CBUSTACEANS . 307 



by their threads ? What is said of the Gossamer Spiders ? Describe 

 the architecture of the Labyrinthic Spider. What is said of the Mason 

 Spiders ? What of the Spider that weaves leaves together ? What 

 of the Spider that builds a raft ? What of the Diving-bell Spider ? 

 What are some of the Arachnida of the second group ? What is said 

 of the Mites ? What of the Harvest-men ? What of the Bock Scor- 

 pions ? 



CHAPTER XXXI. 



CEUSTACEANS, AST) THE "WOEM AND LEECH TEIBE. 



521. The class of the Articulata called Crustacea has 

 its name from the Latin word a-usta, a crust or shell. 

 It includes Lobsters, Crabs, Prawns, Shrimps, Sowbugs, 

 Sand-fleas, Barnacles, etc. Lobsters and Crabs are the 

 most perfect animals of the class. 



522. There is considerable resemblance to insects, and 

 also to Spiders, in most of these animals. Like the In- 

 sects, they may be divided into two groups — the mandi- 

 bulate and the haustellate. The eyes of the Crustacea 

 are generally compound, like those of the Insects. They 

 have also antennae. But the Crustacea differ from in- 

 sects in the character of their i-espiratory apparatus. 

 They are aquatic animals, and breathe by gills. There 

 are a few species that are formed to live in air. The 

 Land Crabs, found mostly in the Antilles^ are an exam- 

 ple. In them there is, above the gills, a spongy appara- 

 tus, from which continually exudes a moisture that keeps 

 t he gills from becoming dry. 



r 523. The legs of the Crustacea often amount to seven 

 pairs, as in the Woodlouse and Sandhopper ; but in oth- 

 er cases there are five pairs, as in the Crab. 'The legs 

 are constructed very difierently in the various Crustacea, 

 according to the manner in which they are to be used. 

 In some they are leaf-like membranes, being thus fitted 

 for swimming ; in others they are columns jointed to- 

 gether, to be used only in walking ; in others they are so 



