STUDIES ON THE EMBRYOLOGY OF THE SIPUNCULID^. 445 



I have conclusive evidence that both the trochophore of P. gouldii and that of 

 P. vulgare at the time of their transformations into the young larvse (forty-eight to 

 fifty-eight hours approximately) shed the zona radiata, for not only have I watched 

 the whole process and made preparations which show clearly the ruptured membrane 

 still clinging to the head, but sections of trochophores from forty to forty-five hours 

 old uniformly show beneath the old yolk-membrane, which still retains its characteristic 

 pore-canals, a well-marked cuticula (Fig. 15). The latter is not as highly refractive 

 as the zona radiata, and at its first appearance is slightly granular. During the pro- 

 cess of shedding the zona radiata, the strong cilia of the postoral band in P. vulgare 

 and the similar cilia of the preoral band in P. gouldii slip through the pores of the 

 membrane. Some of the flagella of the apical plate do likewise, but only remnants 

 of the prominent apical cilia of the trochophore remain upon the larva. 



IV. DISSOLUTION OF THE PROTOTROCH IN PHASCOLOSOMA AND COM- 

 PLETION OF THE DEFINITIVE BODY-WALL OF THE LARVA. 



At the age of about forty-four to fifty hours the trochophore is still enclosed 

 within the thick zona radiata and covered by a thin cuticula. The thickness of this 

 cuticula is greatest at the posterior end of the body, where it is about half that of 

 the zona radiata. It entirely covers the anterior end of the body, including the pro- 

 totroch cells, which are soon to disappear. 



The retractor muscles have already made their appearance at this time, with 

 their origin in the ectoderm of each side of the posterior end of the trochophore and 

 their insertion in each side of the apical plate (Figs. 15, 16). These muscles soon 

 begin to operate, repeatedly drawing the head backward against the endoderm of 

 the newly formed archenteron. This process results in a pressure upon the surround- 

 ing prototroch cells, which are of extraordinary size and completely enclosed by the 

 thin cuticula (Fig. 15). 



The somatic plate of ectoderm at this time forms a continuous layer over the 

 subumbrellar region of the trochophore, which we may henceforth call the trunk. It 

 is extended forward on the dorsal side and is united in P. vulgare to the apical plate 

 by the dorsal cord of ectoderm, which has been already described as composed at its 

 narrowest part at first of only two rows of cells. On the ventral side it consists at 

 this time of a band of cells which extends forward on each side of the stomodseum. 

 Thus the prototroch cells occupy two large areas, one on each side of the anterior 

 part of the body, and these are connected ventrally in front of the stomodseum 

 (Figs. 12, 14). 



