368 WILSON. [Vou. IX. 
2. Comparison between ‘the Egg Larva and Gemmule Larva of 
Silicious Sponges. 
Resemblance of the two 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 1) 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 tht 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 1) 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 
