310 TEXT-BOOK OF ZOOLOGY 



of a simple tube, of varying shape and size (see illustration, p. 308), by 

 ■which the blood is driven through the body. It always lies above the 

 intestinal canal, on the dorsal side of the body, so that in the transparent 

 larvae of aquatic insects, in the transparent lower crustaceans (e.g., water- 

 fleas), and in many hairless caterpillars and maggots, its movements 

 may be seen through the body wall. The blood flows through the body 

 slowly, and owing to the simple structure of the heart the arterial and 

 venous blood are mixed. Consequently assimilation proceeds at a low 

 rate, and only small quantities of heat are liberated. This explains why 

 arthropods are animals of varying body temperature, whose vital activities 

 are dependent in a high degree on the heat of the sun. (Discuss this 

 more fully ; see Part II., p. 229.) Hence arthropods do not, like warm- 

 blooded birds and mammals, require any special provision for the conserva- 

 tion of heat. (For the course of the circulation through the body see 

 insects and crustaceans.) 



5. Respiration. — In those Arthropods whose life is exclusively or 

 principally passed in the air, respiration is effected by means of air-tubes 

 or trachea (see insects) ; in those living chiefly in the water, by means of 

 gills or branchiae (see crustaceans). 



6. Reproduction. — By far the larger number of Arthropods are 

 oviparous. The young, as a rule, differ in their structure markedly from 

 the parent ; they are, in fact, larvce (see Part II., p. 254), and only attain 

 to the form of the parent animals after a more or less prolonged meta- 

 morphosis. 



The chitinous envelope, which covers the whole body, being in- 

 elastic and incapable of undergoing enlargement, it follows that the 

 growth and metamorphoses of the animal must be accompanied by 

 repeated castings of the skin, or moults (the chitinous skeleton being by 

 turns cast off and renewed). 



CLASS I. : INSECTS (INSECTA). 



Aeticulatb animals breathing by air-tubes (tracheae) ; body com- 

 posed of three distinctly separated divisions (head, thorax and abdomen) 

 with a single pair of antennas ; three pairs of oral appendages, three 

 pairs of legs, and generally two pairs of wings. 



1. Divisions of the Body.— As in all the Arthropoda (see p. 308), 

 the body of an insect consists of a number of rings or somites placed 

 one behind the other (hence the name insects, i.e., cut or divided). 

 From the following considerations, however, it will appear that these 

 somites cannot all be of uniform shape. The mouth is placed at the 



