TEE SPONGES 



15 



yomig. Some Infusorians also undergo a process common 

 in low plants, called "conjugation."* 



FtG. \2.—A, Epistijlis flavir.ans Ehr., a single, many-forked colony of bell an> 

 malcules, slightly magnified. B, one of the animalcules magnified 250 diam 

 eters, p, the stem; d, the flat spiral of vibrating cilia at the "edge of the disk: 

 WIS, the muscle; ni tp s, the depth of the digestive cavity: m, the mouth; y, y', 

 the throat; cv, the contractile vesicle; n, the reproductive organ. 



CHAPTER II. 



Bkakch II. — PoEiFERA {Spongesy. 

 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. Upon 

 cutting a dry sponge in half there are to be seen large canals 

 which have large openings called oscula ; these are really 



* See Kent's Manual of Ihe Infusoria, London, 1880- 83. 



