298 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



likewise, in large part, external, so that while the 

 endoskeleton of Vertebrates is characterised by having 

 the -muscles external to it, the exoskeletal parts of 

 an Arthropod are moved by muscles that lie inter- 

 nally to them. The chitinous skeleton may be thin 

 and soft, as in the simpler Eiitoinostraca, or 

 the parts of the different somites may fuse, to 

 form, as in the Crayfish, a firmer cephalothoracic 

 carapace ; or, as in the Ostracoda and other Ento- 

 mostraca, give rise to two more or less dense lateral 

 valves ; in other cases certain parts may become 

 very strong and thick, as is the case with the elytra, 

 or wing-covers, of the Coleoptera (Beetles). The 

 chitinisation of the epithelial layer is not confined to 

 the surface, and, just as in Echinoderms, spicules or 

 plates may be found in the walls of the digestive 

 tract, or in the generative glands ; so, too, chitin may 

 invade the stomodeeal and proctodseal portion of the 

 alimentary tract of Arthropods, or give rise to a 

 definite series of internal supporting pieces, the 

 endosternites. 



These chitinous layers are not formed of cells, but, 

 like the cuticle of a protozoon, are shed out by cells, 

 which they invest with a continuous layer ; the layer 

 is often seen to be laminated, or made up of a number 

 of superimposed secondary layers, laid down in suc- 

 cession. They are traversed by vertically-running pore- 

 canals, and are often strengthened, especially in the 

 Crustacea, by the deposition of calcareous salts.* A 

 firm outer coating of this kind, moulded to the form 

 of the body, would speedily limit the growth of an 

 Arthropod, were it not for the process of shedding, or 

 exuviation, which obtains during growth, and much 

 more frequently in young and rapidly-growing forms 

 than in those which have attained to their full size. 



7 In the Crayfish more than half of the whole weight of the 

 exoskeleton is due to the presence of calcareous salts. 



