/.OOLOGY FOR MEDICAL STUDENTS CHAP. 



the Cr lie extreme hinder end of the body may be marked off 



i nun : > a telson (Figs. 98, 99, 102, t) : in the Scorpion this 



and encloses a poison gland which opens close to its tip 

 tin- whole constituting a hypodermic syringe for the injection of venom. 



Some of the most characteristic features of the Arthropods are 



.-i-d with the great development of the cuticle over the surface of 



the body. This has, as already mentioned, become greatly thickened 



while i me hardened and stiffened by its conversion into chitin, 1 



which in the case of the Crustacea is rendered still harder and more rigid 



\ becoming infiltrated by calcium carbonate. 



This development of the cuticle would naturally tend to interfere 

 with two of the most important vital activities of the animal (I) move- 

 ment and (II) growth, and some of the most striking secondary charac- 

 teristics of the group have their functional significance in the eliminating 

 or at least counteracting this interference. 



I. Tin 1 increase in hardness and thickness of the cuticle is not con- 



FIG. 96. 

 Section through joint of an arthropod's exoskeleton. 



tinuous over the whole surface. Along certain lines it remains thin and 



flt-xiMr. and these portions of cuticle are folded in below the general 



level (Ki-. 96). The result of this arrangement is that the rigid armour 



is divided hy joints at which the edges of the rigid portions can approach 



"in one another, and in this way flexibility is given to the 



\\hole and movement rendered possible. The degree to which this 



jointing t the exoskeleton is carried out is directly related to (i) the 



thi< knc- and hardness of the exoskeleton and (2) the need for flexibility 



particular j.art of the body. Thus it is specially marked in the 



tl- limlis, and the jointed character of the limbs is regarded as 



1 haracteristic of the phylum that it has been made use of 



hnirul name Arthropoda. 



II . I hiriiur Hie earlier part of an animal's life, as it progresses towards 



n.lition, it as a rule undergoes (i) a gradual increase in size 



Wfcal change of form. In the typical Arthropod both of 



interfered with 1,\ the presence of the rigid exoskeleton. 



M in 01 her groups, the amount of living substance in the 



lual increase during the earlier stages of the life- 



1 See below, p. 216. 



