Phytogeny of the Organization of the Cestoda. 427 



Vertebrata were already in existence in Palseozoic times, and 

 that for this reason alone the argument adduced loses its 

 cogency, another circumstance seems to me sufficient to refute 

 his view, at any rate as far as the Platyhelminthes are con- 

 cerned. I refer to the remarkable agreement existing between 

 Trematodes and Dendroccele Turbellaria in the organization 

 of the fully developed sexual forms, an agreement which, if 

 the view in question were correct, could only be explained by 

 means of a convergence of development, which is highly 

 improbable, especially in view of the contrast between the 

 conditions of life in the two cases. 



At the same time, however, it by no means follows that we 

 are bound to regard the intermediate hosts of the larval stages 

 as only later intruders into the life-history of the Helminthes ; 

 far rather may we well maintain the notion that the young 

 worms found their way into the bodies of Invertebrates at 

 the very beginning of the phylogenetic process, but were 

 there unable to arrive at full development and sexual maturity. 

 On the other hand, owing to the altered conditions of sub- 

 sistence, they underwent a necessary change of form, by virtue 

 of which they, either themselves or in the persons of their 

 pa?dogenetically-produced offspring, were enabled to leave 

 their intermediate host once more by means of active or 

 passive migration, and, being now transferred into the body 

 of a Vertebrate under more favourable conditions of nourish- 

 ment, they underwent in their new host, as the definite carrier 

 of the sexual animal, their full morphological and digenetic- 

 sexual development. In this manner the regular occurrence 

 of an intermediate host in the life-history of the Helminthes 

 and the distribution of the developmental phases between two 

 (or more) hosts may find an unstrained explanation. It will 

 also appear quite comprehensible that in the case of nume- 

 rous intestinal worms not one single, but many *, generally 



* Many of the intestinal worms appear to possess an especially great 

 adaptability to the conditions of nourishment in the bodies of their hosts, 

 which renders intelligible the occurrence of one and the same species of 

 Entozoon in different and even widely distant hosts. For instance, 

 JDistomum echinatum, which is developed from the Cercaria echinata of 

 various species of mollusks, is found sexually mature not only in the 

 intestine of the duck and other waterfowl, but also in that of the dog, the 

 rat, and the mouse. The Cysticercus celluloses of Tcenia solium lives not 

 only in the body of the pig, but also in the most dissimilar organs of the 

 human subject, and has also been found in the muscles of the roe, the 

 dog, and the cat. Taenia elliptica occurs not only in the intestine of the 

 cat, but also (cucumerina) in that of the domestic dog and of man. We 

 may further instance the distribution of Echinococcus and of numerous 

 Nematodes, especially of Trichina spiralis, in the bodies of the most 

 widely different mammals. 



