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.