ARTHROPODA — ARACHNIDA 309 



CLASS C, MYRIOPODA 



Body worm-like, of many similar segments, each bearing one 

 (in centipedes) or two (in millipedes) pairs of legs (whence the 

 name from Greek myrioi, 10,000, -\- pons {pod), foot). Skin 

 stiffened with chitin, legs jointed. Head distinct, bearing a 

 pair of eyes. Respiration by tracheae like those of insects. 

 Found from the Devonian to the present day. 



The Myriopoda include {a) the centipedes {e.g. Lithobius, 

 of world-wide distribution, living on earth worms, insects, etc.); 

 {b) the millipedes, {e.g. Julus, the thousand-legged or galley- 

 worm, carnivorous like Lithobius ; when at rest it coils its body). 



CLASS D, ARACHNIDA 



Body usually divided into cephalothorax and abdomen ; 

 six pairs of legs (except in order Limulava) about the mouth, 

 four of which at least are used in walking, the first usually bear- 

 ing claws. No antennae present (except in Limulava) ; eyes 

 sessile, usually simple ; in orders i and 2 both simple and 

 compound eyes occur, in order 3 compound eyes only. Respi- 

 ration by tracheae (see page 317), book-lungs, or book-gills. 



In the hook-lung each opening leads into an internal chamber, 

 the lining membrane of which has been folded into very many 

 delicate laminae, lying parallel with one another like the leaves 

 of a book. The air circulates between the leaves, thus giving 

 oxygen to the blood which circulates through each leaf. Book- 

 gills are similar to book-lungs, but are external, attached to 

 the abdominal appendages. 



Relationship. — The class Arachnida typically includes a 

 water-breathing division, — the orders Xiphosura, Eurypterida 

 and Limulava (often placed together in the sub-class Mero- 

 stomata) and an air-breathing division — the remaining orders. 

 That these two divisions are intimately related is best shown in 

 comparing Limulus, a water-breather, with Scorpio, an air- 

 breather. These agree in the possession of the following char- 



