168 M. Agassiz on the Relations between Animals 



less it were shown tliat the shortened body of the Brachyural 

 Decapods is the result of a retrograde metamorphosis, which I 

 am however not inclined to suppose, as we have some crabs 

 which are in the habit of leaving the water to dwell upon the 

 main land. The occurrence of parasitic Crustacea upon fresh- 

 water fishes, again, seems to indicate that here the parasitism pre- 

 vails over the influence of the surrounding media ; and we should 

 not wonder at this circumstance, as a parasitic mode of develop- 

 ment dependent upon the prior existence of organized beings is 

 not only a prominent feature in the mode of existence of so many 

 Worms and Crustacea, but also even of many of the Insects, 

 especially of the tribe of Arachnida and Diptera, at least in some 

 earlier periods of their existence. In this connection it is an in- 

 teresting fact to notice that the American freshwater Crustacea, 

 the craw fishes, have fewer pairs of gills than the other represen- 

 tatives of the class. 



Again, it may be, that to appreciate truly natural relations of 

 this type of animals, it will be necessary to consider separately 

 each of their minor divisions rather than the whole class as a 

 unit ; as we shall have to do also among the reptiles, where the 

 peculiarities of the primary divisions overi'ule the influence of the 

 media in which they are developed. 



However obscure these relations may be among Crustacea, 

 owing to the parasitism of some of their types, or the peculiar 

 metamorphosis of others, if we now consider the insects proper 

 we shall find here again a strict accordance with the results we 

 have already derived from the investigation of the lower classes. 

 Having acknowledged the superiority of the sucking insects over 

 the chewing tribes, we cannot fail to perceive that the Neuro- 

 ptera, which must be considered as the lowest, inasmuch as their 

 body still preserves the elongated form of worms, are aquatic in 

 their larval condition and have even external gills, as their respi- 

 ratory organs during that period ; next the Coleoptera, among 

 which also we find aquatic larvae, and a number of terrestrial types ; 

 and highest the Orthoptera, which undergo a less extensive, but 

 entirely terrestrial development, whilst the Hymenoptera have a 

 more diversified metamorphosis, and assume even in their larval 

 condition in some of their types, the higher forms which charac- 

 terize the larvae of Lepidoptera. 



Among the sucking insects we begin again with various aquatic 

 types or aquatic larval forms, — next rise to the Diptera with other 

 aquatic larval conditions but a constant aerial mode of life in the 

 perfect state, and finally to the type Lepidoptera, in which all larvae 

 are terrestrial, and even highly organized in their earliest state 

 in the higher groups ; so that the class as a whole does not only 

 rank above the Crustacea for its structure, but consists chiefly of 



