PHYLUM PLATYHELMINTHES 



207 



In 



exterior by an anal aperture. All the Turbellaria (except some 

 Accela) and Trematoda have an alimentary apparatus consisting 

 of two well-defined parts a muscular pharynx and an intestine. 

 The pharynx is usually a rounded muscular bulb, but is sometimes 

 (some Turbellaria) of a cylindrical shape ; it is usually capable of 

 aversion and retraction. Actinodadylella (Fig. 202) is exceptional 

 in having in addition to a large muscular pharynx, an extensile 

 proboscis with a pin-shaped style, which becomes retracted within 

 the opening of the mouth. Uni- 

 cellular glands open into the pharynx 

 in most cases. 



The mouth is always ventral, but 

 varies greatly in its position on the 

 ventral surface, being sometimes 

 central, sometimes situated behind, 

 sometimes in front of, the middle of 

 the length of the body. In the 

 most lowly organised group of Tur- 

 bellaria (the Accela} the intestine is 

 represented merely by a vacuolated, 

 nucleated mass of protoplasm with- 

 out, or with only an irregular, lumen. 

 In the others it is sometimes a simple 

 sac (Rhabdocoele Turbellaria Fig. 

 211, a few Trematoda), with or 

 without short lateral diverticula. In 

 the majority of the Trematodes it 

 consists of a pair of simple canals ; 

 but in some, as in the Liver-Fluke, 

 there is a pair of canals which give 

 off numerous branches. In the Poly- 

 cladida (Fig. 212) there is a central 

 cavity from which numerous branch- 

 ing canals are given off. In the 

 Tricladida (Fig. 213) one median 

 canal passes forwards from the 

 pharynx, and a pair of canals back- FIG 913 

 wards from it, all three giving off 

 branches which again branch. In 

 some Polydadida there are minute 

 pores, by means of which certain of 

 the canals are placed in communica- 

 tion with the exterior. A number 

 of unicellular glands, which probably produce a digestive secretion, 

 open in many Trematodes and Rh'abdocoeles at the junction of 

 pharynx and intestine. 



A bilateral nervous system is developed in all the Platy- 



u 



od 



General plan of the structure of 

 a Triclad. en. brain ; e. eye ; <j. 

 germarium : i. median limb of the in- 

 testine ; \i. right limb ; ig. left limb ; 

 In. longitudinal nerve-cord ; m. mouth ; 

 od' oviduct ; ph. pharynx ; t. testes ; 

 te. tentacles ; v. vitellaria ; vd. vas 

 deferens ; u. uterus ; <$ ejaculatory 

 duct ; 9 vagina ; ? common genital 

 aperture. (After Von Graff.) 



