240 



PLATHELMLXTHES 



composed of a scries of tul)es, the protoncphridia or 'water-vascular 

 !r.\'stem,' wliich branch and ramifv the parenchyma, and open to the 

 exterior Ijy one or more openings \ariously arranged, ^lost llatworms 

 are hermaphroditic and the reproductive organs take up considerable 

 space. There is a small paired or unpaired o\'arv and vilcllaria, 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 ^'ilellaria. At the point where oviducts and yolk duels 

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

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

 which also have to do with the nutrition of the em])ryo. Oi the 'com- 

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

 embr3-o and is the true ovum. 



The riatodes 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 Hatworms, but as nearer the Annelids. 



Class I. Turbellaria. 



The Turbellaria are small, only a few beinp; measured by inches, 

 while many are almost microscopic in size. 1 he name Turljcllaria refers 

 to the currents produced by the ciliated ectoderm wlrich covers tlie body 

 (fig. 59), and which serves at once for motion and for res]iiration. Most 



a In tn m 



Fig. 215. — Digestive anrl nervous systems of SvJiri}li:liinii ftiiliicidiim * (after 

 Wheeler), a, alimentary Iraii; h. hrain; lii, lun,L;iUi(linal (voiitrab iier\es; »;, marginal 

 nerve; /)/, longitudinal ner\-e of ])liaryn\; /)C, ring nerve of iihar\-nx; In, transverse nerve; 

 ii, uterine ostium. 



species are aquatic (fresh water or marine"), only a few land plauarians 

 living in moist earth. In the ^vater t1ie\- either creci> slowly o\er 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 enlo- 

 parasitcs and often show adaptations to their life in the ticgeneration of 



