506 



portion of the shell, and, before hardening, adheres to the fine particles 

 with which it comes in contact. Into this case the eggs are laid, and 

 in it the developing embryos are carried for many days. The eggs 



of this species are few, 

 20—70, large, 0,21 mm 

 in diameter, and develop- 

 ment proceeds slowly. 



Both Y. limatula 

 and N. proxima throw 

 the sexual products free 

 into the water. The 

 eggs are very numerous, 

 smaller than those of 

 N. delphinodonta (those 



Fig. 8. Adult specimen of Nucula delphinodonta with egg case attached, 

 egg case is torn open to show the eggs inside. 



The 



of Y. limatula are 0,15 mm in diameter and those of N. proxima are 

 0,09 mm in diameter) and development is quite rapid. In all three 

 forms, the cleavages are typical. Gastrulation of Yoldia, and probably 

 of the other forms, is formed by epibole. At a rather early period 

 the surface cells, or at least part of them, acquire cilia, and the em- 

 bryos begin to roll around. In the case of N. delphinodonta, where 

 the embryos are carried in a case, the embryos remain evenly ciliated, 

 and are never able to swim freely. 



The embryos of both of the other species finally rise to the sur- 

 face of the water and become quite active. 



Fig. 9. Surface view of a forty-five hour embryo of Yoldia limatula. ac apical 

 cilia, bl blastopore, x depression where the cells that form the cerebral ganglia come 

 to the surface. 



