7° 



FIRST LESSONS IN ZOOLOGY 



the use made of it but in its arrangement and in the part 

 of the body from which it is mainly developed. The 

 skeleton of the invertebrate is developed from the skin, 

 and forms a hard casing over the whole or part of the 

 body. It is therefore called an exoskeleton. How this 

 outside skeleton serves as levers, fulcra, and points of at- 

 tachment may well be seen in the case of insects (fig. 38), 



Fig. 38.— Left middle leg 

 of cockroach, with exo- 

 skeleton partly re- 

 moved, showing mus- 

 cles. (Much enlarged; 

 after Miall and Denny.) 



v.n.c. 



Fig. 39. — Diagram of cross-section through 

 the thorax of an insect to show the muscles 

 of the wings and legs; /;, heart; a/.c, 

 alimentary canal; t'.n.c, ventral nerve 

 cord; lu, wing; /, leg; tit, muscles. 

 (Much enlarged; after Graber.) 



or crayfishes. The wall of the thorax, or carapace 

 (fig. 39), is the central strong portion from which the great 

 pulls are e.xerted, while each joint of a leg is a lever, a 

 fulcrum fr^r the next part, and a point of attachment for 

 muscles. The whole system of muscles is so arranged 

 inside the exoskeleton that the flying, crawling, and 

 swimming of the various animals, as well as the particular 



