CHAPTER VII. 



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



General Characters of Arthropods. — To this group 

 belong those segmented animals which have jointed append- 

 ages, i.e., antennae, jaws, maxillas (or accessory jaws), palpi, 

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

 being more plainly symmetrical than in the lower animals. 

 The skin is usually hardened by the deposition of salts, 

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

 stance called chitine. The segments or rings composing 

 the body are usually limited in number, 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) pairs of feelers (an- 

 tennas), from two to four pairs of biting mouth-parts or 

 jaws, and two compound eyes (except in the spiders, etc.), 

 besides simple eyes. Most Arthropods pass through a series 

 of changes of form called a metamorphosis; 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 Arthropoda. 



Class 1. A head-thorax and abdo- 

 men; two pairs of antenna?; breath- 

 ing by external gills Crustacea : lobster, crab. 



Class 2. Body with few or many seg- 

 ments; no antennae, all the ap- 

 pendages like legs; with gills. . . . Podostomata : king crab, trilohites. 



Class 3. Body worm-like, tracheate, 

 with two antennae ; fleshy legs 

 armed with claws Malacopoda : Pcripatus. 



Class 4. Body many - segmented, 

 many -footed, tracheate; with a 

 pair of antennae Myriapoda : centipede. 



Class 5. Body in two regions; no an- 

 tennae, four pairs of legs; tracheate. Arachnida: spider, scorpion. 



Class 6. Body divided into a Lead, 

 thorax, and abdomen ; breathing 

 by internal air tubes; with wings. Insecla : beetle, butterfly. 



