336 WILSON. Vor. EX. 
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, ec. mem., 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, pm. ¢h., Pl. 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, mes. 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 (7. 2. ini Pl. 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, f. 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 (mes. indicates outline of the 
parenchyma — the whole sponge is supposed to be surrounded 
