THE GROUPS OF THE ARTHROPODA. 225 



In the air-breathing series no completely apodal forms are 

 known. The Tardigrada and the Pentastomida appear to 

 have no jaws ; but the presence of oral stilets in the former, 

 and the position of the hooks which represent the limbs in 

 the latter, throw some doubt upon this point. 



In the Arachnida and the Peripatidea the gnathites are 

 completely pediform. But in the Myriapoda, and still more 

 in the Insecta, the gnathites lose the character of legs, and 

 are completely converted into manducatory organs. Thus 

 we arrive at the following arrangement of the Arthropoda : 



Aetheopoda. 



I. Without Gnathites. 



Trilobita. Tabdigbada (?) Pentastomida (?) 



II. With Pediform Gnathites. 

 Meeostomata. Aeachnida. Peeipatidea. 



III. With Maxilliform Gnathites. 

 Entomosteaca. Myeiapoda. 



Malacostbaca. Insecta. 



"Water-breathers. Air-breathers. 



For the most part. 



Of the four great groups, the Crustacea are those which 

 present the greatest and the most instructive variations upon 

 the fundamental type of structure; while the modi6cations 

 of the Insecta, Arachnida, and Myriapoda, are less exten- 

 sive, and may be regarded as of secondary morphological im- 

 portance. The Crustacea will, therefore, be treated of at 

 some length, while the other groups will be passed over more 

 lightly. 



THE CRUSTACEA. 



The Trilobita. — These ancient Arthropods, which have 

 been extinct since the latter part of the Palaeozoic epoch, oc- 

 cur in the fossil state in great numbers, and in conditions 

 very favorable for their preservation ; but, up to this time, no 

 certain indications of the existence of appendages, nor even 

 of any hard, sternal body-wall, have been discovered, though 



