358 THE DEVELOPMENT OF CESTODES, 



occurs at an earlier stage, even before the formation of the head has 

 begun, and then it is complete up to the blind epd of the caecum. The 

 metamorphosis of the head must in such cases of course take place 

 inside the bladder, from which even the perfectly formed heads may- 

 be seen for a while projecting, with the hooks and suckers directly 

 outwards. When these are subsequently drawn back to the cervical 

 region, the heads, which have originated in this way, look like the 

 others. 



If we suppose that the brood-capsules represent a pushing in of 

 the Uchinococcus-'wall, as is diagrammatically represented in Fig. 205, 

 then the difference between the Echinococci and the ordinary bladder- 

 worms, and particularly the Ccenurus, is essentially limited to this 

 point, that the heads are budded off from these invaginations, instead 

 of from the proper bladder-wall. This supposition does not as a 

 matter of fact seem so incorrect or unreasonable, although the brood- 

 capsules arise, as we shall afterwards see, without any participation on 

 the part of the cuticle. At any rate we can thus very simply express 

 the essential peculiarities of the Echinococci, and their relation to the 

 other bladder-worms. 



As to the fact that the cuticle takes no part in the formation of 

 the brood-capsule, this may perhaps find some explanation in the very 

 unusual thickness which we found it to possess in the Echinococci. 

 There are some whose cuticle is a millimetre thick, and, besides 

 such instances which occur in the larger forms, even the smaller 

 Echinococci are provided with a very thick cuticle ; and not only is 

 it very thick, but it also shows a characteristic lamination, which 

 particularly demands our attention, since it not unfrequently happens 

 that new Echinococcus-h\a.ddeTS are found between the layers, whence 

 they burst forth either outwards or inwards, often at an early stage, 

 and become independent bladders.'- 



According to the precise nature of this proliferation, various forms 

 of Echinococcus have been distinguished, as we shall afterwards see, 

 when we come to discuss in detail this interesting bladder-worm. 



As a rule, daughter-bladders and brood-capsules are only formed 

 at a later stage, when the bladder proper has attained a considerable 

 size. Sometimes they seem never to occur. We know at least of 

 numerous cases in which the Echinococci produce no daughter-bladders, 

 in which we look in vain for brood-capsules or heads, and yet we are 

 not warranted in distinguisliing these — though it has been done — as 

 distinct species. 



1 I may take this opportunity of mentioning that I have to thank Dr. A. Schmid, in 

 Frankfort on the Main, for a Oystkerous tenuicollis, whose bladder encloses a few isolated 

 daughter-bladders, „. ... ,, ... „„ 



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