160 GENERAL ZOOLOGY 



the single locomotor organ, to be extended to the outside. 

 At the posterior end of the mantle-cavity is situated the 

 double-tubed siphon (Fig. 82, 6). Only the lower tube of 

 the siphon is connected directly with the mantle-cavity. 

 This siphon enables Mya to lie buried, anterior end down, 

 in the mud and sand, still maintaining communication with 

 the food-laden and air-laden water above. In large specimens 

 of this clam the siphon is over 25 cm. (10 in.) long. 



The Digestive System. The ventral opening of the siphon 

 (Fig. 82, 7) is surrounded by many short tentacles which 

 guard the passage. An ingoing current is created in the 

 water by cilia on the gills. Ordinarily only microscopic food 

 passes through this incurrent opening of the siphon. In the 

 mantle-cavity the particles of food are carried forward over 

 the gills and along the mantle till they come within range 

 of the waving palps, or mouth-appendages (Fig. 82, 9). 



The mouth-opening (Fig. 82, 10) is situated between the 

 four palps, and is very small. It has no organs of any kind 

 for seizing or chewing food ; none are needed. The food 

 once swallowed passes through the short oesophagus to the 

 stomach (Fig. 82, 11). Surrounding the stomach is the large, 

 paired digestive gland (Fig. 82, 12). which secretes the diges- 

 tive fluid. Situated in the end of the stomach, and in the 

 anterior end of the intestine, we find, in Mya arenaria, and 

 in many species related to it, an organ called the crystalline 

 style (Fig. 82, 14). In Mya this structure is three or four 

 inches long. It is soft and clear like thick, colorless jelly, 

 and lies in a long, thin-walled sac opening into the stomach. 



Many theories have been advanced to account for the exist- 

 ence of the crystalline style. Recently (1901) the nature of 

 the structure was investigated by Professor Mitra, a physio- 

 logical chemist living in Calcutta, India. He has published 

 an account based on experimental evidence, which appears to 

 be satisfactory. His conclusion is that the crystalline style 



