THE SPONOEa 
15 
young. Some Infusorians also undergo a process common 
in low plants, called conjugation,"* 
d m 
Fro. 12.— A, Epistylis flavicans Ehr., a single, many-forked colony of bell ant 
malcules, slightly magnified. B, one of the animalcules magnified 250 diam- 
eters, p, the stem ; the flat spiral of vibrating cilia at the edge of the disk ; 
ms, the muscle; m to s, the depth of the digestive cavity; m, the mouth; g, flri, 
the throat; cv, the contractile vesicle; 7i, the reproductive organ. 
CHAPTER II. 
Brakch II. — PoRiFERA {Spouges)* 
General Characters of Sponges.— Sponges are now 
known to be composed of numerous cells, arranged in three 
layers, the embryo arising from an egg, and passing through 
a blastula and a gastrula stage, as in all the higher animals. 
A sponge, then, is a cellular sac (Fig. 13) with digestive 
chambers or minute rude stomachs lined with ciliated cells, 
the whole sponge-mass being propped up by an irregular 
basket-work of needle-like bodies called spicules. Up9p 
cutting a dry sponge in half there are to be seen large canals 
which have large openings called oscula ; these are really 
* g^e Kent's Manual of the Infusoria, London, 1880-82. 
