THE LIVING WORLD 235 



which embraces all insects, hundred and thousand legs, 

 scorpions, spiders, tics, and inites, all lobsters, crayfish, 

 crab and shrimp-like creatures. As an example, we may 

 select the crayfish, the body of which is evidently in 

 part composed of a longitudinal series of similar seg- 

 ments, while numerous pairs of lateral appendages 

 successively appear (from before backwards) as feelers, 

 jaws, claws, feet, and swimming paddles. 



There are two conspicuous eyes, each borne on a stalk. 

 A nervous system, composed of longitudinal bands and 



FIG. 49. 



CRAYFISH (Actacus fluviatiUs). 



ganglia, runs along the body inside its lower or ventral 

 surface, whereas in vertebrates it is situated in the 

 dorsal region or back. It is there, in the crayfish, 

 that the heart, a single-chambered organ, is placed, 

 whence blood-vessels proceed, the blood being purified 

 by the aid of gills which project upwards from the bases 

 of the legs. In this form of body, serial symmetry is 

 carried to a far greater extent than in vertebrate 

 animals, while bilateral symmetry is no less obvious. 



The class of insects are remarkable for their power of 

 flight; but their wings have no resemblance, save as 

 regards the function they perform, to the wings of 

 vertebrate animals, whether bats or birds. Insects 



