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 suc- 

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

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

 PcripatuS) 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. Crustaceans, 

 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 (Limulus\ and 

 the extinct Eurypterids and Trilobites may be conveniently 

 referred to a separate class Palseostraca. 



General Characteristics of Arthropods (to which primitive, 

 parasitic, and degenerate forms present exceptions) 



The body is bilaterally symmetrical, and consists of numer- 

 ous segments variously grouped. Several or all of the segments 



