106 THE ORIGIN OF PARASITES. 



Cestodes, have no alimentary canal. 1 In the case of a large free- 

 living animal such a want would, of course, be a very remarkable 

 circumstance, since the possession of mouth and intestine is, according 

 to our present knowledge, a most necessary requisite of such animals. 

 But the relations of parasitic life, which permit of nutriment being 

 taken up through the skin, render the possession of these organs un- 

 necessary, or at least not indispensable (p. 18). Even in Nematodes 

 we see the intestinal canal become atrophied in a few cases. This 

 proves no more than that the parasites in question are so completely 

 adapted to the conditions of their existence, that they have no further 

 need for an intestine, and hence we can only interpret the absence 

 of this organ in the Cestodes as meaning that they are much further 

 removed from the conditions of free life than the Trematodes. 



But the absence of hooks in the proglottides, like the absence of 

 an intestine, results from the relations given above. They do not 

 stand in such need of them as the solitary living Trematodes, since 

 they belong to a community which is sufficiently firmly fastened by 

 means of a hook apparatus, with which the so-called head is provided 

 (Fig. 4) ; the individual segments of the chain have thus a certain 

 share in the hook apparatus situated on the head. 



If further proof of this assertion were required, it might be found 

 in the existence of certain unsegmented Cestodes, which, like Gary- 

 ophyllceus, Amphiptyches, &c., represent in their simple body both head 

 and proglottis, that is, unite in themselves both a hook-apparatus 

 and sexual organs like the Trematodes. That which in the common 

 tape-worm was spread over two generations (head and sexual animal) 

 has in these animals again become united in a single individual : and 

 this has been pointed out above to be a frequent occurrence among 

 these groups which present alternation of generations for it is an 

 alternation of generations which manifests itself in the mode of de- 

 velopment of the tape-worms. 



The above-mentioned facts leave no room for doubt that the Ces- 

 todes are very closely related to the Trematodes, that they represent 

 in a certain sense Trematodes without an intestine, in which the 

 organism has, according to the law of alternation of generations, 

 separated itself into two genetically combined individual forms. 

 That this affords certain advantages of great importance, especially to 

 animals exposed to so many vicissitudes, as is the case with the intes- 

 tinal worms, is apparent, especially when we remember that the young 

 tape-worm (Scolex) is rendered capable, through the alternation of 



1 Such is the case, according to a letter which I have received from Prof. Glaus, in a 

 Trematode allied to Distomum from the intestine of Delphiiius dclphis, as also, according 

 to van Beneden, in Distomum Jillicollc. Dr. Tasehenberg will shortly prove that these 

 examples by no means complete the list of anenteric Trematodes. 



