|Class Cephalopoda: the Head-Feet
The final molluscan class you'll consider is the Cephalopoda (head-foot). Refer to the generalized mollusc diagram and compare it to the cephalopod in the left hand corner. Notice the shell is present, although reduced, and the mantle cavity and the ctenidium are still represented. Although not visible in the diagram, a small radula is still present in most cephalopods.
(A) Entering the 3 dimensional world of the water column
Zoologists contend that a snail-like cephalopod ancestor evolved a gas-filled chamber in its shell as a means of buoying up the awkward, heavy visceral mass. As the chamber system evolved, the ancestral cephalopod was able to adjust the gas-water mixture of the chamber and float off the bottom. Free of the bottom the animal was confronted with living in a 3-dimensional world, the open water column, and being vulnerable from the top, side and bottom. Survival in this habitat required a significant increase in all the molluscan systems. Selective pressures increased with the evolution of fishes as competitors and predators in this pelagic realm. Modern cephalopods are molluscs that have successfully met these challenges and remain viable competitors. In the process they also have become the most intelligent and the most specialized of the invertebrates. Viewed in this perspective, cephalopods modification from the basic molluscan plan is easily understood.
(B) Three extant types of cephalopod molluscs
Of the three types of cephalopods, the most primitive is the chambered nautilus. It retains a large molluscan shell for protection and the housing of the flotation chambers mentioned above. Nautilus occurs in tropical seas and its swimming is achieved by drawing water into the mantle cavity and forcing it out through the funnel or siphon. The siphon is considered the only cephalopod structure derived from the ancestral molluscan foot. Around the nautilus' head are a series of tentacles (up to 90) that are used to crawl along the bottom and capture and manipulate prey. These tentacles are thought to have evolved from structures of the head region, and not from the molluscan foot (as was once the belief, which was in vogue when these animals were named the head-foots). The nautilus has large simple eyes and is a nocturnal creature restricted to a small range of habitats. Note the flotation chambers and large body chamber where the animal resides.
These animals lack a shell and the mantle cavity is carried bag-like behind the head that rides on 8 large arms. The arms have suction cups that allow the octopus to climb along surfaces or move quickly across the bottom. The arms are capable of very delicate and refined movement and can detect shape and texture, as well as manipulate prey. At the center of the tentacles the octopus' mouth is a sharp, parrot-like beak used to kill and dismember prey. A small radula is found associated with the beak and is used to pull pieces of prey into the mouth. The octopus also uses venom to dispatch prey, which is injected into the wound (created by the beak) from glands in the mouth. The most noticeable feature of a living octopus are the large eyes. The eyes follow you with an obvious awareness and suggested intelligence that is unnerving. The octopus' eye is considered on a parallel with ours and capable of color vision. When disturbed or pursued the octopus can swim rapidly for short distances. The mantle cavity is inflated and the water expelled vigorously through the siphon, pushing the animal along with its arms folded gracefully behind.
These groups have 8 arms plus 2 very long and protrusible tentacles. Cuttlefish are squatter than squid and remain close to the bottom; they utilize an internal, chambered shell (the cuttlebone in your parakeet's cage) to adjust their buoyancy. Squid are the true pelagic members of the class, swimming free of the bottom, sometimes in great schools, rivaling the fishes for food and, indeed, even feeding on them. The squid has only a remnant of the molluscan shell, called the squid pen, that is used for reinforcement of the expanded and heavily muscled mantle cavity. Squids inflate and contract their mantle cavities at a rapid rate forcing water out the siphon and propelling them swiftly through water. Squids and cuttlefish possess the same excellent vision described for the octopus and, like the octopus, are able to adjust their color to match their background. Color change in these animals is under direct nervous control and thus very rapid. Besides using color for camouflage, the cephalopods use color for mating rituals and to confuse prey and predators.
Obtain a squid for observation of the basic cephalopod characteristics. Identify the tentacles, arms and siphon. Open the mouth and remove the beak. Investigate the mantle cavity and determine how it functions as a jet propulsion unit. Investigate the structure of the large eyes and look at the skin under the microscope to see the chromatophores, which allow the squid to adjust its color. Visualize this animal alive and moving sleekly through the water, the ultimate mollusc and a fitting way to conclude an investigation of adaptive radiation in this majestic group of soft-bodied animals.