CHAPTER XI. 



SIPUNCULID^. 



Our knowledge of the development of the Sipunctdidx is still 

 very meagre. Concei'ning Siptmculus and Phascolosoma, 

 the embryology of which has been studied, we know that 

 they possess larvae which may be compared to the Trocho- 

 phore. The development of Sipuncidus, which has been 

 thoi'oughly dealt with by Hatschek, presents many peculiari- 

 ties, above all the formation of an embryonal membrane. 

 The Troc/iop/iore-like larva does not arise directly from the 

 embryo, but the latter is surrounded by a cellular mem- 

 brane, as if by an amnion. 



I.— The Development of Sipunculus. 



The fii'st stages in the development of Sipunculus are not 

 known. By pelagic fishing, Hatschek captured the embryos 

 in the blastula stage. In these embryos the fundaments of 

 the three gex'm-layers can already be recognized (Fig. 158 A). 

 The thickened part of the spherical blastula consists of tall 

 cells, the entoderm; there is prominent among these a 

 particularly large cell, which, in contrast to the other 

 (prismatic) cells, retains a more spherical shape. This is 

 the first mesoderm cell. It lies in the median plane between 

 the ectoderm and entoderm, and marks the posterior part of 

 the embryo (Fig. 158 A). The free space which existed 

 between the embryo and the egg-membrane — the latter being 

 traversed by radial pores — disappears during the blastula 

 stage, owing to the fact that the cells apply themselves to 

 the egg-membrane. They send out cilia through the pores 

 of the membrane, so that the embryo, together with the egg- 



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