212 ZOOLOGY FOR MEDICAL STUDENTS chap. 



the Crustacea the extreme hinder end of the body may be marked off 

 from the rest as a telson (Figs. 98, 99, 102^ /) : in the Scorpion this 

 forms the sting and encloses a poison gland which opens close to its tip — 

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



Some of the most characteristic features of the Arthropods are 

 associated with the great development of the cuticle over the surface of 

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

 while it has become hardened and stiffened by its conversion into chitin,i 

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

 by its 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. The increase in hardness and thickness of the cuticle is not con- 



FlG. 96. 

 Section through joint of an arthropod's exoskeleton. 



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

 flexible, and these portions of cuticle are folded in below the general 

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

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

 or recede from one another, and in this way flexibility is given to the 

 whole and movement rendered possible. The degree to which this 

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

 thickness and hardness of the exoskeleton and (2) the need for flexibility 

 in the particular part of the body. Thus it is specially marked in the 

 case of the limbs, and the jointed character of the limbs is regarded as 

 so sahent a characteristic of the phylum that it has been made use of 

 in giving the group its technical name Arthropoda. 



II. During the earlier part of an animal's life, as it progresses towards 

 the adult condition, it as a rule undergoes (i) a gradual increase in size 

 and (2) a gradual change of form. In the typical Arthropod both of 

 these processes are interfered with by the presence of the rigid exoskeleton. 

 Although, as in other groups, the amount of living substance in the 

 body undergoes a gradual increase during the earlier stages of the life- 



' See below, p. 216. 



