362 WILSON. [Vol. IX. 



sharply differentiated from each other as they are in the higher 

 animals (Metschnikoff, Spong. Studicn, p. 378). 



Origin of the Oljntlms. — The prevalence of the solid larva 

 in sponges and hydromedusae, coupled with the widespread 

 presence of intracellular digestion in the lowest metazoa, led 

 Metschnikoff years ago to the belief that the solid larva repre- 

 sents the ancestral form of the metazoa, while the gastrula is a 

 coenogenetic modification (12, 13). To my own mind, all the 

 facts that we know indicate this conclusion to be well founded. 

 This hypothetical ancestral form was named Parenchymella 

 (changed later to Phagocytella). I may be permitted to recall 

 its leading features as deduced by Metschnikoff. The animal 

 consisted of an outer layer of flagellated cells and an inner 

 mass of amoeboid cells. The digestion was intracellular, the 

 food being taken in through openings scattered over the 

 surface. A central cavity having a special opening to the 

 exterior was a later acquisition, the opening being in all prob- 

 ability one of the small apertures especially enlarged. This 

 solid ancestor of the metazoa, Metschnikoff derives from 

 colonial forms like Protospongia. Barrois (i), as early as 1876, 

 stated his belief that the ancestor of sponges was a solid 

 animal composed of two layers, the outer representing the 

 ectoderm, the inner mass representing a parenchyma, from 

 which have developed the ectoderm 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 appearing as a perforation 

 at the upper end of the sac. In these forms, Oscarella and 

 Sycandra, we have to suppose that the Parenchymella stage is 

 skipped, the central cavity (which properly belongs to the 

 Olynthus stage) being precociously developed coincidently with 



