108 



CHARACTERS OF ARTHROPODS. 



the head. Eunning down the ventral surface of the thorax 

 and abdomen is the ventral nerve-cord, which is double. This 

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

 sures in each of the three thoracic segments (fig. 43, cl, 3-5), 

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

 visceral nervous system (fig. 41, Ve). 



rss: 



H< 



Flc. 43. — Nervous System. 



u, 8upra-(i.-sophageal g.anglion ; b, nerve-collar ; c, sub-ttaophagi'al ganglion ; 

 d, 3-5, thoracic ^^anglia ; i.-vii., abdominal ganglia. 



Characters of the four Grouts 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 tlie Lobsters, Crabs, Shrimps, 



Prawns, Crayfish, Wood-lice, &c. They are nearly all water-breathing 

 Arthropods, respiring by means of branchite or gills. There are always 

 two pairs of antenna) and compound or facetted eyes. On the abdomen 

 are organs of locomotion. 



2. Arachrwidca — These are the Spiders, Red Spiders, Scab Mites, and 



Scorpions. Respiration is various : some respire by lungs, others by 

 tracheae, 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. Antenna? 

 are never present. There are always four pairs of legs (except in 

 Eriophyida)), which are never carried by the abdomen. Eyes never 

 compound, ocelli only present. 



