Download 9. Cephalopod Anatomy

Cephalopod Anatomy
As their name implies, members of the class Cephalopoda have modified “head-foot”
which bears an array of prehensile tentacles and arms at the cranial end of the body. The
visceral mass is located toward the caudal end. Only the nautilus possesses an external
shell; the shell is completely lacking in octopods and is reduced and internal in squids
and cuttlefish. While squids and cuttlefish typically use their muscularized fins for
leisurely locomotion, they also possess the ability to maneuver quickly by jet
propulsion—rapidly expelling water from their mantle cavity through their tubular
siphon. The evolutionary recruitment of the mantle cavity as the fluid reservoir for jet
propulsion was incompatible with a hard external shell and explains why this feature is
absent or highly reduced in most cephalopods. The consequence of this evolutionary
trend was increased vulnerability to predation due to the lack of protection afforded by a
shell. As a result, many octopods and squids developed the ability to rapidly change their
skin color through the use of specialized epidermal cells allowing them to blend in
perfectly with their surroundings. This, in turn, made them more effective predators. In
addition, most cephalopods (except the nautilus) have an ink sac which can discharge a
dark, cloudy liquid through the anus to confuse potential predators.
1.
Obtain a preserved squid and position it in your dissecting pan so that the side with
the siphon is toward you.
2.
Examine the external anatomy of the squid and identify the following structures:
tentacles, arms, fins, siphon, mantle, eyes and collar.
3.
Using your scissors, make a shallow, longitudinal incision through the mantle
starting at the collar and extending to the caudal end of the body tube (near the
fins). You may need to use pins to keep the body tube open.
4.
Use the illustrations and Table 11.2 to identify the following selected internal
structures of the squid and familiarize yourself with their functions: gills,
esophagus, stomach, pancreas, liver, anus, testis (male only), ovary (female
only), ink sac, siphon retractor muscles, kidney, systemic heart, branchial
hearts, caudal (posterior) vena cava, and cranial (anterior) vena cava.
Cephalopods have the most efficient circulatory system of all molluscs, supporting the
relatively high metabolic needs associated with swift-moving, active predators. They are
unique among molluscs in having a closed circulatory system in which the blood is
permanently contained within a network of arteries and veins. This advancement allows
the blood to contain an unusually high concentration of oxygen-binding respiratory
pigments. In addition to a single systemic heart, which receives oxygenated blood from
the gills and sends it back to the tissues, squids have a pair of branchial hearts
associated with the gills to increase blood pressure and thus blood supply to the
capillaries of the gills.
Perhaps the single most remarkable adaptation that squids possess is their well-developed
eye. The striking similarity between cephalopod eyes and the eyes of vertebrates (birds,
mammals, fishes, etc.) is one of the most beautiful examples of convergent evolution
among animals; each group has independently evolved acute, image-forming eyes that
are amazingly similar in structure. The eye of the squid contains a lens, cornea, iris,
ciliary muscles and a retina, just like the eyes of vertebrates. Due to their independent
evolution, the cephalopod eye does differ in a few important ways from the vertebrate
eye. In vertebrate eyes the lens is elastic and visual images are focused on the retina by
altering the shape of the lens. In cephalopod eyes the lens is rigid and images are focused
on the retina by altering the shape of the lens. In cephalopod eyes the lens is rigid and
images are focused on the retina by altering the shape of the lens. In cephalopod eyes the
lens is rigid and images are focused on the retina by altering the distance between the lens
and retina (just like in a camera).
Another major difference between the two eyes is the way the light is received by the
photoreceptors in the retina. In the vertebrate eye, the rods and cones point toward the
back of the eye (away from the pupil) so light must pass through the photoreceptors and
other associated nerve cells and bounce off the back of the retina (back toward the pupil)
before it is detected by the photoreceptors! An unfortunate consequence of this design is
that all of the neurons are naturally on the inside of the retina and where they exit the eye
as the optic nerve they come together in a large, cable-like nerve fiber and “push” the
rods and cones aside to make a path through the back of the eye creating a blind spot in
our visual field.
In contrast, the cephalopod eye has the light sensitive end of its photoreceptors oriented
toward the front of the eye, so light entering the pupil passes through the lens and directly
stimulates the photoreceptors. Due to the arrangement of neurons being positioned
behind the retina, cephalopods have no blind spot in their visual. If you think about it,
this is a much more logical design for an eye than the architecture of the vertebrate eye!
This is a good illustration of how different evolutionary means may be employed to reach
the same end—in this case a functional, image-forming eye. Remember, natural
selection can only operate on existing variation within populations, so the most logical
design may not always be achievable. Without natural variability in a trait (e.g., the
elasticity of the lens), that trait will forever remain unchanged and other traits in which
variability within the population does exist are modified to improve survival (e.g., ciliary
muscles that move the lens back and forth rather than change the shape of the lens).
Table 11.2 • Anatomy of a Cephalopod (Squid)
Structure
Function
Collar
Fleshy border separating head-foot from visceral mass (mantle)
Eyes
Image-forming organs for detecting visual stimuli
Siphon
Hollow tube through which water is expelled from the mantle cavity at high
velocity to propel the squid through the water
Mantle
Body tube encircling visceral mass forming a hollow chamber in which water is
collected and used for propulsion
Arms
Shorter appendages (8) used to manipulate captured prey and act as a rudder for
navigating while swimming
Tentacles
Long, extensible, prehensile appendages (2) for capturing prey
Fins
Triangular-shaped extensions of the caudal end of the body tube that are used for
leisurely swimming and for maneuvering during locomotion
Gills
Feathery organs used for respiration
Esophagus
Thin tube connecting the mouth to the stomach
Stomach
Small sac located at caudal end of the body tube where food is stored and
digested; digestion is entirely extracellular in cephalopods
Pancreas
Small, granular digestive gland that secretes enzymes into the stomach to assist in
the breakdown of food
Liver
Large, elongated gland that releases secretions into the stomach to facilitate
enzymatic digestion of food
Anus
Terminal portion of digestive tract located near siphon
Testis
Produces sperm; located in caudal end of body tube
Ink sac
Large sac that opens into the anus and secretes a dark brown or black fluid when
the animal is alarmed
Siphon retractor muscles
Long muscles which control the contraction of the siphon
Kidneys
Adjacent excretory organs located between the gills
Systemic heart
Large, muscularized chamber that receives oxygenated blood from the gills and
pumps it throughout the body
Branchial hearts
Smaller, muscularized chambers that receive deoxygenated blood from all parts
of the body and pump blood to the gills
Cauda vena cava
Drains deoxygenated blood from the body tube and mantle back to the branchial
hearts
Cranial vena cava
Drains deoxygenated blood from the head-foot back to the branchial hearts