236 IVILSOA'. [Vol. IX. 



immediate neighborhood of the mesoderm, as is shown in the sec- 

 tion, PL XXII, Fig. 94, the membrane, ec. mem., consists of two 

 layers, the upper and lower ectoderm respectively, but farther 

 out it is one-layered. The ectodermal membrane extends for 

 some distance beyond the body of the sponge, and is more 

 or less covered with debris. It is essentially like the corre- 

 sponding structure in Esperella. The membrane is shown in 

 sections in Fig. 93, and in the surface view. Fig. 90, its 

 outline, cc. man., is partly indicated. The sponge shown in 

 Fig. 90 is only partially surrounded by the ectodermal mem- 

 brane, retaining its earlier character in the region a, where 

 the ectoderm has as yet taken no step towards forming a 

 membrane. Nuclei could be made out here and there in the 

 membrane and in the ectoderm proper, but the cell outlines 

 I could not distinguish. The same deeply staining thickenings 

 which were found in Esperella, are again found in the basal 

 ectoderm and membrane of this sponge, fr. th., PI. XXII, Figs. 

 93 and 94. The only construction to be put upon them seems 

 to be that they are nuclei surrounded by protoplasm. 



As the ectoderm grows out to form the membrane, the 

 peripheral mesoderm throws out lobes and processes, its out- 

 line becoming jagged and irregular, as in Fig. 88, vies. The 

 cells of this part of the mesoderm gradually form a peripheral 

 zone, distinguishable from the rest of the body by the fact 

 that they are much less closely packed than the cells else- 

 where (/>. z. in PI. XXII, Figs. 90, 91, 93). The cells of this 

 peripheral mesodermic zone develop slender processes, and 

 form a net -work (sections. Figs. 93 and 94, /. z.), which, how- 

 ever, is not nearly so open and exquisite as in Esperella. 



During the formation of the ectodermal membrane, and 

 afterwards during all the time I kept the young sponges, they 

 underwent an incessant change of shape, which was more 

 conspicuous during the first three or four days than it was 

 later. This change of shape, though gradual, was greater and 

 more rapid than in Esperella, and the little sponges were much 

 disposed to assume peculiarly irregular shapes, such as that of 

 the sponge shown in Fig. 91 {mcs. indicates outline of the 

 parenchyma — the whole sponge is supposed to be surrounded 



