96 Invertcbrata. 



c. Creeping by the foot or swimming, but 

 not by fin-like processes = Sub-class 

 III. Gasteropoda. 



C. Head-bearing, with the fore-shortened foot around 

 the head, and modified either into tentacles 

 or sucker-bearing arms = Class III. Cephalo- 

 poda, 



CHAPTER XVIII. 



STTB-KINGDOM VII. ARTHROPODA (jOINTED ANIMALS). 



General Characters. This sub-kingdom includes 

 those animals whose bodies are furnished with an ex- 

 FIG ternal hard protective layer, and which 



bear jointed limbs appended to each 

 segment of the body (fig. 55). The 

 armour-plating of the body is known 

 by the name exoskeleton, to distinguish 

 it from the bones, which form the axis 

 of support for vertebrate animals, to 

 San TLiUms. r which the name endoskeleton is given, 

 The exo-skeleton consists of chitin, a 

 horny substance, which is capable of resisting all re- 

 agents except the most powerful corrosives. This 

 layer is usually coloured, laminated, and to the micro- 

 scope shows very little structure except the numerous 

 fine canals which pierce it from within, to which the 

 name pore-canals is given. 



The body of an arthropod consists of a chain 

 ot segments, all built on a common pattern, and each 



