Life of the Snails 23 



from which they obtain small algal cells and diatoms. The common Atlantic 

 Shpper 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 com- 

 rminis 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-fertili- 

 zation. 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 alimen- 

 tary 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 stom- 

 ach 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. 



