7« 



CHAPTEK VII. 



Branch VII. — Arthropoda {Crustaceans, Insects, etc.). 



General Characters of Arthropods. — To this groui^ 

 belong those' segmented animals which have jointed apj)end- 

 ages, i.e., antennse, jaws, maxillje (or accessory jaws), j)alpi, 

 and legs arranged in' pairs, the two halves of the body thus 

 being more plai nl v'^s v mmetri cal than in the lower animals. 

 The skin is nsuallv*'ha rdene d by the deposition of salts, 

 mostly carbonate of lime, and of a peculiar organic sub- 

 stance called *c ///f't HP. The segments or rings composing 

 the body are usually'lim ited in nu mber, there being usually 

 twenty in the Crustaceans and seventeen or eighteen in most 

 insects, though in the Myriapods there may be as many as 

 two hundred. The head is usually distinct from the body, 

 with one (insects) or two (Crustacea) jiairs of feelers (an- 

 tenna?), from two to four pairs of biting mouth-j)arts or 

 ii^s. and two com pound eyes (excejjt in the spiders, etc.), 

 besides si mple ev es. Most Arthrojoods pass through a series 

 of changes of form called a%et aniorph osis ; the young of the 

 butterfly being called a caterpillar or larva, the succeeding 

 stage a pupa or chrysalis, and the mature stage the imago. 



Classes of Aktheopoda. 



Class 1. A head-lhorax: and abdo- 

 men; two pairs of antennae; breath- 

 ing ^}J external gills Ontstacea : lobster, crab. 



Class 3. Body with few or many seg- 

 ments ; no antennae, all the ap- 

 pendages like legs; with gills Podoslomata: king crab, twlublKw i. 



Class 3. Body worm-like, tracheate, 

 wilh two antenna ; fleshy legs 

 armed with claws Malacopoda : Pcripatus. 



Class 4. Body many - segmented, 

 many -footed, tracheate; with a 

 pair of antenna; Myriajwda : centipede. 



Class R. Body in two regions; noan- 

 tenna?,four pairs of legs; tracheale. Jlrac/oiirf-a, .• spider, scorpion. 



Class 6. Bodj' divided into a head, 

 thora.x, and abdomen ; lireathing 

 by internal air tubes; with wiugsj/isccto ; beetle, butterfly. 



