BY W. A. HASWELL AND J. P. HILL. 371 



All these parts are formed almost simultaneously, for the origin- 

 ally compressed and solid bud increases in length, then becomes 

 hollow inside, and. becomes jointed by the development of the 

 hook-apparatus in front and a bladder-like expansion behind. 

 When the suckers and hooks are completely developed, the 

 anterior part of the body draws back into the caudal bladder by 

 invagination of the neck, so that at the end of its development 

 the won* has exactly the same position as we formerly observed 

 in Cysticercus arionis.''* 



Mecznikoff thus, it would appear, describes the embryo as 

 elongating and becoming divided into three parts, on the most 

 anterior of which the hooks become formed, this anterior part 

 subsequently becoming invaginated, with the suckers, into the 

 posterior part or caudal bladder. This is totally at variance 

 with what we have found to occur in the parasite of Didymo- 

 gaster, and, taking into account certain resemblances between the 

 two forms, we are inclined to think that it is erroneous. It is 

 probable that, if our supposition is correct, the error arose from 

 an attempt to reconcile the stages observed with the known 

 developmental history of Echinococcus. As regards the earlier 

 stages, MecznikofF's figures a and b, as reproduced by Leuckart 

 (fig. 213, p. 366), do not represent any stage that occurs in the 

 parasite of Didymogaster, and it is difficult even to reconcile them 

 with Leuckart's description. The former represents the wall of 

 the cyst as raised up internally into three thickenings, and the 

 latter shows three oval masses, developed from those thickenings, 

 attached together by narrow strands, though no longer connected 

 with the cyst wall. 



, Leuckart,! as already noticed, refers Polycerciis to the type of 

 Echinococcus, and so does Moniez.j From this view Yillot§ 



* L.c. pp. 366 and 367 : second German edition, pp. 464 and 465. 

 t Tonu cit. p. 366. 

 J'*Essai Monographique sur les Cysticerques." Travaux de I'lnstit. 

 Zool. de Lille, T. iii. (1880). 



§ " Memoire sur les cystiques des Tenias." Ann, des Sci. Nat. Zool. 6me 

 s^rie, Tome xv. (1883). 



