CHAPTER XIII. 



CRUSTACEA. 



Series Arthropoda : — Classes CRUSTACEA. Prototracheata. 

 — Peripatiis. Myriopoda. — Centipedes and Millipedes. 

 Insecta. Araciinoidea. — Spiders, Mites, Scorpions. 

 Pal/eostraca. King crab, Eurypterids, and Trilobites. 



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, 

 Peripatiis, 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 

 classes. Crustaceans, most of which are aquatic and 

 breathe by gills, are often opposed to the others (Tracheata), 

 most of which are terrestrial or aerial, and breathe by 

 trachese or air tubes, or possible modifications of these. 

 But the King crab (Ltmulus) is aquatic, and so were 

 the extinct Eurypterids and Trilobites. In other respects 

 these three types are very divergent, and as they have 

 been much bandied about from Crustaceans to Arachnoids, 

 it seems convenient to keep them in a separate class as 

 Palceostraca. 



