Animah and the Elements in which they Live. 211 



ranking highest among them, as well for their more ex- 

 tensive metamorphosis as for the characteristic division of 

 the body, the reduction of fcheir locomotive appendages to a 

 peculiar region, the complication of their chewing apparatus, 

 and the development of their wings. The true arrangement 

 of the different members of this class, however, is readily in- 

 dicated by the remarks already made upon this class, and we 

 shall not hesitate to consider Myriopoda as their lowest type, 

 and to place Arachnida next above them, and then true in- 

 sects, among which the sucking tribes rank highest. 



If we now consider the connection of these three classes 

 with the elements in which they are developed, and in which 

 they permanently live, we cannot fail to be struck with the 

 fact that two of their classes are either parasites or entirely 

 aquatic, for even the terrestrial worms live in moist ground, 

 or oh the bark where moisture is constantly accumulating ; 

 and these two classes we have seen to be the lowest of the 

 type, while the class of insects which, in their perfect deve- 

 lopment, are all terrestrial or aerial, constitute the highest 

 type. 



Reviewing tl^ secondary groups of all these classes also 

 in the same connection, we find that the lowest of all not 

 only live in a fluid medium, but require the existence of other 

 animals in whose cavities they find shelter and means of 

 subsistence ; and, among those which have an independent 

 mode of life, we find that the marine worms are probably 

 lower than the fluviatile and terrestrial, — at least, if the view 

 expressed above respecting the relative position of Lumbrici 

 and branchiate Annelides be correct. 



In the class of Crustacea we have exclusively aquatic ani- 

 mals, and we find that, among them, those which live as para- 

 sites upon other animals rank lowest. The distinction, 

 however, between fluviatile and marine types in this class 

 does not seem to be in strict accordance with their gradation, 

 for we have fluviatile Decapods which cannot be considered 

 as higher than the crabs, unless it were shewn that the 

 shortened body of the Brachyural Decapods is the result of 

 a retrograde metamorphosis, which I am not, however, 

 inclined to suppose, as we have some crabs which are in the 



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