TREMATODA. 151 



Gimda segmentata (marine) with internal segmenta- 

 tion. 

 Geodesmus and Bipalium (in damp earth). 

 These two orders are often called Dendrocoel (with branched gut) 

 in contrast to the following Rhabdoccel forms (with straight gut). 



Order 3. Rhabdocoelidea. Small fresh-water and marine forms. 

 The body tends to be cylindrical. The food-canal is either 

 very slightly branched, or quite straight, or absent. 

 e.g.. Vortex. 



Microstoma, unisexual, forming temporarily united asex- 

 ual chains, sometimes of sixteen individuals, sug- 

 gesting the origin of a segmented type. 

 Stenostoma, also unisexual. 

 Graffilla, parasitic in habit (cf. next class). 

 Convoluta, without any food-canal, but containing green 

 cells which some regard as symbiotic Algje. 



Relationships. — Lang maintains that there are marked affinities between 

 these simple animals (especially Cceloplana and Ctenoplana) and the 

 Clenophora. 



The Turbellarians are also related to the next class — the Trematodes. 



Class Trematoda. Flukes, etc. 



The Trematodes are leaf-like or roundish external or 

 internal parasites. With their mode of life we may associate 

 the absence of cilia on the surface of the adults, the well- 

 formed and apparently cellular "cuticle," the presence of 

 attaching suckers with or without hooks, and the rarity of 

 sense-organs. It is likely that they have arisen from free 

 Turbellarian-like ancestors, and they certainly resemble the 

 former class in being all of one piece (unsegmented), in 

 having a pair of anterior nerve-centres from which nerves 

 pass backward and forward, in the rudimentary nature of 

 the body-cavity, in the ramifying system of fine excretory 

 canals, in the hermaphrodite and usually complex repro- 

 ductive system. The alimentary canal is usually forked, 

 often much branched, and always ends blindly. In many 

 cases, at least, the animals are self-impregnating. The 

 development of the external parasites is usually direct, of 

 the internal parasites usually indirect, involving alternation 

 of generations. They occur in or on all sorts of Vertebrates, 

 but those which have an indirect development and require 

 two hosts to complete their life-cycle, often pass part of 



