53° 



SPONGES. 



(inner layer), and the layer of cells on the outer surface, or ectoderm (outer 

 layer). The usually flat cells of the ectoderm are now considered to be 

 simply a superficial layer of mesoderm 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 



