ARTICULATA. 221 



muscles. The normal number of joints is twenty-one, 

 but two or three are often blended. To each of these 



FIG. 123. Circulating Apparatus of Lobster : a. Heart. 3. c. Arteries to the eyes 

 and antennae, d. Hepatic artery, e.f. Arteries to thorax and abdomen, gg. Venous 

 sinus. Ji. Gills, z. Branchial veins. 



joints, except the last, there is attached a pair of mem- 

 bers, the forms and uses of which vary in different spe- 

 cies, and at different ages. These members are jointed, 

 and covered with a similar envelope, or crust, to that of 

 the body. As the body grows the carapace does not 

 grow in the same proportion, rendering frequent molt- 

 ings necessary. The entire covering is thrown off from 

 body, feet, and antennae in the most perfect manner. 

 The Crustacea differ in habits as well as in structure. 

 Most live in the water, but the Land-crabs inhabit the 

 land. The Hermit-crabs (Pagurida) live in the empty 

 shells of Mollusks, which they seize, often killing the in- 

 habitant. The majority of Crustaceans have jaws and 

 organs of mastication, but some have no such organs, 

 but live as parasites, especially on fishes, sucking their 

 juices, and becoming strangely deteriorated. The ali- 

 mentary canal in this class consists of a short gullet, a 



gizzard-like stomach, and a straight intestine. Crusta- 

 19* 



