Looking at the molluscs for the first time it is difficult to see much in common between a relatively immotile clam or oyster and a fast-swimming pelagic squid. The fossil record offers little help in trying to determine the relationships among living molluscan groups due to the failure of soft molluscan bodies to fossilize. So although mollusc fossils date back to the Cambrian period (600 millon years before present) with rare exceptions only shells are represented

To understand the adaptive radiation in the molluscs with the lack of good fossil evidence, zoologists have developed a hypothetical organism they call the archetype mollusc. This organism possesses all the molluscan features that zoologists consider were present in the ancestral molluscan stock that gave rise to the modern groups. You'll examine the ancestral mollusc and then explore the fate of these characters in the major molluscan classes. Many features of the ancestral mollusc, such as the foot or gill structure, are encountered among primitive members of living molluscan classes.

(A) Archetype body plan

Refer to the diagram of the archetype mollusc.

The head-foot region is the locomotory and sensory part of the body. Locomotion is achieved by undulations produced by peristaltic contractions of the broad, muscular foot. The head contains sensory tentacles or antennae, the nerve collar or brain, the mouth and a unique feeding structure known as the radula. The radula has been described as a "flexible file" and consists of many small, sharp chitinous teeth mounted in transverse rows across a long flexible membrane. The radula rides over a cartilaginous base that can be protruded through the mouth and pressed against the substratum. The radula is then drawn back and forth over the substratum scraping loose food and sweeping it into the mouth. To visualize this, imagine yourself licking an ice cream cone; apart from having no teeth on your tongue, you are eating much like a snail or chiton. When not in use the radula is retracted into a radula sac that lies behind the mouth.

The visceral mass rides on top of the foot and contains the excretory, reproductive, digestive and circulatory organs. The third part of the body, the mantle, forms a fleshy cover over the visceral mass and hangs like a skirt over the foot. The mantle secretes the shell that lies on its outer surface. Zoologists consider the shell was originally a cuticle that was gradually thickened and strengthened by secretions of calcium carbonate into a proteinaceous matrix. Modern shells are several layers thick with new shell being secreted by the mantle along its edges and also beneath the existing shell so that it becomes larger and thicker as the mollusc grows.

The mantle-shell has little overhang except in the posterior region where a space between the mantle and foot houses a pair of molluscan gills, or ctenidia. This space is known as the mantle cavity and besides housing the gills it receives the products of the digestive and excretory system via the anus and renal pore, respectively. The ctenidium is a feather-like structure consisting of a central axis with alternating wedge-shaped filaments projecting from either side. The gill surfaces are ciliated and the coordinated beat of the cilia produces a current that flows over the posterior of the foot, through the gills to the upper part of the mantle cavity and back out the posterior again. As the respiratory current exits the mantle cavity it washes over the renal pore and the anus and removes their products. The mantle and the mantle cavity play many important roles in molluscs.

To recapitulate, the ancestral mollusc consists of the following five characteristics: foot, shell, mantle, radula and ctenidium. Now you will explore the role of these structures in the major extant mollusc classes.