116 John H. Gerould, 



the dorsal cord in the older trochophore widens into a broad band. 

 It is this region, especially, which spreads forward on each side 

 beneath the edges of the prototroch. Dorso-lateral proliferations of 

 the apical plate also extend backward beneath the prototroch, and 

 take part in this closnre. 



Thus the whole mass of the prototroch, which has been graduallj^ 

 converted into yolk or similar metaplasmic matter, is passed into 

 the coelom, and the sides of the introvert, which it formerly occupied^ 

 are now covered by small flat cells. The shape of these cells bears 

 evidence that the process of concrescence is aided by the flattening 

 of the neighboring cells, accompanied by cell division. A similar 

 process of flattening and spreading superficially was observed in the 

 primary prototroch cells, immediately after the cell division which 

 gave rise to them. The cuticula, which covers the spaces under 

 which the degenerating prototroch cells lie, is slightly thickened and 

 folded after the remnants of the cells have been swept ofl" into the 

 coelom, but the process of breaking away of the cells is so gradual 

 that rarely, if ever, are vacant spaces left beneath it. It adapts 

 itself to the definitive ectoderm, as fast as the latter spreads out 

 beneath it. 



The metamorphic process and particularly the dissolution of the 

 prototroch are attended with considerable danger to the young larva. 

 Individuals in which rupture of the head region has taken place 

 are found not infrequently. 



9. Development of the Larva. 



Third and Fourth Days. 



The metamorphosis of the trochophore into a cjiindrical worm^ 

 in which introversion of the anterior part of the body constantly 

 occurs, is completed before the middle of the third day, i. e. less 

 than sixty hours after fertilization of the egg. 



The young larva of Ph. gouldii (Fig. 65) at the sixtieth hour 

 has already acquired worm-like habits, and moves along the bottom 

 somewhat like the caterpillar of a geometrid moth, alternately 

 attaching the ventral side of the head and a small ventral area 

 near the posterior extremity of the body; and it sometimes creeps 

 along by the action of the preoral circlet and of ventral cilia of 

 the prostomium. The larva of Flu vulgare, however, (Fig. 53 — 55), 



