53° 
SPONGES. 
(inner layer), and the layer of cells on 
layer). The usually flat cells of the 
simply a superficial layer of mesoderm 
the outer surface, or ectoderm (outer 
ectoderm are now considered to he 
cells, and not to constitute a special 
carpenter’s glass-sponge, Pheronema (nat. size). 
separate layer distinct in itself. Whether this be the case or not, the terms 
mesoderm and ectoderm may be conveniently retained here. The ground- 
substance, or mesoderm, contains cells of various kinds, namely, irregularly-shaped 
