THE LIFE-HISTORY OF NUCULA DELPHINODONTA. 371 



be distinguished as a group soon after the surface cells that 

 form the test become ciliated (fig. 15, eg.). They frequently 

 remain as surface cells for some time, and they doubtless 

 originate as surface cells in all cases. The group of cells is 

 not distinctly paired, and does not invaginate as it does in 

 Yoldia. Each cell becomes much larger at the inner end than 

 at the end that comes to the surface. Although a test cell lies 

 between the cerebral ganglia and the apical plate they still 



Text-fig. V. — Transverse section of a Ibrfy-five hour embryo of Yoldia 

 liniatula, tukcu througli the cerebral pouches, ry. Cerebral pouches. 

 ?iiff. Anterior wall of the mid-gut. L Test. 



remain in contact beneath this cell. Little change occurs in 

 the appearance, size, or position of the cerebral ganglia until 

 the test is cast away, and until then no other part of the 

 nervous system can be distinguished. 



When the test cells break apart and accumulate near the 

 anterior end of the embryo (fig. 34) a portion of the body of 

 the embryo is carried dorsally at the expense of the large 



