THE CLASSES OP AETHROPODA. 7 



beetle is an animal which has no inner skeleton or bones whatever, 

 but only a hard crust on the surface which surrounds the muscles 

 and vital organs. This crust is composed of separate rings, placed 

 end to end. 



Animals whose bodies are thus composed of rings are called Ar- 

 ticulata. They -are in turn divided into two great groups, the 

 Vermes and the Arthropoda. The Vermes (worms) have all the 

 rings composing the body very nearly alike, not hardened into an 

 outer crust or exoskeleton, and without paired limbs which are 

 jointed. The Arthropods have a part of the rings bearing paired 

 jointed appendages, and have the cuticle or outer surface consisting 

 largely of a peculiar substance called ' ' chitin, ' ' which is secreted or 

 exuded by the cells which compose the cuticle. Chitin itself is in- 

 soluble and is not composed of cells, but consists of fine, irregular 

 plates. It hardens the cuticle and thus aids the latter in protecting 

 the delicate vital organs within, and also in forming a framework 

 to which the muscles of movement may be attached. Between the 

 joints the cuticle is devoid of chitin and is thin, delicate and flex- 

 ible, thus allowing the necessary freedom of motion. 



The Arthropoda are divided into four classes, as follows : 



(a) Crustacea (crayfish, lobster, etc.), mostly aquatic ; having 

 the head and thorax usually united and distinct from the abdomen ; 

 breathing by means of gills or directly through the skin, the exo- 

 skeleton with carbonate and phosphate of lime in addition to chitin. 



(&) Aritchnida (spiders, mites, etc.), terrestrial; head and 

 thorax usually combined, and bearing four pairs of legs; breathing 

 by means of tracheae. 



(c) Mijriapoda (myriapods, centipedes, etc.), terrestrial; usu- 

 ally worm-like, with only the head distinct ; legs numerous ; breath- 

 ing by means of trachea^. 



{d) Insecta (grasshoppers, flies, beetles, etc.), in great part 

 terrestrial; legs six; adults usually Avith one or two pairs of wings; 

 breathing by a system of tubes called trachea?, which branch and 

 ramify through every portion of the Imdy. and which open exter- 

 nally in about ten places on each side of the body instead of at the 

 front end. The rings of the bddy are grouped in three regions; 

 the head, the thorax and the ahdmven. In general it may be said 

 that the head contains or bears the organs of sense and of pre- 

 hension and mastication of food ; the thorax the organs of locomo- 

 tion, and the abdomen those of reproduction. 



