TlIK CIjASSKS ok AKTIlHOl'OnA. / 



beetle is an animal wliicli has no innci- sUclclon (tr hones whatever, 

 but only a liacd ei'iist on the sm-race which snrnmnds the muscles 

 and vital oi-.uans. This crnst is coniixjsed of separate rin<4-s, placed 

 end to end. 



Animals whose Ixtdics are thus ('oiiii)ose(l of rinus are called A r- 

 tk-ulata. They are in 1urii divided into two ,i;reat groups, the 

 Vermes and the ArlhrojKxht. Tlie V< mus (worms) have all the 

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

 outer crust or exoskeleton, and without paired limbs which are 

 jointed. The Arihropods 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- 

 solul)le and is not composed of cells, but consists of fine, irregidar 

 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 nuiscles 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) (')-usfacca (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 w^ith carbonate and phosphate of lime in addition to chitin. 



(b) Ardcluitdd (spiders, mites, etc.). terrestrial; head and 

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

 by means of trachea*. 



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

 ally worm-like, with only the head distinct; legs numerous; lu'cath- 

 ing by means of trachea'. 



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

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

 breathing by a system of tubes called tracheae, which branch and 

 ramify through every portion of the body, and which open exter- 

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

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

 the liead, the thorax and the ahdomoi. 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 rei)roduction. 



