368 WILSON. [Vol. IX. 



2. Comparison between the Egg Larva and Gemmide Larva of 

 SilicioHS Sponges. 



Rescfnblancc of the tzvo kinds of larvae. — A comparison of 

 the gemmule larvae I have described, with the egg larvae of 

 silicious sponges, reveals the fact that the two are similar in 

 essential respects. These essential points are the presence 

 and character of the germ layers, and the peculiar differentia- 

 tion of one pole. The similarity between the two sorts of 

 larvae will be seen after a brief survey of what is known 

 concerning the egg larvae of silicious sponges. 



The larva of Isodyctia (Barrois i) is a solid oval larva (paren- 

 chymella). Except at the posterior pole it is everywhere 

 covered by a layer of columnar ciliated cells, the ectoderm. 

 At the posterior pole, according to Barrois, the ectoderm is 

 absent and the inner mass (mes-entoderm) is laid bare to the 

 exterior. In this larva as in others in which it has been 

 claimed that the entoderm is exposed to the exterior through 

 a break in the ectoderm, recent investigations (especially 

 Maas's study of the flattening of the columnar epithelium in 

 Spongilla, and the facts recorded in the descriptive part of the 

 present paper) make it probable that the mes-entoderm is 

 really not laid bare but is covered by a layer of flat ectoderm 

 cells. The unciliated posterior pole of the Isodyctia larva is 

 made further noticeable by a deposition of red pigment in its 

 cells, and the cilia immediately surrounding it are unusually 

 long, forming a conspicuous circle. In this larva the posterior 

 pole ("calotte") is at no time ciliated nor, as I understand the 

 author, is it ever covered (not even before birth) with columnar 

 ectoderm. The resemblance between this larva and the gem- 

 mule larva of Esperella is, to say the least, striking. 



The larva of Desmacidon (Barrois i) is very like that of 

 Isodyctia. As in the latter sponge, the columnar ciliated 

 ectoderm is lacking at the posterior pole which is further 

 distinguished by its pigment and by a circle of long cilia 

 immediately surrounding it. Unlike Isodyctia, the larva of 

 Desmacidon when set free is ciliated all over. But pigment 



