Mussel dissection – Geukensia, Brachidontes or Mytilus – live/pickled
Pair or triple up to study a live mussel
Before you start to dissect note the concentric growth rings and the byssus extending from the shell. How
many growth rings are visible in your specimen?
The byssus is produced by the byssus gland at the base of the foot, hardening in a groove that runs midventrally
along the foot. Test the strength of the byssal threads by trying to pull them apart. As the byssus is produced, it
is attached to the hard bottom on which the animal lives. Producing and attaching byssal threads is the main
function of the foot in mussels. As the byssus is used near the anterior, the anterior end of the shell is narrowed
and reduced, while the posterior, through which water is exchanged, remains wide. This is reflected in the much
smaller anterior than posterior adductor muscle – a heteromyarian ("different-muscle") condition.
You may also be able to see one of the siphons extended.
After examining the exterior, insert the blades of scissors in the widest part of the shell and move back toward
the narrow part of the shell, making sure you are not damaging any internal organs. Once a few of the muscles
are cut try prying open the shell.
Place in dish provided and then add dilute solution of ethanol.
matches the diagram below.
Place the shell down so that the orientation
Locate the foot and labial palps, better seen in the photo below. The byssal threads should be extending from a
pouch at right angles to the mouth. The gonads will be visible as reddish material on top of a visceral mass.
Take a photograph of the labial palps, which are used to sort food material. When feeding the food will extend
between the palps. The mouth opening can be found just at the end of the foot.
Try to find the siphons and note the filamentous gills. Remove part of the gill and examine it under the
microscope. The gills of mussels are very simple in construction. Photograph the gill. In freshwater forms,
small larval glochidia may be contained inside brood pouches in the gills. This does not occur in marine
gastropods where the larval stage is free swimming. Prepared slides of glochidia are available however for
you to view. Glochidial larvae are ectoparasites on the gills of freshwater fish.
Two diagrams with membranes surrounding visceral mass removed and gills removed to see heart and internal
organs. Try to be careful and remove or at least cut some of the membranes. The very dark structure under the
foot is the kidney and the greenish brown structure above is the digestive gland. The intestine coils around,
even runs through the pericardial cavity in some species and difficult not to cut. You may try to remove the
gill and other structures to see the heart. Again it may be difficult to do this without damaging this structure. If
you are careful in removing the gills and locate the stomach, you may be able to trace the intestine back to the
heart in the pericardial cavity. If you are successful, please share your specimen with others in the lab.
Cephalopoda dissection.
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!
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
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.
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 mature male
and it is mating season, 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.
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. 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
You will be challanged to ry to identify as many of the diagrammed structures in the following diagram
and photo as possible.
Internal anatomy of a squid. The photo is of a preserved specimen. Your fresh specimen will look very
different.
Protocal:
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 with a bit of alcohol in it. 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 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.
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. Locate the ink sac and see if you can
see a thin sac next to it which is the rectum. It may not be visible in all species. Try not to puncture the ink
sac and if you do, try not to get ink all over yourself. The ink produced will stain clothes and impossible to
remove. 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, it is fairly amorphous in fresh specimens, so identify it by
location. .
Reproductive system Identify the sex of your specimen, and be sure to see both.
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 structures underneath, underneath.
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 packages the sperm into the spermatophore which is then passed to the female by the
hectoctylus arm. At the anterior end, You may be able to locate the penis which lies to the left and behind the
rectum.
Circulatory and respiratory system Unlike other molluscs, the cephalopod circulatory system is closed. You
have already located the paired gilla. The branchial hearts collect blood from the body through the single, large
anterior vena cava that splits into left and right precava that pass bythe 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 gills or 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.
Identify the branchial hearts and gills. The systemic heart sometimes is visible at this time. If not, try
pulling slight on the branshial hearts. They are connected to a brownish structure just below them that
is the systemic heart. Why furnish the gills with hearts of their own? Cut a piece of the gill and view it
under high power. Compare its structure to that of the mussel’s gill.
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 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.
Carefully remove any remaining lgonadal tissue, using water and gently moving it aside. Underneath the heart
is the U shaped pancreas with a granular appearance. At the end of the mantle cavity, locate An equally large
the caecum. This thin walled structure appears as a large sac at the end of the mantle cavity and may be filled
with recently ingested food and secretions of the liver. You can then trace by using the intestine leading to it
to stomach.
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 (or beaks) are easy to identify. Pry them
apart and inside you will find that the ancestral radula is still present.
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. Try to trace again the intestinal system using these landmarks. Again some of the
system may have been damaged during shipping or dissection. Once done, remove the beak and try to
examine the radula under higher power.
Examination of internal anatomy
Here are more pics to help you. Note that these pics are in the reverse orientation to the first photo.
You will have to remove the kidney to see the systemic heart.