308 TEXT-BOOK OF ZOOLOGY 



Most of these animals, however, are terrestrial in habit, and therefore 

 obliged to carry the weight of their own body (contrast with aquatic 

 animals) ; many others must be capable of nimble and rapid movement 

 in order to be able to capture their prey (contrast with parasites) ; whilst, 

 finally, one whole large division, the insects, are capable of raising their 

 bodies into the air by flight. In all, consequently, the body needs a 

 certain amount of firmness or solidity and the muscles firm points of 

 support. Both of these needs are provided for by the presence of an 

 envelope, sometimes more or less soft {e.g., in spiders), but sometimes 

 very hard {e.g., many insects), which covers all the parts of the body, 

 and which is hence aptly described as a cuticular skeleton (see also 

 illustrations on pp. 311, 312). Moreover, in the ease of animals, such 

 as nearly the whole host of insects, whose life is passed almost entirely 

 in the air and sunshine, this skeleton further provides protection 

 against desiccation (see Part II., pp. 230, 253) ; whilst, finally, in all it 

 serves to protect the internal parts of the body. This skeleton is com- 

 posed of a horny substance called chitin, which is very resistant to decay 

 (as may be seen in insect collections), and, being very light, does not 

 seriously increase the body weight. 



The animals are thus cased in armour, like the knights of old. This 

 armour, however, must not impede their free movements (why?), and 

 therefore, instead of forming one single rigid mass, must, like knightly 

 armour, consist of separate portions capable of sliding upon each other. 

 These portions are, from their shape, described as rings (segments). 

 (Compare with the articulation of the skeleton in vertebrates and the 

 body integument of most reptiles.) These rings, however, are not com- 

 pletely separated from each other, but are connected by portions of the 

 chitinous skeleton which envelops the whole body. In order that the 

 segments should be movable, it is therefore necessary that these 

 connecting portions should remain soft and pliant (articulating mem- 

 branes). In this way joints are formed (contrast with the joints of 

 vertebrates). 



The internal parts (muscular and nervous system and respiratory 

 organs) of the body likewise exhibit a segmentation corresponding to 

 that of the external portions (somites). In regard to the coalescence of 

 separate rings into larger body segments, see under the separate classes, 

 and especially p. 311. 



2. Limbs. — Bach body-segment may carry {e.g., in myriapods) on its 

 under surface a pair of movably articulated limbs which may be employed 

 for the most various purposes (examples). Several of the limbs in the 

 adult animal almost always function as locomotor organs or legs. 

 However, if the chitinous envelope which envelops the legs (as it does 



