CRUSTACEANS. 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. 
CRUSTACEANS, AND THE WORM AND LEECH TRIBE. 
521. Tue class of the Articulata called Crustacea has 
its name from the Latin word crusta, 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 antenne. But the Crustacea differ from in- 
sects in the character of their respiratory 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 
the gills from becoming dry. 
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 differently 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 
