THfcJ LIFE-HISTORY OF NUCULA DELPHINODONTA. 321 



of which are rather indistinct (fig. 15, a^.). They bear cilia 

 that in size and distribution resemble those that cover the 

 test cells. 



Under favourable conditions the test cells can be seen to 

 be arranged in five rows ; occasionally part of a sixth row is 

 present. As in other stages, the boundaries between the 

 test cells are poorly marked, and it is quite impossible to 

 sketch them accurately. Text-fig. E shows their general 

 arrangement, but it must be understood that this is quite 

 diagrammatic. The cilia on the test cells of this species are 

 not collected into bands as they are in Yoldia (Text-fig. F), 



Text-fig. F. — 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. 



but are evenly scattered over their surfaces. The embryo 

 becomes free from the egg membrane about the time that 

 the shell-gland becomes covered by the test, but the cilia are 

 barely powerful enough to slowly move the embryo on the 

 bottom of a dish. The absence of the bands of cilia, and of 

 the long tuft of apical cilia, is probably due to the protected 

 life of the embryo. Nucula proxima lays its eggs free in 

 the water, where they are fertilised and develop. These 

 embryos have to shift for themselves, and are very active. 

 Here, as in Yoldia, the cilia on each of the three intermediate 

 rows of test cells are long and collected into a band (Text* 



