Download phylum mollusca

PHYLUM MOLLUSCA
Introduction
Because the Mollusca are designated as a phylum we know that all the organisms in the taxon
must share a common body plan that they have inherited from the ancestral mollusc; it’s the
very definition of a phylum after all. But, if we were to put a representative from each of the
different classes, both extinct and extant, on the lab bench we would have a tough time trying
to find the characteristics of the ancestral body plan that unifies the classes of the phylum.
That’s because each class is a spectacular examples of invertebrate adaptive radiation with the
result that seemingly unrelated animals, such as snails, slugs, clams, the octopus and squids are
all related to each other and share a single common ancestor.
Whatever it looked like the ancestral Mollusc’s body plan was sufficiently flexible that it
allowed its descendents to adapt with a range of behaviors, and habitats. From sedentary filter
feeders to rapid swimming active predators, and just about everything in between, there is a
mollusc that does it. The adaptive potential of the ancestral molluscs is also seen in where you
find molluscs; freshwater, marine and terrestrial environments.
Their general appearance wouldn’t necessarily give us any obvious clues to why these animals
are all included in one phylum but there are a variety of traits, or characters, that unify the group
and you should look for these in each of the specimens. These include the presence of a
muscular foot, involved in locomotion, and a sensory head that was also associated with food
acquisition by the unique molluscan feature of a radula. Another is the visceral mass, which is
dependent on cilia for its function, and positioned dorsally surrounded by the protective shell.
The shell is secreted by the underlying mantle extending out from the body creating a mantle
cavity inside which you’ll find the ctenidia, the principle respiratory structures of molluscs.
Polyplacophora
These animals are referred to as chitons and their body form is specially adapted for the rough
conditions associated with the intertidal zone of the oceans. When chitons are active they slowly
creep across the rocks feeding on encrusted algae and other organic debris. If threatened they
can roll up into a ball surrounded by the protective armor of their shell.
Fig. 1. External features on the dorsal surface
of a chiton. © BIODIDAC
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External anatomy
The most obvious external feature of these molluscs is the set of eight,overlapping shells, or
valves, on dorsal surface covering and protecting the visceral mass underneath. The posterior
edge of each shell overlaps the front of the shell behind it. As a result, the shells are articulated,
and can move relative to each other allowing the chiton to cling tightly to the rocky surfaces on
which it crawls or curl up in an armored ball when attacked. Around the edge of the chiton is a
muscular girdle with lateral edges of the eight valves embedded in it. The girdle is an exposed
part of the mantle, the rest is underneath the plates and, depending on the specimen,you will be
able to see the needle-like calcareous spicules embedded there. The most primitive molluscs
didn’t have a shell and were protected by spicules much like those around the edge of the chiton.
It’s believed that when a secreted shell did appear it took the form of plates like those seen in
the Polyplacophora. Later modifications of the shell solidified it in a single structure consisting
of only one valve, univalve, that could be wound or folded in a variety of ways. Ultimately the
shell disappears in some molluscs.
Fig. 2. Major anatomical features on the ventral
surface of a chiton. © BIODIDAC
The large oval foot dominates the ventral surface of a chiton and along its lateral edges are the
mantle cavity includes grooves formed from a trough between the foot on the inside and the
fleshy girdle. Inside the mantle cavity you can see the multiple ctenidia used for gas exchange.
The mouth is easy to see at the anterior end but there are neither eyes nor tentacles associated
with it. At the opposite end, the anus is located on the roof of the mantle cavity, on the tip of a
small papillae. Like all molluscs cilia on the surface of the ctendia propel water through the
mantle cavity pulling it in at the anterior end surrounding the mouth, down the two mantle
cavities on each side, and over the ctenidia. At the back the left and right mantle cavities fuse to
form a single exhalent canal where the anal opening is located. If you look closely in the region
of the last few ctenidia you may also be able to see nephridiopores or gonopores that open into
this posterior part of the mantle cavity.
Gastropoda
Gastropod shells
One gastropod trait that is easy to recognize is their spiralled shell; the most effective way to
package the increasing size of the visceral mass as the snail grows. The shell consists of only
one piece, a univalve, molded into a cone wound on itself. Essentially the snail winds its shell
up and lives underneath it. A snail’s shell is a protective safe haven where it can pull in its whole
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body when threatened. Some snails have an extra piece of shell on the dorsal surface of the foot,
an operculum, that closes to door behind them. Overwintering snails secrete a thin layer of
calcified slime across the opening.
Fig. 3.
Major structures of a gastropod shell. © BIODIDAC
Examine the intact and sectioned conch shells to understand the spiralled nature of the shell. The
main parts of the body includes the head and the foot extending out of the opening of the shell,
the aperture. Not all apertures open in the same direction. Place the apex of the shell away from
you. Is the aperture’s opening on the left or right? If it’s on the right is dextral, on the left
sinistral just like right and left-handedness. One complete circle of the shell is a whorl and the
edges of each whorl are connected to the next by suture lines. These lines are often sculptured
and can form spines. Like all mollusc shells, growth lines are visible on the surface of the shell
and the oldest part of the shell is the apex. In the shells that have been cut open identify the
columella. Are there individual chambers in the shell, or is it one single continuous
chamber?
Not all gastropods have shells; slugs have discarded the cumbersome shell.
Helix
Helix is a terrestrial herbivore that prefers to feed at night, or after a rain when conditions are
damp. Many gardeners battle the nocturnal dining of snails and slugs in their gardens. They
overwinter by crawling under rocks or digging into the soil.
External Anatomy
Take a relaxed specimen; its body extends outside of the shell. Locate the apex of the shell and
the anterior and posterior ends of the animal. Only the head and foot extend from the shell, the
visceral mass remains protected inside. The mantle’s edge is thickest in the region of the collar.
The pneustome, the opening to the mantle cavity, is located on the surface of the collar. One
of the best ways to find the pneustome in preserved specimens is to gently squeeze the body of
the snail and watch the collar for where preservative comes out. These are terrestrial animals
and the mantle cavity has been modified as a lung that we will see later when we remove the
shell.
The anterior position of the opening to the mantle cavity is a consequence of torsion, another
is the anal opening positioned in front of the animal instead of behind. Locate the anus just
beneath the pneustome, and the mouth with its three lips. All three openings are located on the
anterior part of the body, and when you see this in a mollusc you know you have a gastropod.
Where are the openings to the anus and mantle cavity usually located in molluscs? The
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gastropods assymetric body plan is best seen by obersving the position of the anus and opening
to the mantle cavity on the side and the displacement of the shell to the side of the foot, rather
the above.
Below the mouth is the opening to the pedal slime gland that lays down the mucus trail used
in locomotion. There are two pairs of retractable tentacles on the head the most anterior are
shorter than the longer posterior pair with an eye at the tip.
The opening to the reproductive system, the genital pore, is below and just behind the anterior
tentacle on the right side of the head. To see it you may want to let the surface of the snail dry
off a little.
To see the remaining external features you will have to remove the specimen from the shell.
This is not an easy task because the snail’s body is wound right up through the shell to the very
apex. Use a hammer to crush the shell by gently hitting the delicate apex of the shell. Be careful
when you do this, too much force and the sharp edges of the broken shell will damage the
specimen. Just enough force and the shell will break and you will be able to remove it to expose
the underlying snail.
The whole body is covered by the delicate mantle that thickens only at the collar. This thicker
part of the mantle results in collar shell being laid down faster compared rest of the mantle edge;
this is what creates the spiral of the shell. The dorsal surface of the front half of the mantle cavity
well vascularized and you will see the blood vessels that carry the haemolymph into the mantle
for oxygenation. How is air pumped in and out of the mantle cavity? The single
metanephridium, kidney, is dark and can be seen in the posterior half of the mantle cavity
occupying the second half of the first visceral whorl. Why are the excretory structures not
paired? Below the junction of the metanephridium and the lung is the heart surrounded by the
pericardial cavity. The heart consists of two chambers, paired auricles, or atria, receiving the
blood from the lung and a single ventricle pumping blood from the heart to the rest of the body.
On the inner edge of the mantle is the rectum. If you were unable to locate the anus in your
previous observations cut a small opening in the rectum and use a blunt probe to find the exterior
anal opening. The digestive gland and reproductive system fills the remaining whorls of the
visceral mass. Near the apex of the coil you will be able to see the albumen gland and in some
cases the ducts leading to the ovotestis found at the apex.
Internal Anatomy
The dissection of the snail is not one of the easiest but if you are careful, and patient, you will
see all of the structures. What makes this dissection so hard is that the spiral of the visceral mass
means that the structures that you need to see are wound around inside the spiral as well.
Fig. 4.
Major anatomical features of the snail. © BIODIDAC
Circulatory and excretory system
Submerse your specimen under water and to one side
of the dissecting dish so that you will be able to look at it under the dissecting scope. Use a pair
of fine scissors and cut through the collar in the region of the pneustomes. Cut along the edge
of the collar towards the rectum and then along the length of the mantle on the inner edge of the
whorl. Pin aside the roof of the mantle cavity and identify the blood vessels; efferent blood
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vessel on the midline of the cavity’s roof is the easiest to see and it carries blood to heart. Open
the pericardial cavity to expose the large atrium and smaller, muscular ventricle. The ventricle
pumps blood into an aorta with two main branches; the visceral aorta supplies blood to the
visceral mass and the cephalic aorta that supplies the head. The metanephridia is also located
on the roof of the mantle cavity and on the piece of mantle you have just exposed. Locate the
metanephridia and the duct that drains it.
Digestive system
Cut through the floor of the mantle cavity and collar along the midline.
Continue the cut forward between the tentacles until you reach the mouth. Be careful that you
don’t cut too deeply and damage any of the underlying structures. Fold the two flaps of tissue
back so that you can see the structures underneath. The body cavity that you have exposed is the
hemocoel. Working back from your first cut carefully cut along the midline and gently remove
the thin skin of mantle that covers the underlying organs. Reproductive structures lie primarily
on the right; digestive on the left and of these the pharynx is the easiest place to start your
observations. A short esophagus connects the pharynx and the buccal mass to the crop. On
either side of the crop are the salivary glands with a duct that empties into the pharynx. The
crop is connected to the thin walled stomach located in the second whorl. The paired digestive
glands that fill the rest of the visceral whorl connect to the stomach. The intestine leaves the
stomach, bends back on itself, and extends towards the anterior before becoming the rectum
opening to the outside through the anus.
Fig. 5.
Major anatomical features of the snail. © BIODIDAC
Later, when you have finished your observations of the reproductive system, open up the
pharynx to see the radula inside. Below the radula locate the jaw. The radula works against the
jaw to grind up the ingested meal.
Nervous system
The main nerve ring may be apparent over the buccal bulb. The remaining
part of the nervous system is difficult to trace as a consequence of torsion and there really isn’t
any reason to try and find it.
Reproductive system Snails are monoecious and you’ll be able to see most of the
structures of the male and female reproductive systems. Be sure to spend a bit of time unwinding
things so that you can see the main features. When you cut through the floor of the mantle cavity
to expose the digestive system you will have also exposed the different parts of the reproductive
system. We will need to find a few landmarks from which we can map the remainder of the
system. The mucus glands, with their filamentous components and the dart sac are the easiest
to find. As its name suggests the dart sac contains a dart that is jabbed into the side of this snails
partner during copulation. Is the dart a part of the male or female reproductive system?
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At the base of the dart sac is the junction of a number of tubules from the male and female parts
of the reproductive system. The junction of the female is the vagina containing the openings to
the seminal receptacle and the hermaphrodite duct. Adjacent to this is the penis, which is in
turn connected to the flagellum. The flagellum is wound up on itself but if you gently tease the
various tubules apart from each other you will be able to find the tip of the flagellum. Follow it
to its base and you will find the penis. Be careful as you expose the flagellum because the
delicate sperm duct connects to the flagellum above the penis.
Expose the common duct extending from the junction of the male and female reproductive tracts
back to the albumen gland. It is easy to identify by the appearance of its two components: a large
globular gland of the female system and the tubular sperm duct attached to it. The albumen
gland attaches to the common duct and differs from the digestive gland by its shape and lighter
color compared to the digestive gland. Extending from the base of the albumen gland is the
hermaphroditic duct that leads to the ovotestis which is buried in the tip of the digestive gland.
There is only one part of the reproductive system that we have not identified, the seminal
receptacle. This is a ball shaped structure, also found in the area of the digestive gland attached
by a filamentous duct to the vagina. It is often difficult to see this intact but you should be able
to find the seminal receptacle itself, and with a careful dissection its connection to the remainder
of the system.
Gastropod diversity
Examine the different gastropod specimens. How have they adapted to the problems associated
with torsion? To answer this question you might want to follow how water moves through the
mantle cavity and note the position of the anal or excretory openings. Specimens if available
include: limpets, the garden slug, Limax, conch, and nudibranch
Scaphopoda
Fig. 6.
Major anatomical features of a scaphopod. © BIODIDAC
There are two plastic mounts that show the internal anatomy of the tusk shelled Scaphopods.
These animals have a characteristic tooth-like shell and the mantle cavity runs the length of the
shell. Compare these specimens to the diagrams showing the internal anatomy of these animals.
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When these animals feed, thin tentaclular captacula reach out from the mantle and pick up
organic matter on their sticky ends. The food is then passed to the mouth. A U-shaped digestive
system opens into the mantle cavity and wastes are passed out through the tip of the tusk. The
gonads are located internally more towards the tip of the shell and also release their contents
into the excurrent flow. The foot is used to burrow in a manner similar to that seen in the
bivalves.
Bivalvia
Clams and their bivalve relatives are one of only a few mollusc classes found in both freshwater
and marine environments. In both places they're specialized as filter feeders, and like most filter
feeders they're essentially sessile. To accommodate their way of life, bivalves have made some
major modifications to the mollusc body plan. The most obvious is that the body is compressed
laterally with the mantle folded down over the sides in two lobe-like sheets. The shell matches
the shape of the mantle's two parts. The shell has two valves that cover each side of the animal
and an elastic hinge ligament connects the sides across the top. The two valves give the class
their name, the bivalvia. But, be careful. These are univalve molluscs because the original single
valve is only folded!
The lateral compression of the bivalve body not only changes the shape of the shell but also the
organs of the visceral mass. Instead of sitting above the muscular foot, the visceral mass is
partially buried in the foot. The only part still visible above the foot is the pericardial cavity on
the dorsal side. Another modification for the filter-feeding lifestyle has resulted in the loss of
the radula and head.
Fresh water clam - Unio or Anodonta
External Anatomy
Identify the dorsal hinge ligament and next to it the paired umbos on the outer surface of the
shell. The umbo is the oldest part of the shell and is used to identify the clam’s anterior end.
Look closely at the surface of the shell and you’ll see a series of concentric rings extending from
the region of the umbo. Just like the rings of a tree, each of the rings on the surface of the shell
represent different rates of growth at different times of the year. Look at the animal from the
anterior and posterior ends. Where are the excurrent and incurrent openings and how are
they positioned relative to each other?
Orient the clam so that its anterior end is on your right and the dorsal surface is uppermost. The
side facing you is the right-hand side of the clam and we’ll be removing the right shell. To do
this, use a scalpel to remove part of the hinge ligament. This will weaken the ligament and make
it easier to open the shell. Commercially available preparations usually have a small wooden
peg holding the ventral side of the shell open. If not, partially open the shell and beginning at
the dorsal posterior region, separate the sheet of tissue, the mantle, from the shell using a blunt
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probe. Once the mantle has been separated from the shell reach in over the mantle and with a
scalpel and cut the large anteriorly and posteriorly located muscles. Carefully lift the shell off
and as you do, separate the tissue that remains attached to the shell.
Fig. 7. Features on the inner surface of a freshwater clam
shell. © BIODIDAC
Examine the inner surface of the shell you removed. Near each end are the large adductor
muscle scars. These muscles run from the left to right sides connecting the two valves to each
other. Contraction of the shell adductor muscles closes the shell and the hinge teeth insure that
the two shells align properly as it closes. How does a clam open its shell? Next to each of the
adductor muscle scars is a scar from the attachment of the foot retractor muscle. The anterior
muscle scar lies a bit above and behind the anterior adductor. The posterior retractor lays next
the posterior shell adductor. As their name implies the foot retractor mucles pull the foot into
the shell. How does a clam extend its foot? The pallial line extends from the adductor muscle
scars and runs along the edge of the shell. This scar is created by the attachment of muscle along
the mantle edge.
Fig. 8. Major structures inside the mantle cavity of a
bivalve. © BIODIDAC
Break off a piece of shell from the valve you have removed and identify its three layers. These
include an inner mother of pearl layer, or nacreous layer; a middle prismatic layer and the outer
periostracum.
Internal Anatomy
If it isn’t already, fold the mantle back over the body of the clam lying inside the left shell. The
mantle is fused to the dorsal surface of the animal and on the ventral side it hangs free and
encloses the mantle cavity. With the mantle still covering the body look at the posterior edge.
The incurrent and excurrent openings are only visible when the two sides of the mantle lie next
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to each other. The incurrent opening has a more ragged edge than the excurrent opening and
this helps to filter the incurrent water. In many bivalves the two sides of the mantle fuse and are
extended as siphons.
Lift the edge of the mantle and remove it by cutting along where it fuses to the dorsal surface of
the body. The two most prominent structures visible underneath are the ctenidia and the
muscular foot.
Respiratory system W-shaped ctenidia are found on both sides of the foot and are covered
with cilia that pump water into the mantle cavity and across the gill surface. The ctenidia are
attached to the visceral mass along their dorsal surface with their ventral edges suspended
inside the mantle cavity. Each ctenidium consists of two demibranchs each formed from the
fusion of two lamella. The lamella are fused at their distal edge and the space between them is
divided into a series of water tubes running from the bottom to the top of the demibranchs
where they empty into the suprabranchial chamber. Water enters the demibranchs through
ostia on the surface and these connect to the water tubes inside. It then moves dorsally in the
filament to suprabranchial canals and out the excurrent opening.
Fig. 9.
Internal anatomy of a bivalve. © BIODIDAC
Cut a piece of gill from your specimen and look at its edge under the dissecting microscope to
see the paired lamella, the connections between them and the water tubes inside. Examine the
side of the same piece under low power to see the grid of ridges and connectives that form the
demibranchs. Blood vessels are embedded in the tissue and as the water flows through, the
blood is oxygenated.
Fig. 10. Detailed structure of a demibranch. ©
BIODIDAC
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In freshwater species the female gills may be enlarged. They are used as brood chambers and
small larval glochidia may be contained inside. This does not occur in marine gastropods where
the larval stage is free swimming. Prepared slides of glochidia are available.
Circulatory system Bivalves have an open circulatory system with a heart lying inside
the dorsal, pericardial cavity, underneath the hinge. The largest cavity of the body is the
hemocoel and blood from the heart is pumped into the hemocoel where it bathes the various
tissues and organs before passing back to the heart through the ctenidia. If it’s not already open,
use a fine pair of scissors to open the pericardial cavity by making a cut along the dorsal edge
of the cavity. The intestine passes through the pericardial cavity and the three-chambered heart
is wrapped around it. Paired thin walled atria, auricles, on either side of the cavity connect the
ctenidia to the heart. These triangle structures are often damaged when the pericardial cavity is
opened and if you don’t see the atria on the side nearest you, gently move the intestine and look
on the opposite side of the pericardial cavity. Blood in the atria passes through large ostia and
into the single ventricle. When the ventricle contracts, blood flows into either the anterior or
posterior aorta carrying the blood to the hemocoel.
Fig. 11.
Circulatory system in a bivalve. © BIODIDAC
Excretory system
The metanephridia are located just underneath the pericardial cavity
and in front of the posterior adductor muscle. They appear as dark glandular structures and the
opening in the wall of the pericardial cavity is difficult to see. The metanephridium is composed
of two parts the glandular part near the heart, and the duct that carries the urine to the
nephridiopores opening on the side of the visceral mass near the base of the inner demibranchs.
This opening is also hard to see. Fold the gills up and see if you can locate it. How do metabolic
wastes pass from the circulatory system to the metanephridia?
Digestive system
The same water used to aerate the gills also contains particulate food.
Cilia on the surface of the gill move the particulate food to the ventral margin of the
demibranchs and cilia along the grooved edge pass it forward to the labial palps. There are
two pairs of labial palps, inner and outer, and these fuse above the mouth. The labial palps are
also covered in cilia and their beating propel food particles into the mouth. We’ve already made
a note of the intestine running through the pericardial cavity; follow it towards the anus that
empties into the suprabranchial chamber.
The rest of the digestive tract is contained in the visceral mass embedded in the foot. To be able
to see this you’ll have to carefully remove the muscular tissue of the foot. This is best done by
using a sharp scalpel and gradually slicing away tissue starting in the region of the labial palps.
The slices should be from dorsal to ventral and parallel to the main axis of the body. With each
slice you’ll expose a bit more of the underlying digestive system starting with the mouth and
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short esophagus that leads to the stomach. The greenish tissue above the stomach is the
digestive gland. The intestine is embedded throughout the foot and as you continue to remove
thin slices of tissue you’ll see sections of the convoluted intestine.
Fig. 12. Structure of the glochidial larva of
freshwater clams. © BIODIDAC
Reproductive system Although the sexes are separate there is no easy way to tell the sex
of your specimen because morphologically the gonads are the same in both sexes. If you’re
specimen has the modified gill with glochidia inside then you’ve got a female. The tissue of the
gonads spreads throughout the foot and is often difficult to distinguish from the spongy
musculature of the foot. You’ll need to use your dissection scope and a well-flooded specimen
to be able to see the difference between the two.
Nervous system
The nervous system is difficult to see and in fresh specimens the various
ganglia will have a pinkish color. It consists of three pairs of ganglia; cerebral, pedal, and
visceral ganglia. The visceral ganglion is located on the surface of the posterior adductor
muscle. The brain is located just above the mouth and behind the anterior adductor muscle.
Three major nerves radiate from the brain and connect to the visceral ganglion, the pedal
ganglion and the mantle, and the pallial nerve. The pedal ganglion is located in the base of the
foot
Cephalopoda
A cephalopod’s visceral mass has been stretched along the dorsoventral axis above the foot,
bringing the head and foot closer together on the ventral side. That’s how they got their name,
Cephalopoda (head, foot). The mantle surrounds the visceral mass, and ancestrally a hard shell
surrounded all of this to form an elongated cone-shaped shell with the head and foot poking out
the open end. It was easier to point the tip of the shell in the direction that the animal was
moving. Cephalopods swim with what was their original dorsal surface pointing in the direction
they move, rather than up. In most animals the surface of the body facing into the direction of
movement would be the functionally anterior surface and this is usually the same side of the
body where the mouth and head are located. In cephalopods the anterior head and mouth has
now become the new dorsal side although the squeezing together of the foot compressed the
anterior/posterior axis and the oral opening and head still have a functionally anterior position
but its not facing in the direction that they swim. The result of this is that cephalopods swim
backwards!
With a larger visceral mass on top of a muscular foot, cephalopods were faced with the same
problem as gastropods had for finding a way to make a more compact body. They used the same
solution, although they wound their shells in a different way.
The fossil record includes cephalopods with unwound, wound, and even partially wound shells,
and the diversity of fully wound ammonite fossils is good evidence of the successful body plan.
At one time these cephalopod predators ruled the ancient oceans, but now only Nautilus remains
to give a hint to what these ancient animals looked like.
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Shelled cephalopods disappeared at about the same time as jawed predatory fishes appeared. It’s
possible that the two events are related. The mollusc adaptation to these more active and agile
newcomers was another modification to the mollusc body plan to become just as active and
agile–the result are the cephalopods we see today.
Nautilus shell
The only living cephalopod with a shell is Nautilus and even in this animal it differs from that
of the other living Molluscs. In Nautilus the pearly shell consists of a series of closed chambers
separated from each other by septa and connected by the small openings created by the
siphuncle. This animal lives only in the very last chamber and the body does not extend back
into the shell. How does this differ, for example, from the spiral shell of a Gastropod?
Changes in the ionic composition of the fluid in the siphuncle move fluids in and out of the
chambers. This affects the levels of gas in the chambers and has the overall effect of altering the
buoyancy of Nautilus. In the other Cephalopods the shell has become reduced and internal. It
may be large such as the cuttle bone in cuttle fish or just a thin “pencil” or strengthening rod, or
pen, as in the squid Loligo.
Fig. 13. Ancestral body surfaces of a
squid as seen from the functionally ventral
side. © BIODIDAC
Loligo
Squids in the genus Loligo are found around the world from warm shallow waters to the deep
abyssal depths of the oceans where they feed on a variety of small crustaceans and fish not fast
enough to escape from these predators.
External anatomy
To better understand how cephalopods have modified the mollusc body plan it’s important to
orient yourself by locating the head, foot and dorsal visceral mass. The squid’s body is divided
into two main regions. The first is the elongate, and somewhat conical visceral mass surrounded
by the mantle. Below this the head and foot that have fused. The last region includes the arms,
and tentacles surrounding the mouth. The mouth is the original anterior part of the body, the
funnel the posterior. Nautilus and the ancient cephalopods wound this visceral mass up, modern
cephalopods just tip over and swim with the ancestral dorsal surface pointing in the direction
that they travel. The result; the anterior side is functionally dorsal, the posterior is functionally
ventral, the ventral functionally anterior, and the dorsal surface functionally ventral and as was
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mentioned earlier cephalopods end up swimming backwards. From here on the terms dorsal,
ventral, anterior and posterior refer to the functional organization of the animal; trust me it will
be less confusing.
If you are using a fresh specimen, take a look at the body surface. The small dots of coloration
are the elastic capsule chromatophores that squids use to change colors.
There are eight arms and two tentacles surrounding a central mouth. Place the dorsal surface
uppermost in your tray, if you’re following the functional orientations we just described that
means funnel down. The five arms on each side are numbered, starting from the dorsal surface
as one to five right, and one to five left. Using this numbering scheme appendages L1 and R1
are the smallest arms and R2, L2, R3, L3, R5 and L5 are larger. Arms have two rows of suckers
and are not retractable. This differs from R4 and L4, the tentacles. These have four rows of
suckers on an enlarged tip of the tentacle referred to as a peduncle. Unlike the arms, tentacles
are retractable being shot out to capture prey and shortened to bring it into the mouth.
Fig. 14. Arms and tentacles surrounding the mouth
of a squid. © BIODIDAC
Take a close look at the suckers of the arms under the microscope. Examine a sucker near the
base of an arm; those at the tip are the youngest and as a result much smaller. The cup of the
sucker is surrounded by a chitinous ring with a central muscular suction cup. Each sucker is
attached by stalk or pedicle. If you have a male, arm L5 will be modified for sperm transfer and
is referred to as the hectocotylus. Its suckers are small and located on the end of much longer
pedicles. The hectocotylus arm is used in mating and sperm, contained in a spermatophore, are
attached to these modified suckers before being passed to the female. In some species the tip of
the arm and its package of sperm breaks off. It’s no great loss to the male; in cephalopods
damaged arms can be regenerated.
Lets turn our attention to the mouth region. Bend back the arms and tentacles attached to each
other by a muscular membrane surrounding the central mouth. Inside this is a second
membrane, the ruffle-edged peristomial membrane. In female squids the peristomial
membrane is modified into a horseshoe shaped seminal receptacle in the middle and below the
mouth. Sticking out from the mouth you should be able to see the beak-like chitinous teeth
used to tear apart captured prey. We’ll take a closer look at the buccal bulb later in the
dissection.
A pair of eyes on the head are remarkably similar to mammalian eyes; an excellent example of
convergent evolution. What is convergent evolution? Identify the cornea, iris, pupal and
lens. How does this eye differ from the mammalian eye? Just behind the eye, and near the
base of the arms is the aquiferous pore that stabilizes fluid pressure on the eye as the squid
dives. In front is a crest of tissue referred to as the olfactory crests and next to them the
olfactory grooves. They are chemosensory and positioned in the incurrent flow of the mantle
cavity.
The cone shaped funnel, siphon, is located on the ventral surface of the head and water is forced
out of the funnel for jet propelled locomotion.
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The mantle, with two lateral fins that are extensions of the mantle cavity, surrounds the visceral
mass of the body. Muscles in the fins allow graceful forward and backwards motion and rapid
changes in direction when necessary. The medial mantle artery is visible from outside and is
located on the midline of the body between the collar and tip. Look at the edge of the mantle,
the collar, and you’ll see three divisions created by three projections around its edge. On either
side of the funnel are the pallial cartilages that form cartilaginous ridges on the inner surface
of the mantle. Below them, on the body, are the cartilaginous grooves into which the ridges
lock when the mantle contracts. The mantle is composed of mainly circular muscle and when
they contract the mantles edge is sealed against the body and any slippage is prevented by these
lock and key structures. The result is all the water passes out through the funnel. The third
projection on the collar is the tip of the chitinous pen, all that remains of the ancestral mollusc
shell. It runs to the tip of the posterior surface and protects the soft tissues that lie inside and
underneath it.
Internal anatomy
To expose the visceral mass inside the conical mantle surrounding it, you’ll have to make a
longitudinal cut up and through the mantle on the side with the funnel. If you’re using
commercially available injected specimens there will already be a cut in the mantle where the
various pigments have been injected to enhance the appearance of the circulatory system, just
extend that opening. It’s good practice in dissection to never cut along the exact midline so as
you make yours do it just off to the side. Why do you never cut along the exact midline of a
specimen?
Fig. 15.
Major internal features of a squid. © BIODIDAC
Mantle cavity Like all soft bodied animals it’s going to be easier to see the various systems
if they are supported by water. Place you specimen in a dissection tray, pin the sides of the
mantle back and flood the specimen with water. By opening the mantle cavity on this side you’ll
see that the visceral mass seems to be floating inside the mantle cavity. Move it to either side
and you’ll see it’s attached to the mantle by a fine ligament on the underside of the body. It’s
on that side where you’ll find the pen, the remnants of the shell. On the inside of the mantle
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surface the pen forms a protective concave structure surrounding the soft body parts underneath
it. In our preliminary observations we’ll identify some of the main structures inside the mantle
cavity before looking at individual systems in detail.
Fig. 16.
Lateral view of the head and mantle in the squid. © BIODIDAC
Split open the funnel and trace the way that water flows through the mantle cavity. Water enters
through the open collar. This is sealed against the body when the circular muscles of the mantle
contract, and force the water out through the funnel. The large funnel retractor muscles,
combined with the circular and longitudinal muscles in the funnel, direct the jet of water from
the funnel; controlling the direction the squid swims. Inside the funnel is a muscular valve that
prevents water from entering through the funnel. Underneath the funnel retractors are the
cephalic retractor muscles. The mantle is primarily circular muscle, but there is a smaller set
of longitudinal muscles that enlarge the mantle cavity. Why are there more circular muscles
compared to the longitudinal? The visceral mass is covered by a thin, transparent membrane;
the body wall.
The paired ctenidia are anchored to the wall of the mantle cavity and where they connect with
the body you will see the paired branchial hearts. The rectum and anus are located next to the
funnel and the opening is shared with the ink sac that lies along side this part of the digestive
tract. What is the function of ink? Underneath the rectum, and between the retractor muscles
is the large liver, a modified portion of the digestive gland.
Reproductive system Identify the sex of your specimen, and be sure to see both. We’ll
make some preliminary observations now. Some of the ducts of the system will be easier to see
once observations of the circulatory system are complete and the branchial hearts have been
removed.
If you have a reproductive female enlarged, paired nidamental glands lie on top of the viscera
near the center of the body, and the single ovary, filled with granular eggs is located at that
posterior tip of the body. The single oviduct runs along the left side of the body and opens into
the mantle cavity through the oviducal funnels. In the area where the oviduct passes under the
gills it enlarges forming an oviducal gland. Gently remove the nidamental gland to reveal the
oviduct and the opening of the oviduct underneath. Removing the nidamental gland will also
expose the accessory nidamental glands underneath. The oviducal gland adds the shell to the
eggs and the nidamental glands add the gelatinous capsule that covers the egg case.
In the male the single testis appears as a tubular structure just off center and near the posterior
end of the squid. The testis lies inside a thin membranous sac and the twisted sperm duct drains
the capsule. There is no direct connection between the two and sperm pass into the space of the
capsule and then into the sperm duct. A larger convoluted, spermatophoric gland may be
visible now, or after the branchial hearts have been removed. The spermatophoric gland
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packages the sperm into the spermatophore which is then passed to the female by the
hectoctylus arm. At the anterior end, locate the penis which lies to the left and behind the
rectum.
Excretory system.
If the delicate body wall remains intact, locate the two openings to the
metanephridia. The triangular metanephridia are in front of the branchial hearts. The
metanephridia are next to the anterior vena cava, and if you have an injected specimen, the
injected dye may make the excretory structures hard to see. The metanephridia filter the fluid in
the pericardial cavity.
Circulatory and respiratory system Unlike other molluscs, the cephalopod circulatory
system is closed. We’ve already located the paired branchial hearts, so let’s start our
observations of the circulatory system there. The branchial hearts collect blood from the body
through the single, large anterior vena cava that splits into left and right precava that pass by
the kidney before entering the branchial hearts. The paired posterior vena cava also drain into
the branchial hearts along with left and right mantle veins. Each branchial heart pumps blood
into the ctenedia on the corresponding side of the body. Blood enters the afferent branchial
artery, crosses the gills where it is oxygenated, and leaves through the efferent branchial veins
that connect with the single systemic heart.
Fig. 17. Major blood vessels in the region of the
branchial and system hearts of the squid. © BIODIDAC
Open the pericardial cavity surrounding the systemic heart that pumps oxygenated blood from
the ctenidia through the anterior aorta to the front of the body. The posterior aorta supplies
the posterior part of the body and divides into three mantle arteries that disappear into the wall
of the mantle. Smaller branches of the anterior aorta may be visible supplying the rectum and
reproductive structures.
Digestive system
The digestive system is complex and it’s worth remembering the general
mollusc digestive plan. The simplest description is a long tubular gut with a blind ended sac, the
gastric or digestive gland. The same applies with the squid, the difference being the digestive
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gland itself has modified into its own separate compartments, and of course these are all going
to have names. Morphologists have often used common vertebrate terms to describe these
divisions; pancreas and liver are only modified regions of the ancestral digestive gland.
Fig. 18.
Major features of the digestive tract of a squid. © BIODIDAC
Carefully remove the heart and metanephridia without destroying the underlying digestive
organs. Underneath the heart is the U shaped pancreas with a granular appearance. The
pancreas is part of the duct that leads to the liver. The liver occupies most of the anterior viscera
and embedded on its side are salivary glands. Loosen and remove the connective tissue
surrounding the liver and gently lift it up to reveal the esophagus and anterior aorta that pass
through the region. An equally large structure, this time forming most of the posterior end of the
viscera is the caecum, or stomach pouch. This thin walled structure fills most of the posterior
part of the mantle cavity and is filled with recently ingested food and secretions of the liver.
Other obvious features are the anus and the intestine leading to it.
Free the funnel from underlying tissue by cutting the two small siphon protractor muscles
between head and funnel and lift the siphon out of the way. Cut through the head between the
two arms immediately underneath the funnel to expose the pharynx modified into the buccal
bulb. The large, tough, interlocking jaws are easy to identify. Pry them apart and inside you will
find that the ancestral radula is still present. Under the radua is the ligula and the two structures
are referred to as the odontophore.
The esophagus leads out of the bulb, extends through the liver, and connects with the stomach.
The stomach has a muscular, thick wall and connects to the caecum, or stomach pouch and its
own diverticulum. The pouch in turn is connected to the liver through the pancreas. The
intestine is connected to the stomach near the entrance of the esophagus.
Nervous system The generalized mollusc body plan has a variety of ganglia positioned in
different parts of the body. In the squid these have all become fused and encased in a
cartilaginous brain case, or skull. The only exception to this are the stellate ganglia on the
inside of the mantle at the tips of the gills. To see the brain cut lengthwise through the head. It
will be hard to distinguish the differences but the supraesophagial ganglion is a single paired
ganglia and the mass of nerve tissue underneath the esophagus, the subesophagial ganglia, is
a fusion of the pedal and visceral ganglia. Circumesophagial connectives connect ganglia
above and below the esophagus. The large eyes are connected to the central nervous system
through the optic nerves attached to the ganglia underneath the esophagus.
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