute opening (which is difficult to find) and with the
suprabranchial chamber via the renal pore, which, along with the gonopore, lies on a
urogenital papilla. The suprabranchial chamber (See Ctenidia) in turn is connected to the
exhalant siphon.
Digestive System: Mytilus
Locate the labial palps, two pairs if triangular flaps that lie anteriorly on either side of the
anterior foot retractor muscle. They are ridged and ciliated on their inner surfaces; the
ciliated surfaces face each other and surround or touch the anterior portion of each
ctenidium when the animal is feeding. They take up the food-laden mucous string which
travels along the edge of the ctenidium. A groove above the transverse ridges on each
labial palp conveys fine particles of food to the mouth, which lies in the middle of a
longitudinal slit in between the two pairs of palps. To find the esophagus, carefully clip
or pick through the large dark-green digestive gland a few millimeters above the mouth.
The esophagus is a brownish-white tube approximately 2 mm wide; it is surrounded by
the digestive gland. As you will notice, there is no radula or buccal mass in mussels--or in
any of the rest of the class Bivalvia/Pelecypoda. You may elect to trace the esophagus
posteriorly, picking digestive gland away as you go , to a white faintly striped bulb, the
stomach.. (At this point it may be necessary to cut the pedal and byssal retractor muscles
away from the shell, if you have not already done so.) Or you can try and access the
stomach directly by poking down into the digestive gland just forward of the pericardium.
The stomach has a pleated inner surface, which increases the area for absorbing nutrients.
If you are lucky you will be able to find the style sac, which lies inside the beginning of
the intestine. A gelatinous crystalline rod, the style , lies within the style sac. The style
liberates enzymes used for the digestion of carbohydrates. Don't be annoyed it you can't
find the style. There seems to be a relationship between the oxygen content of the
animal's environment and the presence of the style. Unless your mussel is very hardy and
it lived in a well-oxygenated environment, it may completely lack a style. You'll have
another chance to see the style when you trace the clam's gut. The intestine loops
through the visceral mass until it doubles back posteriorly and runs through the
pericardium. Finally, the rectum wraps around the posterior adductor and dumps feces
into the excurrent region of the mantle. To finish tracing the intestine, you will probably
have to remove the animal from its shell completely, by cutting all the muscles where
they attach to the shell.
Digestive System: Venus
One of the most noticeable differences in the body plans of the clam and mussels is how
their digestive tracts are arranged: coiled and compartmentalized in the clam, compared
to a single elongated coil in the mussel. But even though the clam's organ systems seem
more "differentiated" than the mussel's, the basic structures are rather similar. Start with
the clam's labial palps, which are nearly identical to those of Mytilus. The short
esophagus leads to the dorsally-positioned stomach, which is mostly buried in the bilobed
brown "liver" (homologous to Mytilus 's digestive gland). If possible, exposed the
stomach, open it up and retrieve the style, which is always present in Venus. It is shaped
like an arrowhead, with two flanges projecting dorsally. The coiled intestine lies
posteriorly and ventrally to the stomach, and is buried partially in the liver, but mainly in
the gonad. After making several coils, the intestine narrows and turns dorsally to leave
the visceral mass and enter the pericardial compartment. Follow the intestine posteriorly
through the pericardium. It curls around the dorsal surface of the posterior adductor, and
the anus empties into a chamber at the base of the excurrent siphon.
Reproductive System
Unlike many of the animals you are studying, bivalve mollusks typically have an
amorphous-looking reproductive system. In Venus, the gonads, which are whitish in both
sexes, invest the visceral mass and surround most of the digestive tract. Although they are
paired, this is difficult to detect. Individuals are male or female, not hermaphroditic (at
least, not simultaneously). Eggs and sperm are shed directly into seawater. The openings
of the genital ducts, the gonopores, lie on the unogenital papilla, within the enclosed
suprabranchial chamber (See Ctenidia). This chamber in turn connects to the exhalant
siphon. Thus, in the clam, solid wastes, kidney wastes, and gametes all leave via the
exhalant siphon. (In Mytilus, the gonopore empties into the branchial chamber, as it has
no separate suprabranchial chamber.) The gonads of Mytilus lie mainly in the mantle
lobes, and form most of the lobes' thickness. A small amount of gonad also forms a
"median lobe" by investing the spaces around the pedal retractor muscles and even the
viscera. "Ripe" adult mussels are easy to sex, as the gonad is pinkish-white in the male
and salmon-pink in the female. Notice that a much larger proportion of Mytilus 's total
body weight (sans shell) is composed of gonad compared to Venus. Mytilus 's gonad is
also riddled with numerous arteries, which are visible to the naked eye. Given that both
forms shed eggs and sperm into the surrounding seawater, why do you think Mytilus
seems to put so much of its energy into reproduction?
Fig 5A: Digestive system of Mytilus
Fig 5B Digestive system of Venus
Phylum: Mollusca
Class: Bivalvia
Lab notebook collection next week! The first three labs will be graded.
Don’t forget scale bars!
Mytilus californianus
Sketch #
Title
1&2
External anatomy, (1)
dorsal and (2) lateral.
These two sketches can
be on the same page.
Est. time
40 min
Notes
LABEL: 1. Right and left
valves 2. Ligament 3. Hinge
4. Umbo/beak 5. Escutcheon
6. Growth lines 7. Byssal
threads 8. Epiboints
3&4
(3) Empty shell with scars
and (4) ctenidium. These
two sketches can be on
the same page.
40 min
LABEL EMPTY SHELL: 1.
Muscle scars 2. Ligament 3.
Hinge teeth/denticles 4.
Pallial line
LABEL CTENIDIUM: 1.
Filaments 2. Demibranchs 3.
Gonad 4. Mantle 5. Byssal
threads
5
Internal anatomy without
ctenidium
60 min
6
Circulatory system
40 min
7
Digestive system
40 min
LABEL: 1. Mantle 2. Siphons
3. Labial palps 4. Foot 5.
Byssus 6. Adductor muscles
7. Pedal retractors 8.
Posterior byssus retractors
9. Digestive gland 10. Kidney
11. Visceral mass 12.
Urogenital papilla 13.
pericardium
LABEL: 1. Pericardium 2.
Ventricle 3. Keber’s organ 4.
Auricle. Note blood
pathway.
LABEL: 1. Labial palps 2.
Esophagus 3. Mouth 4.
Digestive gland 5. Stomach
6. Intestine 7. Kidney 8.
Rectum 9. Crystalline style, if
present and you have time
Venus mercenaria
Sketch #
Title
1&2
External anatomy, (1) dorsal
and (2) lateral. These two
sketches can be on the
same page.
Est. time
40 min
Notes
LABEL: 1. Right and left
valves 2. Ligament 3. Hinge
4. Umbo/beak 5. Escutcheon
6. Growth lines 7. Lunule
3&4
(3) Empty shell with scars
and (4) internal anatomy.
These two sketches can be
on the same page.
90 min
LABEL EMPTY SHELL: 1.
Muscle scars 2. Ligament 3.
Hinge teeth/denticles 4.
Pallial line
LABEL INTERNAL
ANATOMY: 1. Ctenidium 2.
Filaments 3. Demibranchs 4.
Mantle 5. Labial palps 6.
Pericardium 7. Siphons 8.
Foot
5
Circulatory system (leave in
shell)
40 min
6
Digestive system
40 min
LABEL: 1. Pericardium 2.
Ventricle 3. Keber’s organ 4.
Auricle. Note blood
pathway.
LABEL: 1. Labial palps 2.
Esophagus 3. Mouth 4.
Digestive gland 5. Stomach
6. Intestine 7. Kidney 8.
Rectum 9. Crystalline style
Conclusion:
Discuss two similarities and two differences in form and two similarities and two
differences in function between Mytilus and Venus (eight comparisons total) in
the context of the environment in which each lives.
AND / OR
Discuss the transition from HAM to bivalve.