ELISHA MITCHELL SCIENTIFIC SOCIETY. 45 



There is another (hypothetical) way of explaining these 

 phenomena, which consists in supposing that ectoderm 

 cells of the larva migrate into the interior, and though in- 

 distinguishable from the surrounding mes-entoderm cells, 

 alone take part in forming the afferent canals. Similarly 

 we ma}' suppose that in the solid mass, which constitutes 

 the parenchyma of Hsperella, there are two radically dis- 

 tinct classes of cells, one of which is potentially gifted 

 with the power of forming efferent canals and flagellated 

 chambers, while the other has not the powder, and must 

 remain as amoeboid mesoderm. But this is pure hypothesis. 



The result of this critical examination seems to be that 

 the Olynthus must be regarded as the common ancestor of 

 sponges (Haeckel, Kalkspongien), and that the entoderm 

 and mesoderm are not sharply differentiated from one 

 another 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 Hydromedusse, coupled with the wide- 

 spread presence of intracellular digestion in the lowest 

 metazoa, led Metschnikoff years ago to the belief that the 

 solid larva represents the ancestral form of the metazoa, 

 while the gastrula is a coenogenetic modification. "^ To my 

 own mind all the facts that we know indicate that Metschni- 

 koff' s conclusion is well founded. This hypothetical an- 

 cestral form is known as the Parenchymella (Phogocytella). 

 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 intercellular openings (pores) scattered over the 

 surface. A central cavity having a special opening to the 



*MetschnikoflF. Spongiologische Studien. Zeit. f. Wiss. Zool. Bd. 32. Metschnikoff. 

 Embryologische Studien au Medusen. Wien., 1886. 



