CHAPTER XI. 



SIPDNCULID^. 



• UR knowledge of the development of the SiptmcuUdse is still 

 |very meagre. Concerning Sipunculus and Phascolosoma, 

 Ihe embryology of which has been studied, w^e know that 

 bhey possess larvae which may be compared to the Trocho- 

 thore. The development of Sipunculus^ which has been 

 thoroughly dealt with by Hatschek, presents many peculiari- 

 ties, above all the formation of an embryonal membrane, 

 'he Trochop}iore-Vik.e larva does not arise directly from the 

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

 ►rane, as if by an amnion. 



I.— The Development of Sipunculus. 



The first stages in the development of Sipunculus are not 



[known. By pelagic fishing, Hatschek captured the embryos 



an the blastula stage. In these embryos the fundaments of 



khe three germ-layers can already be recognized (Fig. 158 A). 



'he thickened part of the spherical blastula consists of tall 



jells, the entoderm; there is prominent among these a 



)articnlarly large cell, which, in contrast to the other 



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



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



bhe ectoderm and entoderm, and marks the posterior part of 



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



)etween the embryo and the egg-membrane — the latter being 



traversed by radial pores — disappears during the blastula 



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



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



)f the membrane, so that the embryo, together with the egg- 



357 



