Ill PLATODES—OEQAKISATION OF CESTODA BODY 165 



and the brain commissures ; or, when we compare the head and pro- 

 glottis mth a Tremaiode, we find that the head has not the trunk, and 

 the proglottis not the head, of the Trematode body. The head and one 

 proglottis together, however, answer to the head and trunk, and thus 

 to the whole body of a Trematode, apart from the fact that an intestinal 

 canal is altogether wanting in the Cestoda. 



We, however, know forms whose body during life consists only of 

 head and trunk, which is not clearly divided into a scolex and a 

 proglottis. Such forms are Amphilina, Caryophylkeus, and Archigetes. 

 These may be regarded either as intestineless Trematoda, or unsegmented 

 Cestoda. They are, in any case, transition forms between the Trematoda 

 and Cestoda. The relation existing between them and the segmented 

 Cestoda is somewhat similar to that between the Acraspeda which are 

 attached throughout life {e.g. Lucernaria), and the polydisc strobila of 

 Aurelia. 



A segmented tapeworm must in fact be considered as a strobila. 

 The young, still unsegmented tapeworm, which attaches itself to the 

 intestinal wall, i.e. the scolex, answers to the young stage of one of the 

 unsegmented Cestoda mentioned above (AmjMlina, Caryophylkeus, or 

 Archigetes), in which the genital apparatus is not yet developed in the 

 slightly developed trunk, the future neck. Now follows the incom- 

 plete constriction of that part of the body of the scolex (the trunk or 

 the first proglottis) in which later the genital organs develop. Re- 

 generation of the constricted part then takes place ; this part is again 

 constricted and again regenerates, and so on. The single parts remain 

 connected for a longer or shorter time, and form the segments of the 

 tapeworm chain or strobila. Finally, like the oldest Medusa discs of 

 a polydisc strobila of Aurelia, the oldest segments of the tapeworm 

 strobila detach themselves. The points in which the process differs 

 in the two groups are essentially the following. The Medusce which 

 detach themselves from a polydisc strobila develop further, and their 

 sexual organs attain development only after detachment. The segments 

 of the Cestoda which detach themselves, however, are already more 

 than mature (sexually) ; they have performed their function, the pro- 

 duction of fertilised eggs, and they make no attempt to regenerate 

 the part which is wanting to make them complete Platodes, i.e. the 

 head. In the Medusa strobila, further, that part of the body by which 

 it is attached, viz. the apex of the exumbrella, is an insignificant part 

 of the body both physiologically and anatomically, while the part by 

 which the tapeworm is attached contains at least the principal part of 

 the central nervous system. 



It does not seem difficult, in the case of the Tapeworm, to trace 

 back strobilation to the phenomenon of regeneration. Proceeding 

 from forms, like Amphilina, capable of regeneration, we can understand 

 that by the peristaltic movements of the intestinal canal in which the 

 animals lived parasitically, and by the outward movement of the ex- 

 crement, the trunk, with the genital organs it contains, would be torn 



