//. TREMATODA. 



271 



marked off from the mesoderm, but it is a question whether these l Accela ' 

 are primitive or degenerate. 



Order I. Polycladidea. 



Marine species of considerable size, in which the digestive caeca spring 

 from a central chamber. Species of Leptoplana and 

 Stylochus on our shores. Thysanozoon, Europe. 



Order II. Tricladidea. 



Alimentary canal with three main trunks, an anterior 

 unpaired and a pair of posterior branches, arising from 

 the pharynx. These trunks bear lateral caecal branches. 

 Among the marine genera are Bdelloura * and SyncoR- 

 lidlum * (fig. 229) (parasitic on Limulus), Gunda,* Poly- 

 clicerus * (fig. 229); in fresh-water occur Dendrocoelum* 

 Planaria* and Polyscelis* Phagocata * with divided 

 pharynx. The land planarians (Bipaliiun,* 10 or 12 

 inches long) are tropical, but have been introduced 

 into greenhouses in various parts of the country. 



Order III. Rhabdoccelida. 



Small, even microscopic, in size, and recalling in 

 habits and appearance the Infusoria; alimentary canal 

 rod-like, without branches. Monops* and Monoscelis* 

 marine. The fresh-water MICROSTOMID.E reproduce 

 almost exclusively by fission, so that sexual individuals 

 are rare. 



Class II. Trematoda. 



These are exclusively parasitic, some living on the skin or 

 gills (ectoparasites) or in the interior of other animals (entopara- 

 sites). In structure they are closest to the triclad Turbellaria, 

 from which they are especially distinguished by characters the 

 direct result of their parasitic life. Thus they have lost the cilia 

 or these only appear in the aquatic larval stages. On the other 

 hand they are armed with structures derived from the skin 

 suckers and hooks for adhesion to the host. The suckers are 

 shallow pits of columnar epithelium, lined with cuticle and fur- 

 nished with a thick muscular layer which by its contraction increases 

 the lumen of the sucker, the edges of which are closely applied to 

 the host. At least one such sucker is present; if but one or two 

 (entoparasites), one is at the anterior end (oral sucker) and sur- 

 rounds the mouth, while a second larger sucker may occur near the 

 month (fig. 232), but may be (Amphistomum) at the posterior end. 

 In the ectoparasites there are a pair of anterior suckers near the 

 mouth; at the posterior end a single sucker, or a number of suck- 

 ers or hooks or both on a sucking disc (fig. 234). 



FlG.231. Gundn loba- 

 ta. (After Schmidt.) 

 <7, cerebral ganglia, 

 with eye spots ; 

 o, mouth (entrance 

 to long pharynx); 

 p, genital pore with 

 male organs be- 

 hind, female in 

 fron t . 



