THE NATURE AND VARIETIES OF 



invests itself with a kind of egg-shell ; during this process 

 the tail is cast off.* 



" Should this process take place within the body of an 

 insect, the encysted Cercaria might be introduced into the 

 body of an insectivorous bird or beast. In the act of di- 

 gestion by the engulpher, the body of the insect is destroyed, 

 together with the capsule of the cercarian pupa ; but this by 

 virtue of its vitality remains unharmed, and is thus trans- 

 planted into a new sphere, and fitted for its farther change into 

 a sexual entozoon of the Trematode or ' fluke-worm' order. 



" Then again commences the strange and complex genetic 

 cycle, from the Harveian point the impregnated ovum. 



" Three different species of animal may contribute two 

 are essential to the successful progress of the ordinary and 

 Parthogenetic processes of propagation, manifested by the 

 three distinct forms of Infusory, Gregarina, and Cercaria, 

 intervening between the egg and the perfect parasite fluke- 

 worm. "( 



If the hypercriticism be excusable of objecting to the 

 accuracy of the figurative language, in which this author 

 gives so lively a representation of these strange mutations, 

 it may be remarked that the terms " coat" and " greatcoat" 

 are out of point here in two particulars. 



1. That such articles of dress never envelope more than 

 one wearer at a time, whereas the Gregarina-form at least 

 encloses a whole brood of Cercarise ; and 



2. That these garments are worn simultaneously, whereas 

 the Gregarina does not develope the Cercarige till it has 

 escaped from its infusorian cyst. Indeed in some cases it 



* Sometimes this appendage appears to be torn off at the first entrance 

 of the parasite, in the act of boring through the integument. It would 

 appear, however, that some Distomata never pass through the Cercaria 

 stage at all. Huxley, in Medical Times, III., 133-134. Siebold's Compar. 

 Anatomy, 118 [note], 



f Address to Brit. Assoc., 1858, 23-24. 



