46 JOURNAL OF THE 



exterior (osculiim) was a later acquisition, the osculnm 

 being in all probability one of the small apertures (pores) 

 especially enlarged. Even after the formation of this 

 cavity the division of the parenchyma into entoderm and 

 mesoderm was not (and is not) in the sponges a rigid 

 division, the primitive power of digesting food intracel- 

 lularly having been retained by both layers. It was only 

 with the appearance of the higher animals that the separa- 

 tion of entoderm from mesoderm became a perfect one. 

 (Spongiologische Studien, p. 378). This solid ancestor of 

 the metazoa, Metschnikoff derives from colonial forms like 

 Protospongia. Barrois as early as 1876* stated his belief 

 that the ancestor of the sponges was a solid animal, com- 

 posed of two layers, the outer representing the ectoderm, 

 the inner mass representing a parenchyma from which 

 have developed the entoderm and mesoderm of higher 

 animals (p. 78). 



According to this view the early development of Plakina 

 (or Reniera, Chalinula, etc.) gives the first chapters in the 

 history of the group of sponges more faithfully than does 

 a form like Oscarella (or Sycandra). In the former sponges 

 it will be remembered there is a solid larva hollowed out 

 to form a three-layered sac, which then breaks open to the 

 exterior, forming the osculum. In the latter there is an 

 invaginate gastrula, which settles mouth downwards, the 

 gastrula mouth subsequently closing and the osculum ap- 

 pearing as a perforation at the upper end of the sac. In 

 these forms (Oscarella, Sycandra) we have to suppose that 

 the Parenchymella stage is skipped, the central cavity 

 (which properly belongs to the Olynthus stage) being pre- 

 cociously developed coincidently with the immigration of 

 the entoderm. The blastopore of the sponge gastrula, on 



*Memoire sur rembryologie de quelques Eponges de la Maiiche. Ann. Sci. Nat. T. 3, 

 VI Ser. 



