Coelomate Animals
Molluscs are coelomate animals, that is, they possess a body cavity between the gut and the body wall that is completely lined by mesoderm tissue. The coelom serves several important functions, such as providing space for the development and function of complex internal organs and providing a hydrostatic skeleton to facilitate the movements of soft-bodied animals.
All of the rest of the phyla we will study this quarter are also coelomate.
Recall that coelomates are subdivided into two groups: molluscs, annelids,
and arthropods are protostome coelomates. The protostomes are distinguished
by the fact that the mouth of these organisms is formed from the first
opening in the embryonic gut. You may remember this first opening is called
the blastopore and is formed during the gastrula stage of development.
In contrast, the echinoderms and chordates are deuterostome coelomates.
In deuterostomes the blastopore forms the anus, and the mouth is formed
from a second embryonic opening. Keep in mind that grouping animals by
these and other developmental similarities gives a clearer picture of the
evolutionary relationships of organisms that externally may appear very
dissimilar. (Review Buschbaum Chap. 24 for this discussion.)
Mollusca Taxonomy
Molluscs are a very large group that includes such diverse animals as chitons (Class Polyplacophora), clams, mussels, scallops, oysters (Class Bivalvia), snails and slugs (Class Gastropoda), and squids, nautiluses and octopuses (Class Cephalopoda). (We are omitting two smaller classes.) The mollusc body typically is soft and unsegmented, consisting of an anterior head, a ventral foot and a dorsal visceral mass. The body is usually enclosed, at least in part, by a thin fleshy layer called the mantle. The mantle secretes a hard shell, though in some molluscs the shell has been internalized and/or reduced. A rasp-like radula is not present in all molluscs, but is unique to this phylum. The radula is somewhat like having a fingernail file for a tongue. In chitons and some gastropods the radula is used for scraping algae, in carnivorous gastropods it is used as a file to scrape through hard-shelled prey. (See demo in lab of bivalve shells that show small round holes--these are evidence of predation by a gastropod.) Generally speaking, all the organ systems found in more complex animals are present in phylum Mollusca.
While the above characteristics are found in most mollusc species, the classes show much variation with regard to feeding and locomotion. The structural and functional adaptations within this phylum provide an excellent way to introduce the concept of adaptive radiation. The evolution of many different types from a single ancestral form is referred to as adaptive radiation. The ancestral mollusc form is thought to be similar to today's chiton, with an elongated, flattened body protected dorsally by eight calcareous plates secreted by the mantle. The ventral foot of the chiton is well-developed and perfectly adapted for clinging to wave-beaten rocks in marine intertidal areas. From this basic body plan the other mollusc classes have undergone modifications over time that allow them to utilize habitats quite different from their ancient ancestor and modem polyplacophoran.
For example, the bivalve body and foot are completely protected when its dorsally hinged shell is closed. Locomotion is limited to digging with the foot, except in the scallop which swims by clapping its shells together. Feeding is accomplished by filtering food from an internally controlled water flow. There are many freshwater as well as marine species.
In comparison, gastropods have a broad foot and are quite mobile, but usually slow. There are terrestrial species as well as those found in all types of aquatic habitat. Most are plant-eaters or scavengers, but as already mentioned, some gastropods are predators. Some gastropods risk pooping on their own heads because of the way the body becomes twisted during development. The anus ends up just dorsal to the head, but the advantage of this strange growth pattern is that the animal is able to withdraw its head into the shell prior to withdrawal of the foot. Thus, the head, with its exposed sense organs, is well protected by the shell and the covering foot. In some snails, a portion of the foot forms a hardened plate called an operculum that completely seals the shell opening when the animal is withdrawn.
The cephalopods are highly modified for speed and active predation. They have image-forming eyes, remarkably similar to ours, and a highly developed nervous system. The foot is modified into several pairs of arms that encircle the mouth and also a funnel (muscular tube) that permits controlled, rapid movement when water is forcibly expelled. Like the polyplacophorans, all cephalopods are marine.
For more information on cephalopods go to the NATIONAL
RESOURCE CENTER FOR CEPHALOPODS (NRCC) home page.
KINGDOM ANIMALIA: Phylum Mollusca
Class Polyplacophora (all marine)