240 



PLATHELMINTHES 



composed" of a series of tubes, the protonephridia or 'water-vascular 

 system,' which branch and ramify the parenchyma, and open to the 

 exterior by one or more openings variously arranged. Most flatworms 

 are hermaphroditic and the reproductive organs take up considerable 

 space. There is a small paired or unpaired ovary and vitellaria, usually 

 paired and branched. The eggs arise in the ovary, and to these are 

 added nourishment in the shape of cells (abortive ova, 'yolk cells') rich 

 in yolk from the vitellaria. At the point where oviducts and yolk ducts 

 join, a single egg cell and several yolk cells are united into an oval body 

 the compound egg protected by a shell, formed by the 'yolk cells,' 

 which also have to do with the nutrition of the embryo. Of the 'com- 

 pound egg' only the egg cell takes a direct part in the formation of the 

 embryo and is the true ovum. 



The Platodes are usually divided into four classes. Of these the 

 Turbellaria are the most primitive, and the others have come from them. 

 The Trematodes and Cestodes have been altered by parasitism, resulting 

 in more or less degeneration. By some the Nemertini are not regarded 

 as allied to the other flatworms, but as nearer the Annelids. 



Class I. Turbellaria. 



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

 while many are almost microscopic in size. The name Turbellaria refers 

 to the currents produced by the ciliated ectoderm which covers the body 

 (fig. 59), and which serves at once for motion and for respiration. Most 



FIG. 215. Digestive and nervous systems of Synccelidium pellucidum* (after 

 Wheeler), a, alimentary tract; b, brain; In, longitudinal (ventral) nerves; m, marginal 

 nerve; pi, longitudinal nerve of pharynx; pr, ring nerve of pharynx; tn, transverse nerve; 



u, uterine ostium. 



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, or they swim freely. The larger species swim by undulations 

 of the body, the smaller by means of the cilia. Several are ecto- or ento- 

 parasites and often show adaptations to their life in the degeneration of 



