MOLLUSCA — PELECYPODS 217 



gastrula yto oi a millimeter in diameter. Up to this time it 

 has remained inclosed in the gelatinous case which covered the 

 egg, but in its gastrula form it is provided vv^ith hair-like ciHa 

 which start it rapidly revolving, and soon it breaks through the 

 transparent egg case and escapes as a trochosphere larva into 

 the water. This trochosphere stage is characterized by a top- 

 shaped body with cilia confined to the blunt anterior end, by a 

 primitive mouth and by the appearance of a shell gland opposite 

 the mouth. 



During the next twenty-four hours a tiny valve, secreted by 

 the shell gland, forms on each side of the animal and slowly 

 increases in size until it completely envelops the embryo. 

 The dorsal portion alone remains uncalcified and by additions 

 of conchiolin it develops into the C-spring ligament. Other 

 changes taking place along with the formation of this shell 

 are the development of a velum, — a pecuUar kind of extensible 

 ciliated swimming organ, the development of a foot and the 

 increasing complication of the digestive tract. This shelled 

 swimming form, succeeding the shell-less swimming trocho- 

 sphere, is called the veliger. All these changes, from gastrula 

 to veliger, are accompanied by but small increase in size ; but at 

 the end of the veliger period, which endures from six to twelve 

 days, according to the temperature of the water, important 

 changes are inaugurated which lead rapidly toward the attain- 

 ment of the adult form. The animal increases in size, and with 

 the complete disappearance of the velum and the loss of the 

 swimming function of the foot, it leaves the free swimming 

 life and sinks to the bottom. 



A gland in the foot secretes the byssus, fine, tough threads 

 by which the animal becomes attached to sand grains, shells, 

 eel-grass or other objects. It is, however, an active animal at 

 this stage and constantly travels from one place of attachment 

 to another, breaking its byssal threads and forming new ones. 

 The addition to the shell from this time on is coarser, whiter, 

 and characterized by well-marked concentric growth lines. A 



