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. 237, 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. 227. 



Fig. 226.— Excretory system of Ceroaria. (After Albert Lang.) 7), limb of bladder ; 

 ?/, same "with urinary concretions; cc, collecting canal; c.^, canals of ventral 

 suclier; cr, collecting vacuole; e, eye; cp, excretory pore; I, lumen of tail; o^^ oral 

 suclier ; v^^ ventral suclcer. 



Fig. 327.— Eggs of LiMiitiiiim nnilvlosurn. (After Schauinsland.) A, before develop- 

 ment; B, later, the yolk broken down, d, yolk cells ; c), egg cell ; €h\ ectoderm ; 

 tw(, entoderm; )j, 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. 237, 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 Ijody, the cilia arising from the single layer of 

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

 for motiou and for respiration. Most sjiecies are aquatic (fresh 

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

 In the water they cither creeji slowly over stones or plants on 

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

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

 of the cilia. 



