190 ESSAY ON CLASSIFICATION. 



Mice with their Worms being eaten up by Cats, the para- 

 sites living in them undergo their final transformation in 

 the latter. Many Worms undertake extensive migrations 

 through the bodies of other animals, before they reach the 

 proper place for their final development. 1 



Among the Mollusks parasites are very few, if any can 

 properly be called true parasites, as the hectocotylized 

 arm of the males of some Cephalopods living upon their 

 own females ; 2 as the Gasteropods growing buried in 

 Corals, 3 and the Lithodomus and a variety of Areas found 

 in Corals. Among Eadiata there are no parasites, properly 

 speaking ; some of them only attaching themselves by 

 preference to certain plants, while the young of others 

 remain connected with their parent, as in all Corals, and 

 even among Crinoids, as in the Comatula of Charleston. 



In all these different cases, the chances that physical 

 agents may have a share in producing such animals are 

 still less than in the cases of independent animals, for here 

 we have, superadded to the very existence of these beings, 

 all the complicated circumstances of their peculiar mode 

 of existence, and their various connections with other 

 animals. Now if it can already be shown, from the mere 

 connections of independent animals, that external circum- 

 stances cannot be the cause of their existence, how much 

 less could such an origin be ascribed to parasites ! It is 

 true, they have been supposed to originate in the body of 

 the animals upon which ^ they live. What, then, of those 

 who enter the body of other animals at a somewhat ad- 



1 See above, p. 114, note 2; SIE- 1858. 



BOLD, Wanderung, etc., p. 116, notel; 2 See above, p. Ill, note 1, KOLLI- 



STEENSTRUP, Generationswechsel, q. KER, MULLER, VERANY, VOGT, STEEN- 



a., p. 69 WEINLAND (D.), The Plan STRUP, FRONTED, etc. 



adopted by Nature for the Preserva- 3 RUPPELL (Eo.), Memoire snr le 



tion of the Various Species of Hel- Magilus antiytnts, Trans. Soc. Strasb., 



minths, Proc. Bost. Nat. Hist. Soc., 1832, i, fig. 



