chaptp:r XIII 



PHYLUM ARTHROPODA 



Chief Classes — Crustacea, Prototracheata, Myriopoda, 

 Insecta, Arachnoidea, Pal/eostraca 



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. Further- 

 more, Peripatus, 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 Kiug-crabs (Limulus), and 

 the extinct Eurypterids and Trilobites — may be con- 

 veniently referred to a separate class — Palaeostraca. 



General Characteristics of Arthropods (to which 

 primitive, parasitic, and degenerate forms present 

 exceptions) 



The body is bilaterally symmetrical, and consists of numerous 

 segments variously grouped. Several or all of the segments 



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