192 GENERAL ZOOLOGY 



edge, the thin part of the mantle folds continues to deposit 

 the nacreous layer, which is sometimes called mother-of- 

 pearl. 



The mantle folds inclose a mantle cavity. An opening at 

 the anterior end of the mantle cavity allows the foot (Fig. 

 101), the single locomotor organ, to be extended to the out- 

 side. At the posterior end of the mantle cavity is situated 

 the double- tubed siphon (Fig. 101). 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 speci- 

 mens of this clam the siphon is over twenty-five centimeters 

 (ten inches) long. 



The Digestive System. The ventral opening of the siphon 

 (Fig. 101) 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 for- 

 ward over the gills and along the mantle till they come 

 within range of the waving palps, or mouth appendages. 



The mouth opening 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 swal- 

 lowed passes through the short esophagus to the stomach. 

 Surrounding the stomach is the large, paired digestive gland, 

 which secretes the digestive 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. In Mya this structure is three 

 or four inches long. It is soft and clear, like thick, color- 

 less jelly, and lies in a long, thin-walled sac opening into 

 the stomach. The style produces substances of use in the 



