The Development of Phascolosoma. 127 



Selenka's observation that the cuticula becomes the egg mem- 

 brane, which I have shown to be untrue of at least three species of 

 Phascolosoma, was supposed by Hatschek to be corroborated by the 

 fact that the cilia of the postoral circlet in Phascolosoma penetrate 

 the zona radiata, project beyond the surface of it, and serve as the 

 organ of locomotion, whereas in Sipunculus this circlet, covered by 

 the serosa, is found within the amniotic cavity. In reality the 

 position of the postoral circlet in S. niidus and Ph. vulgare is iden- 

 tical. It surrounds the trunk in both forms immediately behind the 

 stomodaeum. It the less highly-modified form, Phascolosoma, it lies 

 behind the prototroch, separated from it by a narrow interval. In 

 Sipunculus the prototroch cells, much before this stage, have slipped 

 past the edge of the somatic plate (vide Hatschek's figg. 19, 23, etc.), 

 which is destined to give rise to the cells of the postoral circlet, 

 and have become the serosa. 



My study of the trochophore of Phascolosoma and conclusions in 

 regard to the serosa indicate clearly that Sipunculus is the more 

 highly modified form and the more divergent from the Chaetopod 

 type. Thus the extraordinary modification of the prototroch in Sipun- 

 ■culus, its early separation from the definitive ectoderm, and the 

 sinking of the latter beneath the yolk membrane could have arisen, 

 it seems to me, only from forms like Phascolosoma and the Chaetopods, 

 in which the prototroch cells are unmodified in the trochophore, and 

 the definitive ectoderm does not sink beneath the yolk membrane. 

 I have elsewhere suggested (1903, p. 449) that these conditions in 

 Sipunculus might have been acquired through the gradual loss of 

 yolk in adaptation to a more active pelagic life. It is entirely con- 

 ceivable that the yolk-laden trochophore of Phascolosoma might thus 

 be transformed into an embryo covered with an amnion, like that 

 of Sipunculus. Moreover the occurrence of two different organs 

 associated with the stomodaeum in Sipunculus, which makes their 

 appearance in the unhatched embryo, and persist in the larva, but 

 which are not found in Phascolosoma, are indications of further 

 differentiation in the embryonic Sipunculus. 



From the point of view to which these studies have led, it is 

 clearly seen that to compare the newly hatched larva of Sipunculus 

 that has lost its prototroch to a trochophore, as Koeschelt & Heider 

 (1890) have done, is to compare stages of unequal advancement, and, 

 as would be expected, these authors note in the larva of Sipunculm 

 a great reduction in the prostomial region and a higher grade in 



