No.3-] DEVELOPMENT OF MARINE SPONGES. 369 
accumulates at its posterior pole, and the cilia and “son 
revétement cellulaire’ disappears (this, as I have said, is prob- 
ably to be interpreted as meaning that the columnar ciliated 
ectoderm of this pole becomes transformed into flat unciliated 
cells). In this last detail the Desmacidon larva is more like 
Tedania than Esperella. It will be remembered that in the 
Tedania embryo all the ectoderm cells become columnar, those 
at the posterior pole subsequently flattening out. In Des- 
macidon and Isodyctia both, the ectoderm is subsequently 
“broken through” at the anterior pole also. But this, it 
would seem, is only the first step in the general flattening of 
the ectoderm. 
The larva of Reniera filigrana (Marshall 18) is a solid oval 
larva with a pigmented pole, and covered at first uniformly 
with columnar ciliated ectoderm. Unlike the two preceding 
sponges, the pigmented pole is the anterior. The ectoderm 
“‘bursts”’ at the pigmented pole, and the mes-entoderm is laid 
bare. Subsequently the ectoderm “bursts”? at the opposite 
pole, and at about the time of fixation the whole ectoderm 
flattens. 
In Chalinula fertilis (Keller 10) there is a solid larva essen- 
tially like the gemmule larva of Esperella and Tedania, in that 
the columnar ciliated ectoderm is absent at the posterior pole. 
According to Keller, this larva is derived from an epibolic 
gastrula, and the cells occupying the posterior pole are a part 
of the mes-entoderm, which is here from the start exposed to 
the exterior. My observations on the way in which this pole 
is formed in Esperella and Tedania, make it probable, I think, 
that the surface cells in this region of the Chalinula larva are 
ectodermic. Indeed, for a short time after birth the posterior 
pole is ciliated, but the cilia are, however, soon lost. Believing, 
as I have said, that the inner mass of cells is from the begin- 
ning exposed to the exterior at the posterior pole of the embryo, 
Keller naturally regards this region as a blastopore. 
Vosmaer (34) describes the development of a larva which 
probably belongs to the genus Myxilla. The larva is solid and 
is covered with a cylindrical epithelium. A portion of the 
surface loses its cilia, the cells becoming cubical. Attachment 
