PARASITIC ANIMALS. 187 



many of the Orcliidese, may be quoted as equally remark- 

 able examples of parasitism. 



There exists the greatest variety of parasites among 

 animals. It would take volumes to describe them and to 

 write their history, for their relations to the animals and 

 plants upon which they are dependent for their existence 

 are quite as diversified as their form and their structure. 



It is important, however, to remark, at the outset, that 

 these parasites do not constitute for themselves one great 

 division of the animal kingdom. They belong, on the 

 contrary, to all its branches ; almost every class has its 

 parasites, and in none do they represent one natural order. 

 This fact is very significant, as it shows at once that 

 parasitism is not based upon peculiar combinations of the 

 leading structural features of the animal kingdom, but 

 upon correlations of a more specific character. Nor is 

 the degree of dependence of parasites upon other organized 

 beings equally close. There are those which only dwell 

 upon other animals, while others are so closely connected 

 with them that they cannot subsist for any length of time 

 out of the most intimate relation to the species in which 

 they grow and multiply. Nor do these parasites live upon 

 one class of animals ; on the contrary, they are found on 

 all of them. 



Among the Vertebrata there are few parasites properly 

 speaking. None among the Mammalia. Among the Birds 

 a few species depend upon others to sit upon their eggs 

 and hatch them, as the European Cuckoo, and the North 

 American Cowbird. Among Fishes some small Ophi- 

 diums (Fierasfers) penetrate into the cavity of the body 

 of large Holothurise in which they dwell. 1 Echeneides 



Ilistoirc natnrollc ties vegetatix para- les animaux vivans ; Paris, 1853, 8vo. 

 sites qui croisseitt sur I'homiue et sur l See above, p. Ill, note I. 



