78 LECTURE IV. 



of mammalia seem to be subject to that accumulation. It is shown in 

 a slight degree in the so-called Cysticercus pisiformis of the liver of 

 the hare, which probably finds its full development as the Tcenia 

 serrata of the dog. But in many instances the dropsical enlargement 

 of the encysted cestoid larva proceeds to such an extent as to make 

 it highly improbable that they can carry out the future phases of 

 their proper life-cycle. 



This we may conclude to be the case with the Cysticercus cellulosce 

 of the human subject, from the number of the adventitious cysts of 

 that parasite which are found to contain the dead hydatids ; these, 

 moreover, have sometimes undergone a decomposition into an adipo- 

 ceral and cretaceous mass, like a tubercle, but the true nature of which 

 can frequently be demonstrated by the recognizable remains of the 

 head, or of the booklets of the abortive parasite. 



In the encysted, probably cestoid, larvae of the ruminants 

 (^Cysticercus tenuicollis, Auct.) the accumulation of the fluid proceeds 

 to a greater extent ; and in that to which the name Echinococcus has 

 been given, it seems to be carried so far as to obliterate entirely both 

 head and neck of the original larval form. The Echinococcus how- 

 ever, like the Coenurus, retains so much of the original spermatic force 

 in the germ-cells that remain unmetamorphosed in its parietes, as to 

 set up a multiplication of its kind by gemmation. But the product 

 never goes beyond the sexless vermicule with the uncinated and suc- 

 torial head, like that which is developed in the egg of the tapeworm. 

 In the Ccenurus, these larvae are retained, as we have seen, in 

 organic connection with their parent-cyst, and bud out from her 

 outer surface. In the Echinococcus they proceed from the inner 

 surface of the cyst, and become free, reminding us of the development 

 and accumulation of the navicellar larvae, within the cyst-like body of 

 the parent Gregarina. 



The parthenogenetic mode of generation being the only one that 

 the hydatid-like sexless progeny of the Cestoidea are able to manifest, 

 the life-period of these dropsical larvae, when not liberated, "seems to 

 be determined by the progressive accumulation of their parenchymal 

 calcareous corpuscles, which have been absurdly described as their 

 ova. The right recognition of the nature of the cystic entozoa of 

 Rudolphi shows the futility of looking for normal organs of genera- 

 tion in any part of their structure. 



The hypothesis of equivocal generation has been deemed to apply 

 more strongly to the appearance of internal parasites in animal bodies 

 than to the origin of animalcules in infusions. But if a tapeworm 

 might be organised from a fortuitous concourse of organic particles, 

 or by the metamorphoses of an organic cell in the animal it infests, 



