164 



GENERAL ZOOLOGY 



3--- 



or the eggs, lies in the midst of the coils of the intestine. 

 Each gland has a short, slender tube, with an external opening 

 (not shown) near the opening of the nephridium, just below 

 the attachment of the gills. 



Development. The very early history of the } r oung Mya 

 arenaria has not been studied. We know, however, that 



after a short period of 

 development and growth 

 young long-neck clams 

 I swim about on the sur- 



face of the water. Soon 

 after the appearance of their shell 

 they sink to the bottom. If they are 

 fortunate enough to fall near the 

 shore-line, they anchor themselves 

 to a seaweed, or to a pebble, by a 

 tough, gelatinous thread (Fig. 84, 3) 

 which is secreted by a gland at the 

 5ck base of the foot (Fig. 84, l). This 

 thread is called the byssus. In cer- 

 tain mussels found in the sea the 

 byssus is a permanent and very com- 

 plicated organ of the adult, but in Mya arenaria it disappears 

 when the clam is about 5 millimeters (l- inch) long. At that 

 time the animal burrows into the mud and sand, where it 

 usually remains permanently. 



Relation to Environment. The adult Mya arenaria lives 

 in soft mud and sand between high-tide line and a few feet 

 beyond low-tide line. The reason the -clam lives in that 

 situation is because food is most abundant there. Lying 

 almost helpless in its mold of mud, the long-neck clam is 

 rendered in a measure independent of conditions outside, as 

 long as the currents of water carry the bountiful supplies of 

 food over its burrow. 



Clam. (After J. L. Kellogg) 



1, foot; 2, siphon; 3, byssus- 

 thread; 4, pebbles 



