and the Elements in which they live. 167 



tamorphosis, and assuming finally an aerial 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 he- 

 sitation in placing the class of insects highest among Articulata, 

 and in comprising in one class the true insects with the Arach- 

 nida and Myriopoda, which are only lower degrees of develop- 

 ment 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 spi- 

 ders with their cephalic and thoracic rings united into a cephalo- 

 thorax representing their chrysalis in a permanent state of deve- 

 lopment ; and the true insects, with their three distinct regions, 

 the so-called head, thorax and abdomen, ranking highest among 

 them, as well for then* more extensive metamorphosis as for the 

 characteristic division of the body, the reduction of their loco- 

 motive appendages to a peculiar region, the complication of their 

 chewing apparatus, and the development of their wings. The 

 true arrangement of the difierent members of this class however 

 is readily indicated by the remarks already made upon this class, 

 and we shall not hesitate to consider the Myriopoda as their 

 lowest type, and to place the 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 ai*e 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 accumulating ; and these two classes 

 we have seen to be the lowest of the type, while the class of in- 

 sects, which in their perfect development are all terrestrial or 

 aerial, constitute the highest type. 



Reviewing the 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 the 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 faiviatile De- 

 capods which cannot be considered as higher than the crabs, un- 



