RELATIONS AND DIVISIONS OF INSECTA. 3 



from side to side always, and not up and down, as in the 

 Vertebrata. 



There is no head-brain in this section, but a series of 

 nervous ganglia (or "depots"), connected by thin double 

 cords of nerves running along the abdominal surface, 

 and giving off nervous radiations on each side. In the 

 lowest conditions there is a separate ganglion to each 

 segment (connected as above), so that there is a centre 

 of vitality in every division of the body, hence the mar- 

 vellous tenacity of life in worms, etc., and the capability 

 of reproducing limbs, when mutilated, in the other 

 classes. From the circumstance of the nervous cords 

 running along the ventral surface in the *Articulata, they 

 have been said to walk on their backs, as the spinal co- 

 lumn is dorsal in all the Vertebrata. The classes of 

 Articulata are the Crustacea, Araclmida, Insecta, My- 

 riapoda, and Annelida. 



The Crustacea have a distinct heart and white blood ; 

 they breathe through branchiae or branchial plates, ge- 

 nerally situated at the base of the legs or lower jaws ; 

 their body is covered with an integument, varying from 

 an earthy hardness to a leathery texture, and in the for- 

 mer case, composed chiefly of carbonate of lime; they 

 have jointed limbs, and are invariably wingless ; and the 

 head, nearly always merged in the thorax, has four ari- 

 tennse and two mandibles, with other jaws varying in 

 number, often ten. Some have but one eye, the rest 

 only two (which are frequently elevated on a retractile 

 foot-stalk), and the legs are generally ten, but sometimes 

 more ; finally, the sexes are distinct, in which they differ 

 from some Mollusca. The Crab, Lobster, Cray-fish, 

 Prawn, Shrimp, Wood-louse, and Brine-shrimp, are 

 well-known members of this class, which has inhabitants 

 of the sea, fresh water, and dry land. B 2 



