from which they obtain small algal cells and diatoms. The common Atlantic Slipper Shell feeds in the same manner as the oyster, and its stomach is found to contain the same diatomaceous food. Just as in the oyster, a food current of water is set up in the mantle cavity and the pectinate gill acts as a food sieve. The food particles are entrapped on the gills by a mucus secreted by an endostyle which is located at the base of the gill. Tiny cilia move the food along a groove on the side of the body to a pouch located near the mouth where it is then taken in up through the proboscis. Turritella communis of Europe buries itself in mud and has a ciliary feeding habit. This snail remains for days in one spot just below the surface of the mud. An inhalant depression in the mud is made by lateral movements of the foot, and the action of thousands of cilia creates a current which brings food-laden water into the mantle cavity. There is a unique exhalant siphon constituted by two overlapping folds, and through this are expelled water and fecal pellets without disturbing the surrounding mud.
The most extreme modifications in the entire molluscan phylum have occurred in connection with the feeding habits of certain parasitic snails. For years the Entoconcha snails found inside the Synapta sea cucumbers were thought to be some form of parasitic worm. The “head” of the mollusk is attached in leech-like fashion to a blood vessel of the host, and its worm-like body is embedded in the gonads of the sea cucumber. The adult parasite has no shell, sensory organs, nervous system or radula. It is little more than a tube adapted to absorbing the blood of the host and carrying on self-fertilization. Were it not for the tiny young found inside the adult with their small shell and operculum, it is doubtful if these creatures would ever have been thought to be mollusks.
The passage of food from the buccal cavity, through the esophagus to the stomach is facilitated by muscular contractions of the wall of the alimentary tract and by saliva produced by the two salivary glands. The hind end of the esophagus may be modified into a gizzard, and in many Bubble Shells, especially Scaphander, there are several large, cucumber-shaped plates armed with hard corrugations which grind the food into small particles. The stomach proper consists of a simple enlargement of the digestive canal. Its wall may be smooth, furrowed, or lined with spines. As in most bivalves, some snails possess a jelly-like crystalline style which projects into one corner of the stomach and dissolves off digestive enzymes. The so-called “liver” of the snail which forms most of the upper part of the soft, coiled viscera is actually a digestive gland where food material is broken down and absorbed into the blood stream.
