TRICLADA. 



221 



Order 4. TRICLADA. 



Enteron with three IraitdK'*, one forward and tiro hark/rant : 

 l>liarynx cylindrical, inserted at the junction of the three I/ranches. 

 Genital pore single, lieliind the mouth. 



The Tridada comprises marine, fresh-water, and terrestrial forms. 

 A description of a typical member of the order has already been 

 given. The testes are numerous and 

 follicular; the vitellarium consists of 

 scattered follicles, and is only exception- 

 ally compact (Otoplana) ; the ovaries are 

 two in number, placed anteriorly. The 

 mouth is usually behind the middle of 

 the body, and the body is more or less 

 flattened. The skin is often provided 

 with glands, the secretion of which in 

 certain land forms (Bipalium, Rhyncho- 

 demus) hardens to a fibrous web. The 

 Tridada have great power of repairing 

 lost parts ; fission occurs in Planaria 

 alpina and cornuta. 



_ C 



a 



FIG. 176. Planaria, polychroa (a), 

 P. Ingubris (b), P. torva (c), 

 about twice natural size (after 

 O. Schmidt). 



Marine forms. Enteric branches hut little ramified, sometimes simply lohed. 

 Mouth placed in the posterior half of the body (except Bdellura). Body flattened. 

 Uterus placed behind the genital opening. 



Fam. 1. Otoplanidae. With otocyst and ciliated pits, without eyes. Otoplana 

 Du Plessis. 



Fam. 2. Procerodidae. Without otocyst and ciliated pits. Cercyra 0. Schm. ; 

 Procerodes Girard (including Ginida, Fovia, Haga) ; Uteriporus Bergendal. 

 (rimda and Fovia recorded from British coast. 



Fam. 3. Bdelluridae. Ectoparasitic on Limit his, provided with a caudal 

 apparatus for fixation. Bdflltirtt, Leidy. 



Fresh-water forms. Enteric branches much ramified. Mouth placed in the 

 posterior half of the body. Body flattened. Uterus placed between the 

 pharynx and the penis. 



Fam. 1. Planaridae. Head without differentiated 

 Planaria 0. F. Mull, (including Ditgcsia), with two eyes 

 P. punctata Pall. ; P. polychroa Schm. ; P. torva M. Sch. ; P. alpina Dana are 

 British. Phagocata Leidy, with 8 or 9 pairs of pharynges additional to the main 

 one, Pennsylvania; Anocclis Stimpson, no eyes; Polycelis Hemprick and Ehrenb. , 

 many marginal eyes ; P. nigra Ehr., P. cornuta are British. 



Fam. 2. Dendrocoelidae. Head with several differentiated organs of fixation. 

 Dendrocoehim Oerst. (incl. Bdelloccphala and Galeocephala), two eyes ; Olicjocelis 

 Stimpson, six eyes ; Procotyla Leidy ; Sorocelis Grube, and Dicotylus Grube, 

 from Lake Baikal. 



Terrestrial forms. Enteric branches generally simply lobed. Position of 



organ of fixation. 

 P. lactca 0. F. M. ; 



