//. TREMATODA. 



271 



marked ofE from the mesoderm, but it is a question whetlier these ' Acoela ' 

 are primitive or degenerate. 



Order I. Polycladidea. 

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

 from a central chamber. Species of Leptoplana and 

 Htijlochus on our shores. Thysanozooii, 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 ceeoal branches. 

 Among the marine genera are Bdelloura* and Syncce- 

 lidiiun * (fig. 239) (parasitic on Limidus), Gunda,* Poly- 

 chwrus * (fig. 339); in fresh-water occav Dendroccehim* 

 Planaria* and PohjsceUs* Phagomta * with divided 

 pharynx. The land planarians (Bipalium,* 10 or 13 

 inches long) are tropical, but have been introduced 

 into greenhouses in various parts of the country. 



Order III. Rhabdocoelida. 



Small, even microscopic, in size, and recalling in 'Fia.^iSi.—Gundaioba- 

 habits and appearance the Infusoria; alimentary canal 

 rod-like, without branches. 3Ionops* and Monoscelis* 

 marine. The fresh-water Microstomid.e reproduce 

 almost exclusively by fission, so tliat sexual individuals 

 are rare. 



Class II. Trematoda. 



These are exclusively parasitic, some living on the skin or 

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

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

 from which they are esiiecially distinguished by characters the 

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

 or these only apj^ear in tlie aquatic larval stages. On the other 

 hand they are armed with structures derived from tlie skin — 

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

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

 nished with a thick muscular layer wliicli Ijy its contraction increases 

 the lumen of the sucker, the edges of wdiicli 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 

 mouth (fig. 232), but may be {Ampliistomuin) 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 <ir hooks or both on a sucking disc (fig. 231). 



ta. (After Schmidt.) 

 (/, cerebral ganglia, 

 with eye-spots ; 

 o, mouth (entrance 

 to long pharyn.x); 

 ?^ genital pore witli 

 niale organs be- 

 hind, female in 

 front. 



