TURBELLARIA 189 



live in a symbiosis (p. 43) with the Turbellarian. The photo- 

 synthetic products of these algal cells provide a source of nourish- 

 ment for the animal. Convoluta henseni, another member of this 

 order, is a rare platyhelminth that has adopted a planktonic habitat. 



Order RHABDOCOELID A 



In these forms (Fig. 147) the gut is straight and the mouth is near the 

 anterior end. The gut may or may not have lateral pouches. In the 

 more primitive forms, of which Microstomum linear e is a common ex- 

 ample, found in fresh water, the germarium and the vitellarium are 

 not separated. Another well-known member of this group is Dalyellia 

 viridisy common in freshwater ponds in Britain and remarkable for 

 the elaborate chitinous structure of the penis. Mesostoma ehrenbergi 

 and M. quadr angular e^ the latter X-shaped in cross-section, both 

 occur in freshwater ponds. They are large and transparent and form 

 the best objects for studying the structure of the group. Plagiostomum 

 lemani is a form with side pouches to the gut.^ It occurs at the bottom 

 of deep lakes in temperate regions. Otoplana also has side pouches to 

 the gut but is chiefly remarkable for possessing an otocyst overlying 

 the brain. 



The Rhabdocoelida occur in both fresh and salt water; marine 

 forms are, however, very small. 



Order TRICLADIDA 



In this group the gut is divided into three main divisions with numer- 

 ous lateral diverticula from each division. The mouth has shifted back- 

 wards to the middle of the body. There are three well-recognized 

 divisions of this order, separated according to habitat : the Paludicola 

 or freshwater forms, the Maricola or marine forms, and the Terricola 

 or land forms. The Paludicola are all fairly large forms in contrast 

 with the Maricola which are small, no more than 2-4 mm. long. To 

 the Paludicola belong the three commonest freshwater Turbellaria 

 in Britain: Dendrocoelum lacteum, a white form, Planaria lugubris, a 

 black form, and Poly cells nigra, a rather smaller black form easily 

 recognized by the ring of eyes round the anterior edge of the body. 

 Perhaps the best known member of the Maricola is Procerodes lobata 

 (= Gunda segmentata) in which the side diverticula of the gut are 

 regularly arranged, with testes and excretory openings between them, 

 giving the appearance of a segmented animal. The Terricola often 

 reach a very large size — as long as 50 cm. They are often brightly 

 coloured with stripes down the dorsal surface. Bipalium kewense is 



^ These forms, with side pouches to the gut, are sometimes placed in a 

 separate order called Alloiocoela. 



