106 RAMBLES OF A NATURALIST. 



exhibits two exactly symmetrical lateral halves of 

 the body. We will now proceed to consider the 

 limits within which these conditions of the ideal 

 type are either modified or fully accomplished. 



The first glance we take at this group of the 

 Articulata shows that it is separated into two great 

 divisions. In the former, namely, that of true Arti- 

 culata with jointed limbs, the rings instead of forming 

 a string of separate segments are united and soldered 

 together, to form groups of organs. The body of 

 the animal is thus divided into three parts, repre- 

 senting the three great regions of the body in the 

 Mammalia, that is to say, the head, thorax, and 

 abdomen. These three parts of the body, which are 

 always very distinct in insects, may to some extent 

 blend into one another. Thus, in the Myriapods or 

 Millepedes, the thorax and abdomen can no longer 

 be severally distinguished ; whilst in the case of the 

 Arachnidans, which include all the spiders and the 

 scorpions, it is the head which is amalgamated with 

 the thorax. These three classes respire atmospheric 

 air, while Crustaceans (lobsters, crabs, &c.) are 

 essentially aquatic. We refer here only to a very 

 small number of characters, but how different would 

 the case be, if, penetrating into the interior, we could 

 comprehend the whole of these organisms ! We 

 should find that in one case air, that fluid without 

 which no living being can exist, is diffused through- 

 out the entire body by means of an admirable net- 

 work of tracheae, or canals, whose structure is 

 almost exactly similar to that of an elastic webbing*, 



* True Tracheae are only to be met with in Insects, Myriapods, 

 and one section of Arachnidans. 



