and the Elements in which they live. 383 
that the ine 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 animals, 
and we, find that among them those which live as parasites 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 De- 
“9 
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 man 
orms and Crustacea, but also even of many of the Insects, espe- 
cially 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 fresh water 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 se I 
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 overrule the influence of the 
ia in which they are developed. 
However obseure theserelations may be among Crustacea owing 
to the parasitism of some of their types, or the peculiar meta- 
morphosis of others, if we now cofisider the insects proper we 
ll find here again a strict accordance with the results we have 
already derived from the investigation of the lower classes. Hav- 
ing acknowledged the superiority of the sucking insects over the 
chewing tribes, we cannot fail to perceive that Neuroptera, which 
must be considered as the lowé@st, inasmuch as their body still 
Preserves the elongated form of worms, are aquatic in their larval 
condition and have even external gills, as their respiratory organs 
during that period. Néxt, Coleoptera among which also we find 
