CHAKAOTERS OF AKTHKOPODS. 



97 



chain is swollen out into a pair of ganglia united by com- 

 missures in each of the three thoracic segments (fig. 36, d, 3-5), 

 and a pair in the first six abdominal segments. There is also a 

 visceral nervous system (fig. 34, Ve). 



Fig. 36. — Nervous System. 



a, Supra-oesopbageal ganglion ; 6, nerve-coUar ; c, sub-oesophageal ganglion ; 

 d, 3-5, thoracic ganglia ; i.-Tii., abdominal ganglia. 



Characters op the four Groups of Arthropods. 



The following are the distinctive features of the four divisions 

 of the jointed-limbed animals : — 



1. Crustacea — This is the group that includes the Lobster, Crab, Shrimp, 



Prawn, Crayfish, Wood-louse, &c. They are nearly all water-breath- 

 ing Arthropods, respiring by means of branchise or gills. There are 

 always two pairs of antennae and compound or facetted eyes. On the 

 abdomen are organs of locomotion. 



2. Arachnoidea — These are the Spiders, Mites, and Scorpions. Eespiratiou 



is various : some respire by lungs, others by trachese, yet others by the 

 whole body surface. The head and thorax are always united into one 

 piece, the cephalothorax, and in some (Mites) the abdomen is also 

 united to the cephalothorax. Antennae are never formed. There 

 are always four pairs of legs (except in Phytopti), which are never 

 carried by the abdomen. Eyes never compound, ocelli only being 

 present. 



3. Myriapoda — Centipedes and Millipedes. Head quite separate from the 



body, but no distinction between thorax and abdomen visible. One 



G 



