CHAPTER XIII. 



CRUSTACEA (First Class of the Arthropod Series). 



More than half the known species of animals are included 

 in the Arthropod series, 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, 

 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. But we cannot, as yet, do more 

 than recognise certain possibilities of pedigree. 



It is also difficult to discern the relationships of the 

 various classes included in the Arthropod series. Crusta- 

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

 often opposed to the others (Prototracheata, Myriopoda, 

 Insecta, and Arachnoidea), most of which are terrestrial or 

 aerial, and breathe by trachea?, or possible modifications of 

 these. But besides the classes named there are three 

 divergent types : — the King-crab (Limulus), and the extinct 

 Eurypterids and Trilobites. These have been much 

 bandied about from Crustaceans to Arachnoids, and it 

 seems convenient to keep them in a separate class as 

 Pakeostraca. 



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 



