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 sacwhich 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, systemicheart, 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)
Fleshy border separating head-foot from visceral mass (mantle)