268 



PL A THELMINTHES. 



the vitellaria. At the point where oviducts and yolk ducts unite 

 a single egg cell together with several yolk cells are united into 

 an oval body the compound egg protected "by a shell secreted 

 by special glands (fig. 227, A). This forms only an apparent 

 exception to the rule that the egg is but a single cell, for the 

 development shows that only the egg cell takes a direct part in the 



FIG. 226. 



FIG. 227. 



FIG. 226. Excretory system of Cercaria. (After Albert Lang.) ft, limb of bladder ; 

 b', same with urinary concretions; cc, collecting canal; cs, canals of ventral 

 sucker; cv, collecting vacuole; e, eye; ep, excretory pore; J, lumen of tail; os, oral 

 sucker ; vs, ventral sucker. 



FIG. 227. Eggs of Dtatomum nodulosum. (After Schauinsland.) A, before develop- 

 ment; U, later, the yolk broken down, d, yolk cells ; ei, egg cell ; eh, ectoderm ; 

 en, entoderm ; p, pigment spot. 



formation of the embryo and is the true ovum, while the yolk cells- 

 break down and furnish food to the growing embryo (fig. 227, B). 



Class I. Turbellaria. 



The Turbellaria are small, only a few being measured by inches, 

 while many are almost microscopic in size. The name Turbellaria 

 has reference to the currents produced by the ciliated ectoderm 

 which covers the body, the cilia arising from the single layer of 

 columnar epithelial cells (fig. 58). This ectoderm serves at once 

 for motion and for respiration. Most species are aquatic (fresh 

 water or marine), only a few land planarians living in moist earth. 

 In the water they either creep slowly over stones or plants on 

 their ventral surface, or they swim freely. In swimming the larger 

 species progress by undulations of the body, the smaller by means 

 of the cilia. 



