212 L. Agassiz on the Natural Relations between 



habit of leaving the water to dwell upon the mainland. 

 The occurrence of parasitic Crustacea upon fresh-water 

 fishes again seems to indicate that here the parasitism 

 prevails over the influence of the surrounding media ; 

 and we should not wonder at this circumstance, as a parasi- 

 tic mode of development 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 Arach- 

 nida and Diptera, at least in some earlier periods of their 

 existence. In this connection it is an interesting fact to 

 notice that the American fresh-water Crustacea, the craw- 

 fishes, have fewer pairs of gills than the other representa- 

 tives 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 so also among 

 the reptiles where the peculiarities of the primary divisions 

 overrule 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 pecu 

 liar 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 Neuroptera, which must be considered as 

 the lowest, inasmuch as their body still preserves the elon- 

 gated form of worms, are aquatic in their larval condition 

 and have even external gills, as their respiratory organs 

 during that period. Next, 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 Hymenoptera have 

 a more diversified metamorphosis, and assume even in their 

 larval condition in some of their types, the higher forms 

 which characterise the larvse of Lepidoptera. 



