CHAPTEE XVI 



ARTHROPODA 



Chaeactekistics. — Bilaterally symmetrical Coelomata, with 

 a chitinous exosheleton. Their body is segmented hetero- 

 nmrwusly. The segments usually hear a pair of jointed 

 appendages, those in the neighhourhood of the moidh are 

 TTwdified in connection with the prehension and mastication 

 of the food. The nervous system consists of a train or 

 sv/pra-oesophageal ganglion, a ring round the oesophagus, and 

 a ventral, usually segmented, nerve cord. A heart is 

 typically present dorsal to the alimentary canal, blood enters 

 it through a series of lateral ostia ; the coelom is reduced, and 

 to some extent replaced hy a hasmocoel. The sexes are typi- 

 cally distinct, and the paired genital glands usually open hy 

 paired ducts. Cilia are universally absent from the group, 

 with the single exception of Peripatus. 



The Arthropoda may be divided into two large groups, 

 according to the nature of the breathing organs : (i.) the 

 Branchiata, which breathe by gills and are typically aquatic ; 

 and (ii.) the Teacheata, which breathe by tracheae or lung 

 books, and are tj^ically terrestrial. 



The Branchiata include but one class, the Crustacea. 

 The Tracheata include four : (i.) the Prototracheata, (ii.) the 

 Myriapoda, (iii.) the Insecta or Hexapoda, and (iv.) the 

 Arachnida. The first three classes may be grouped together 

 as the Antennata, and opposed to the last class, the 

 Arachnida. 



