362 WILSON. [Von. IX. 
sharply differentiated from each other as they are in the higher 
animals (Metschnikoff, Spong. Studien, p. 378). 
Origin of the Olynthus. — 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 (1), 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 
