CHAPTER XIII. 



PHYLUM ARTHROPODA. 



Chief Classes CRUSTACEA, PROTOTRACHEATA, MYRIOPODA, 

 INSECTA, ARACHNOIDEA, PAL^OSTRACA. 



MORE than half the known species of animals are included 

 in the Arthropod phylum, for of insects alone there are said 

 to be more species than of all other animals taken together. 



The Arthropods are in some ways like Annelids in the 

 bilateral symmetry; in the division of the body into successive 

 segments, some or all of which bear appendages ; in the 

 plan of the nervous system ; and so on. Furthermore, 

 Peripafus, which has air-tubes or tracheae somewhat similar 

 to those of Myriopods and Insects, has nephridia like those 

 of some Annelids ; and the biramose appendages of a 

 simple Crustacean like Apus may be compared with the 

 parapodia of an Annelid. 



It is difficult to discern the relationships of the various 

 classes included in the Arthropod phylum. Crusta- 

 ceans, most of which are aquatic and breathe by gills, are 

 often opposed to the Prototracheata, Myriopods, Insects 

 and Arachnoids, most of which are terrestrial or aerial, 

 and breathe by tracheae, or possible modifications of these. 

 Three divergent groups the King-crabs (Limuhis\ and the 

 extinct Eurypterids and Trilobites may be conveniently 

 referred to a separate class Palreostraca. 



General Characteristics of Arthropods (to which primitive, 

 parasitic, and degenerate forms present exceptions). 



77/6' body is bilaterally s\m metrical, and consists of numerous 

 segments variously grouped. Several or all of the segments 

 dear paired jointed appendages variously modified. The 



