No. 3-] DEVELOPME^'T OF MARINE SPONGES. 369 



accumulates at its posterior pole, and the cilia and " son 

 revet ement 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 



