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MANUAL OF ELEMENTARY ZOOLOGY 



mites, ticks, etc.), and Centipedes belong to this group. 

 Crustaceans have two pairs of antennae and are almost all 

 water animals, breathing by gills. Insects have one pair of 

 antennas and are usually land animals, breathing by air 

 tubes and provided with wings. They have six legs. 

 Arachnids have no antennas, nearly all breathe air by 

 various devices, and have no wings, but eight legs. The 

 Arthropoda are far more numerous than any other group 

 of animals, and are of the greatest importance to man, 



Fig. 172. — The Garden Spider 

 {Epeira diademata). — From 

 Parker and Haswell. 



Note, from behind forwards : abdomen 

 and prosoma ; on latter, four pairs 

 of legs, one pair of pedipalpi, one 

 pair of chelicera (barely shown). 



Fig. 173. — A diagram of a 

 vertical, longitudinal section 

 through a lung-book. 



a.s., Air space ; ./"., anterior end ; 

 h., hinder end; //., "leaves" 

 of book in which the blood flows ; 

 o., opening, on D.J., ventral sur- 

 face of body. 



partly because some of them serve him as food, but more 

 because they damage his crops, annoy him as parasites, and 

 in sucking his blood convey to him the germs of very 

 serious diseases. 



In spiders (Araneida) the first pair of limbs (chelicene) 

 are sharp poison-claws, and the second {pedipalpi) are tactile 

 organs. These and the four pairs of legs are borne upon 

 a so-called " cephalothorax " or prosoma. The animals 

 breathe by means of lung-books, which are pits of the wall 

 of the under side of the abdomen, containing a number of 

 leaflets in which the blood circulates and is thus exposed 



