382 Agassiz on the Relations between Animals 
in their parasitic mode of existence so extravagant forms, and an - 
to the relative position of Crustacea among Articulata ; they rank 
higher than worms; though they must be placed below the in- 
sects notwithstanding their perfect circulation and their otherwise 
highly developed structure ; for, in every respect, insects consid- 
ered as a whole class, are more highly organized, their higher 
types assuming a division of the body into three distinct regions ; 
—undergoing also far more extensive metamorphosis, and assum- 
ing finally an aérial mode of respiration, to which the Crustacea 
do not reach. For these reasons, which I have illustrated more 
fully on another occasion, I have no hesitation in placing the 
class of insects highest among Articulata, and in comprising in one 
class the true insects with Arachnida and Myriopoda, which are 
only lower degrees of development of the more special types of 
true insects; the Myriopoda representing in a permanent state of 
development, and with the structure of true insects, the form of 
their caterpillars ; the spiders with their cephalic and thoracic rings 
united into a cephalo-thorax representing their chrysalis in a 
permanent state of development; and the true insects with their 
three distinct regions, the so-called head, thorax and abdomen, 
ranking highest among them, as well for their more extensive 
metamorphosis as for the characteristic division of the body, the 
reduction of their 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 indicated by the remarks already 
made upon this class, and we shall not hesitate to consider Myri- 
opoda as their lowest type, and to place Arachnida next above 
them, and then true Insects, 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 on the bark 
where moisture is constantly accufmulating ; and these two classes 
we have seen to be the lowest of the type, while the class of 
insects, which, in their perfect development, are all terrestrial or 
aérial, constitute the highest type. 
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