444 STUDIES ON THE EMBRYOLOGY OF THE SIPUNCULID^. 



general the prototroch of Phascolosoma. The arrangement of the cells of these struc- 

 tures in two or at most three rows in an equatorial band, separated in front, dorsally, and 

 behind by tracts which correspond essentially in the two forms, as well as their large 

 size and uniformly ciliated condition in each form, make the general homology certain. 



Before describing the fate of the prototroch in Phascolosoma, it will be well to 

 point out the fact that in P. vulgare a zone of prominent cells bearing a postoral circlet 

 of long cilia is formed behind the prototroch and separated from it by a narrow interval 

 (Figs. 13, 14). This postoral circlet is quite independent of the prototroch proper, 

 and is retained long after the latter has ceased to exist. Thus it develops earlier than 

 the postoral circlet in Sipunculus, which appears in a similar position only after the 

 prototroch cells have slipped back over the somatic plate and formed the embryonal 

 envelope. In Sipunculus the postoral circlet is formed within the amniotic cavity, 

 and appears from Hatschek's observations to become functional as a locomotor organ 

 only after the casting off of the serosa; in Phascolosoma, on the other hand, the cilia, 

 like those of the aboral band covering the prototroch proper, penetrate the zona 

 radiata and serve even before the shedding of that membrane as the organs of loco- 

 motion for the trochophore. During the shedding of the zona radiata they either 

 slip through its pores like the flagella of the apical plate in both Sipunculus and 

 Phascolosoma, or the membrane itself splits open along the line of their connection with 

 the body. The postoral circlet in Phascolosoma gouldii is vestigial and in most indi- 

 viduals entirely . absent ; but a preoral circlet in front of the adoral band of the pro- 

 totroch (Fig. 15) serves in this species for a short time as the chief organ of locomotion. 



Our entire knowledge of the development of Phascolosoma, with the exception 

 of a few scattering notes, has been based upon a single brief paper by Selenka (75) 

 in which he describes in an excellent manner for that time, but in a primitive and 

 incomplete way, a few of the cleavage stages, the trochophore and two stages in the 

 development of the young larva. In this paper he asserts that the zona radiata 

 (Dotterhaut) is never shed, but becomes transformed gradually into the cuticula of 

 the larva. Similar statements that have been made in regard to various annelids 

 seem to me to be open to the suspicion that there has been a failure to observe that 

 critical stage in which the zona radiata and cuticula are both present. My experience 

 with Phascolosoma has shown how readily this stage may be overlooked, and I quite 

 agree with Eisig ('98, p. 98) that "sobald nur das Augenmerk speciell hierauf gerichtet 

 wird, auch noch weitere Falle von Hautungen des Embryos zur Beobachtung gelan- 

 gen und dementsprechend die Angaben iiber die Verwandlung der Eihaut in die 

 Cuticula der Larve oder des Wurmes allmahlich aus der Litteratur verschwinden 

 werden." 



