188 ESSAY ON CLASSIFICATION. 



attach themselves to other fishes, but only temporarily. 

 Among the Articulata the number of parasites is largest. It 

 seems to lie in the very character of this type, so remark- 

 able for the outward display of their whole organization, 

 to include the greatest variety of parasites. And it is 

 really among them that we observe the most extraordinary 

 combinations of this singular mode of existence. 



Insects in general are more particularly dependent 

 upon plants for their sustenance than herbivorous animals 

 usually are, inasmuch as most of them are limited to par- 

 ticular plants for their whole life, such as the Plant-lice, 

 the Coccus, the Gall Insects. In others the larvae only 

 are so limited to particular plants, while the larvae of 

 others again, such as the Bots, grow and undergo their de- 

 velopment under the skin or in the intestines, or in the 

 nasal cavities of other animals. The Ichneumons lay their 

 eggs in the larvse of other insects, upon which the young 

 larvae prey until hatched. Among perfect Insects, there 

 are those which live only in community with others, such 

 as the Ant-Hill Insects, the Clavigers, the Cleri and the 

 Bees. Different kinds of Ants live together, if not as 

 parasites one upon another, at least in a kind of servitude. 

 Other Insects live upon the bodies of warm-blooded ani- 

 mals, such as the Fleas and Lice, and of these the number 

 is legion. Some Hydrachnas are parasitic upon aquatic 

 MoUusks. 1 



Among the Crustacea there are Crabs which constantly 



1 NITZSCH (CHR. L.), Darstellung vols. 4to., fig. CLARK (En.), Obser- 



der Familien und Gattungen der vations on the Genus Oestrus, Trans. 



Thierinsekten ; Halle, 1818, Svo. Lin. Soc., iii, p. 289, fig. KOCH (C. 



HAYDEN (C. v.), Versuch einer sys- L.), Die Pflanzen-Lause, Aphiden, 



tematischen Eintheilung der Acari- Niirnberg, 1846, 8vo., fig. DUGES 



den; Isis, 1826, p. 608. RATZEN- (ANT.), Recherches sur 1'ordre des 



BURG (J. S. C.), Die Ichneumoncn der Acariens, Ann. Sc. Nat., 2de ser., 



Forstinsekten ; Berlin, 1844-52, 3 1834, i, p. 5 ; ii, p. 18, fig. 



