515 



The embryos of N. proxima cast their tests as quickly as do the 

 embryos of Y. limatula. The embryos of N. delphinodonta take several 

 hours for the process. The embryos of both species of Nucula cast 

 the test, the stoinodieum, and apparently part of the cells of the apical 

 plate. 



In all of the species, the shell-valves remain gaping ventrally until 

 about the time the tests are cast, and the tissues of their bodies extend 

 completely across between the valves. When the shells are closed, 

 there is no longer sufficient space for the organs in their original 

 positions. 



The principle changes of importance, due to this crowding, takes 

 place in the pouches of the digestive glands. These are forced into 

 the space dorsal to the intestine, and the cells of their walls become 

 scattered, Fig. 20 z. Reformation of the digestive glands is accom- 

 panied by the disappearance of these cells. When the digestive glands 

 appear again, the left lobe, Fig. 21 M, is much larger than the right 

 lobe, rl. 



When the test is cast, the foot of Yoldia is capable of feeble 

 movements, and, within a few hours, it becomes quite active. Loco- 

 motion is chiefly effected by long and powerful cilia, which are 

 arranged along the sides of the foot, and are very active whenever 

 the foot is thrust out of the shell. By means of these, an animal is 

 enabled to glide over the surface on which it lies, but it Cannot rise 

 in the water. 



The test in each species of Nucula is cast while the foot is very 

 immature, and for two or three days afterward the animals are quite 

 motionless. The foot then begins its activities, and grows quite ra- 

 pidly. The foot of each species is supplied with cilia, but the cilia 

 are too small to be of much service in locomotion. It is of interest 

 to note that the young of N. proxima must lie on the soft muddy bottom 

 for some days in quite a helpless condition. 



The foot at first shows no indication of the modification later 

 known as a sole. This is formed by ventral growths of the sides of 

 the foot. 



At the same time the postero-ventral portion of the foot becomes 

 more and more prominent, Fig. 21, and resembles a heel. The use 

 of this outgrowth is not known. It remains in the adult animal as 

 a small projection just behind the opening of the byssal gland. The 

 byssal gland makes its appearance rather early, but does not seem to 

 be functional in any of the forms. 



The gills begin as thickenings of the posterior portions of the 



