248 THE STUDY OF ANIMAL LIFE CHAP. 



sheath or cuticle of a material called chitin, and this 

 firm sheath explains the need for the limbs being well- 

 jointed. The chitin seems antagonistic to the occurrence 

 of ciliated cells, for none occur in this large series except 

 in the strange type Peripatus. Moulting or cuticle- 

 casting is general throughout the series, being necessi- 

 tated by the fact that the cuticle is non-cellular (and 

 often quite rigid) and cannot grow of itself. As the body 

 grows, it has to be cast and a new one made. Finally, 

 Arthropods have a nervous system like that of Annelids 

 -a double dorsal brain connected by a ring round the 

 gullet with a double chain of ganglia along the ventral 

 surface. But the life of most Arthropods is more highly 

 pitched than that of Annelids. The sense-organs are 

 more highly developed, brains are larger and more com- 

 plex, the ganglia of the ventral chain tend to become 

 concentrated ; there is division of labour among the 

 appendages ; there are new internal organs such as a 

 heart ; the whole body is better knit together. A cray- 

 fish may part with a limb and groAV another in its place, 

 but the animal will not survive being cut across the 

 middle as some kinds of Annelids do. 



The series includes several classes :- 



Crustacea, almost all aquatic, and breathing by gills. 



Onychophora, represented by Peripatus and other 

 genera. 



Centipedes or Chilopoda, Millipedes or Diplopoda, often 

 united as Myriopods. 



Insecta, more or less aerial. 



Arachnida, spiders, scorpions, mites, etc. 



Crustacea. Except the wood-lice, which live under 

 bark and stones, the land-crabs which visit the sea only 

 at the breeding time, and some shore-forms which live 

 in great part above the tidemark, the Crustaceans are 

 aquatic animals, and usually breathe by gills. Each 

 segment of the body usually bears a pair of appendages, 

 and each appendage is typically made up of a basal piece 

 (protopodite) and two branches (exopodite to the outside 

 and cndopodite toAvards the middle line). Among these 

 appendages much division of labour is often exhibited, 



