TURBELLARIA. 



'57 



Cestoda) have arisen from Turbellarian-like ancestors, which adopted 

 parasitic habits, as a few marine Turbellaria have done. 



Classification. — A. Rhabdoccelida. — Small fresh-water and 

 marine forms. The food canal is very slightly branched, or quite 

 straight, or absent. 



(i) Accela. Without intestine, e.g. Convohita, which contains green 

 cells, regarded by some as symbiotic Algse. 



(2) Rhabdoccela. With straight intestine, e.g. Vortex ; Microstoma, 



Fig. 68. — Diagrammatic figure 

 of a simple Turbellarian. 



m., Mouth ;ph., pharynx \g., digestive 

 part of gut ; I.e., longitudinal excre- 

 tory vessels; e.p., excretory pore; 

 Ect., ciliated ectoderm ; Ms., meso- 

 derm ; End., endoderm. 



Fig. 69. — Diagrammatic figure of 

 part of the structure of a simple 

 Turbellarian. 



Ect., Ciliated ectoderm ; e.g., cerebral 

 ganglion; l.n., lateral nerve; T., 

 testes ; ov., ovary. 



a unisexual fresh-water genus, with stinging cells, which forms tempor- 

 arily united asexual chains, sometimes of sixteen individuals, suggesting 

 the origin of a segmented type ; Graffilla and Anoplodium, parasitic 

 (cf. next class). 



(3) Alloioccela. With lobed or irregular gut. All marine except 

 one from Swiss lakes (Plagiostoma Lemani). 



B. Tricladida. Elongated flat ' ' I'lanarians " ; the mouth and 

 tubular pharynx lie behind the middle of the body ; intestine with three 



