No. 3-] DEVELOPMENT OF MARINE SPONGES. 36 1 



diverticula, may be formed from groups of mesoderm cells 

 (Esperella, Tedania, Chalinula fertilis, Myxilla). 



3. The afferent canals, including the subdermal cavities, 

 instead of being formed as invaginations from the ectoderm, 

 arise as lacunae in the mes-entoderm (Esperella, Tedania, 

 Esperia, Stelletta, Myxilla). In Reniera filigrana (Marshall) 

 they are formed as entodermic diverticula. 



The coenogenetic development of the flagellated chambers 

 and efferent canals suggests, as I have said, an essential 

 similarity of nature in the so-called entoderm and mesoderm of 

 sponges. This belief, so long upheld by Metschnikoff, derives 

 some of its strongest support from this author's physiological 

 investigations (see ante, p. 359), as well as from the fact, first 

 emphasized by Metschnikoff and Barrois, that in the most 

 common sponge larva (the solid larva) mesoderm and entoderm 

 form a single indivisible layer. 



And likewise the development of the afferent system of 

 canals, in some sponges from the ectoderm, in others from the 

 mes-entoderm, may possibly be taken as meaning that even 

 these two primary layers (the outer and the inner) are not 

 distinctly differentiated from each other in the sponges ; or, in 

 other words, that the mes-entoderm is still enough like the 

 ectoderm to form organs ordinarily produced by the latter 

 layer. 



There is another (hypothetical) way of explaining these 

 phenomena, which consists in supposing that ectoderm cells of 

 the larva migrate into the interior, and, although indistinguish- 

 able from the surrounding mes-entoderm cells, alone take part 

 in forming the afferent canals. Similarly we may 'suppose that 

 in the solid mass which constitutes the parenchyma of Esperella 

 there are two radically distinct classes of cells, one of which is 

 potentially gifted with the power of forming efferent canals 

 and flagellated chambers, while the other has not this power 

 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 ancestor of sponges (Haeckel, 

 Kalk-spongien), and that the entoderm and mesoderm are not 



