THE EMBRYOLOGY OF THE PORIFERA. 215 



microscleres, such as Reniera, than to the genera possess- 

 ing chelae {Desmacidonidce). 



From larva of the second type, with a ring of larger 

 flagellated cells at the hinder end, there is an easy transi- 

 tion to the larvae of horny sponges with their uniform 

 covering of cilia. In the latter the ring of large cells has 

 grown inwards to form a circular area, completely cover- 

 ing the hinder end. The larvae of horny sponges, such as 

 Hircinia and Euspongia, are thus more nearly allied to 

 the larvae of Reniera and Chalinula than to those of other 

 Monaxonida, a relationship parallel to that shown by the 

 structural characters of the adult sponges. In Aplysilla, 

 however, the larva, as described by Delage, is without the 

 pigment ring, and approaches more in other peculiarities 

 also to the first type of larva ; a fact which would point 

 to a polyphyletic origin of horny sponges from different 

 families of Monaxonida. In Spongilla also the uniform 

 covering of cilia is to be explained by an overgrowth of the 

 cells at the posterior pole, but its cells are different from 

 those of horny sponges. The larva of Aplysilla differs, 

 according to Yves Delage, from the other larvae described 

 by himself and by Maas, in that the inner mass is uncovered 

 at the anterior pole. 



The structure of the larvae of siliceous sponges can only 

 be determined by sections, on account of their opacity, and 

 the statements of Maas and Delage differ somewhat with 

 regard to this point. They are agreed upon the funda- 

 mental point that the larva is composed of two layers of 

 cells — (1) an external layer of flagellated cells ; (2) an inner 

 mass containing spicules and several kinds of cells imbed- 

 ded in a jelly-like substance. A cavity may persist at the 

 anterior pole, probably a remnant of the segmentation 

 cavity, or the larva may be compact. The two authors 

 differ considerably, however, as to the exact composition 

 of this inner mass and its relations to the outer layer. 

 According to Delage the inner mass consists of a layer 

 of epidermic cells placed immediately beneath the ciliated 

 cells, and a central nucleus composed of amoeboid cells 

 and "cellules intermediates ". According to Maas the 



