MECHANISM OF THE ROSTELLUM. 403 



This is perhaps connected with the notable thickness and rigidity 

 of the cuticle, which would hardly permit of vigorous contraction. 

 The worm exhibits so many peculiarities, especially in the develop- 

 ment of its head, that we are bound to regard it as the type of 

 a special group of cystic worms. As we have mentioned above, the 

 occurrence of a special brood-capsule for the budding heads is very 

 characteristic, and perhaps also finds its explanation in the excessive 

 thickness of the cuticle. 



Nor can we avoid noticing that, in regard to the cystic tape-worms, 

 an assertion has been lately made by Megnin 1 which throws doubt on 

 all we have been affirming as to the nature and development of these 

 animals, and even of all the Cestodes. According to his assertion, 

 the cystic tape-worms are in no wise independent forms, but represent 

 merely the heteromorphic condition of hookless tape-worms. The 

 latter, says Megnin, are the true typical Tcenice, which require for 

 their development neither change of host, nor cystic stage, but rather 

 result directly from the six-hooked embryos whenever these find 

 from the first conditions entirely favourable for their development. 

 In other cases the animals develop only circuitously through a 

 Cysticercoid stage, and may, according to circumstances, become a 

 cystic tape-worm, or return by metamorphosis to the typical form. 

 Thus, when the Echinococcus of a herbivore is transferred to the intes- 

 tine of a carnivore, it becomes Tcenia echinococcus, but it is developed 

 into T. perfoliata whenever the head, as sometimes happens, reaches the 

 digestive canal by perforation of the intestinal wall of the host. In 

 the same way Cyxticercus pisiformis develops in the dog to Tcenia 

 serrata ; but in the rabbit itself may become Tcenia pectinata. (!) 



The two species here named are the only ones which Megnin 

 specially discusses, although he is of opinion that every hookless 

 Tcenia has its representative armed form. For any proof of this state- 

 ment one looks in vain. Our author has neither demonstrated the 

 asserted relationship by experiment, nor made it probable by the 

 existence of intermediate types. He has apparently no conception of 

 the extent of the positive observations on the development and struc- 

 ture of the Cestodes, which are quite enough in themselves to disprove 

 the conclusions which he has tried to establish on merely subjective 

 grounds and analogies. 2 



1 Comptes rendus, t. Ixxxviii., p. 88, and in detail, Journ. anat. et physiol., t. xv., 

 p. 225, 1879. 



2 The opinions and conclusions of Me'gnin on the Cestodes have been decidedly rejected, 

 not only by me (Archiv /. Naturgesch., Jahrg. xliii., Bd. i., p. 199,11877), but also by 

 Moniez (Bull sci. dep. Nord., p. 233, 1880). The latter is, indeed, as we have mentioned 

 (p. 280, note) so far in agreement with Me'gnin, since, according to his opinion, a great 

 many of the Tcenia undergo a continuous development within the same host. 



