80 LIFE-HISTORY OF PARASITES. 



mediate stage passing directly, and without migration, into the sexual 

 condition. 



All this, however, is quite exceptional, and the rule for the life- 

 history of parasites may be stated as follows : The life-history of para- 

 sites is divided into two stages (1) the larval, and (2) the sexually mature 

 adult; and each of these is passed in the body of a separate host. Sometimes 

 these two hosts may be merely two individuals of the same species, as 

 in the case of Trichina ; but generally they are quite different, and 

 may belong even to separate orders or classes. Tcenia crassicollis 

 inhabits the liver of the mouse while in the young condition, and the 

 intestine of the cat when adult ; Tcenia maryinata, the connective tissue 

 of sheep and oxen when young, and finally the intestine of wolves and 

 dogs ; the adult Tcenia solium of man is found in the young condition 

 in swine. In a similar way the life of Ligula is divided between fish 

 (Cypriiiidse) and water-birds ; of Echinobothrium typus, between rays 

 and Gammarina ; of Distomum echinatum, between ducks and Paludince; 

 of Amphistomum subclavatum, between the frog and Planorlis ; of Fen- 

 tastomum tcenioides, between the dog and rabbit, and so forth. These 

 examples do not merely prove the justice of the general principle just 

 enunciated, but also bring out prominently the fact that the host of 

 the young parasite is frequently an animal which serves as food for 

 the definitive host ; thus the mouse yields to the cat not only its flesh, 

 but its parasites, and the like happens with the rabbit and dog, the 

 fish and the sea-gull. And this fact is not difficult to understand from 

 a physiological as well as a teleological point of view. If one animal 

 select as its food a certain other animal, it evidently follows that the 

 latter is best suited to its nutritive requirements, hence the conditions 

 of nutrition in both must be somewhat similar, and a parasite capable 

 of living in one would probably also find the other in a great measure 

 favourable to the conditions of its life. This idea, however, must not 

 be pushed too far, since we find, for example, the young of Tcenia 

 crassicollis in many animals which are not preyed upon by cats ; so 

 also the human tape-worm is occasionally found in the asexual 

 state in man himself, a fact which, on the principles just enun- 

 ciated, would seem to justify cannibalism from the stand-point of 

 natural history. The presence of the young stages in Carnivora is 

 certainly to be looked upon in the above light. The Herbivora also 

 often contain parasites which live in the young stage in bodies of 

 other animals; 1 but in these cases, the latter inhabit the same 



1 The statement of Von Siebold (" Handworterbuch d. Physiol.," Bd. ii., p. 647), 

 repeated recently by Ercolani, that the Herbivora become infected with their parasites 

 through the medium of their food, because the parasitic Nematodes of many plants develop 

 in their bodies, has no foundation. The Nematodes of plants are independent species, 

 which are never parasitic upon animals. [On the other hand, the recent researches of 



