THE CLASSES OP ARTHROPODA. 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 .1 r- 

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

 }'< nncs and the Artliropoda. The Venues (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 v' i ' the muscles of movement may be attached. Between the 

 joint, 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) Cri'sfacca (crayfish, lobster, etc.), mostly aquatic; having 

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

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

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



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

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

 by means of trachea?. 



(i .:. Myriapoda (myriapods, centipedes, etc.), terrestrial; usu- 

 ,,:' -v -mi-like, with only the head distinct; legs numerous; breath- 

 : i . ny means of trachea 3 . 



(('!'} Insccta (grasshoppers, flies, beetles, etc.), in great part 

 : ; !,-t ' : legs six ; adults usually with one or t\vo pairs of wing.-; ; 

 breathing by a system of tubes called trachca j , which branch and 

 IT .lify thi'iiiigh every portion of the body, and which open exter- 

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

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

 the >!(<s-.l, the th.ora.r- and the alxlonx H. In general it may be said 

 that t);i ii";id contains or bears the organs of sense and of pre- 



; 



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

 tion, .^r^ t'.i iibdomen those of reproduction. 



