112 John H. Gerould, 



of well-preserved individuals, fixed immediately after the yolk 

 membrane was moulted, I have found even these cilia still present, 

 althoug-h the deeper portions of the prototroch cells to which they 

 were connected were in an advanced stage of degeneration. The 

 preoral cilia of Ph. gouldii are retained (Fig. 65); they appear to 

 slip easily through the attenuated zona radiata. The ruptured yolk 

 membrane usually clings longer to the prototrochal and apical regions 

 than to the trunk, though the reverse is sometimes true. Meanwhile 

 the retractor muscles are beginning to act, and they gradually bring 

 about an oft-repeated introversion of the apical plate, or as we may 

 now call it, the head region. These movements aid materially in 

 sloughing off the membrane from that part of the body. I have 

 often seen a helmit-like remnant of the membrane clinging to the 

 head (Fig. 51 and 68) which reminds one of the conditions shown in 

 Hatschek's fig. 49 and 50 of the shedding of the ^^g membrane 

 in Sipunculus, except that, unlike Sqnmculus, none of the prototroch 

 cells in Fhascolosoma are cast oft' with it. The flagella of the apical 

 plate are often partially broken oft" during the process of hatching, 

 but in sections of Ph. vulgare., fixed soon after the loss of the yolk 

 membrane, I have found them beautifully preserved (Fig. 95. See 

 also Fig. 52, 65). The rapidly repeated movements of introversion 

 of the head soon remove the last tattered shreds of yolk membrane, 

 which may cling to it. 



I have already called attention in the description of the trocho- 

 phore (pag. 105), to the finely granular cuticula, which was formed 

 underneath the yolk membrane prior to its rupture. This can be 

 seen both in the living animal (Fig. 59, 60, 62, 64) and in sections 

 (Fig. 67, 86 — 88). The e^^ membrane, therefore, does not become 

 the cuticula of the larva. Selenka was doubtless in error in stating 

 that in Ph. elongcdiim the q^^ membrane is not shed, but persists as 

 the cuticula. I was for a time inclined to accept Selenka's view, 

 even after considerable study of the living animals, and until, by a 

 close examination of the growing trochophores of each species, I 

 observed in each the whole process of casting the e,gg membrane, 

 and saw the pore canals in the curled-up shreds, that were clinging 

 to the body and lying in the surrounding water. The persistance 

 of the e^g membrane as the larval cuticula in the Chaetopods has 

 been noted by several excellent observers, but my experience with 

 Phascolosoma leads me to be skeptical in regard to all such obser- 

 vations (see also Eisig, 1898, p. 95 — 98). 



