320 



OILMAN A. DREW. 



nected with a space that is formed between the gut and the 

 shell-gland. 



At no stage in its development is the shell- gland invagi- 

 nated. From the time of its formation it arches dorsally to 

 some extent (figs. 7 and 9, sg.). Just before it becomes 

 covered by the test it flattens somewhat (figs. 11 and 12), but 

 it soon arches dorsally again and becomes quite convex (fig. 

 17,.^.). 



The cells that give rise to the cerebral ganglia are few in 

 number (fig. 15, eg.), and lie ventral to the anterior end of 

 the stomodaeum. They frequently come to the surface, but 



Text-fig. E. — Surface view of a young embryo of 

 Nucula delphinodonta. 



they may be entirely covered by test cells. A more or less 

 distinctly recognisable test cell lies between the cerebral 

 ganglia and the apical plate, but beneath this test cell the 

 cerebral ganglia and the apical plate are in contact. The 

 two cerebral ganglia seem to originate from a single mass of 

 cells. There is no indication of the formation of cerebral 

 pouches, as in Yoldia (Text-fig, V). The position occupied by 

 the developing body of Nucula does not make it necessary 

 for the cerebral ganglia to shift their position from the point 

 of their formation until the test is shed. 



The apical plate is composed of a number of cells, the walls 



