SIXTH BRANCH OP ZOOLOGY. 



CRUSTACEOLOGY. (Lat. crustaceq, ; Gr. tyos, logos, a 

 discourse.) 



CLASS CRUSTACEA. (Lat. from crusta, a shell or crust.) 



SECTION I. 



THIS Class includes animals, some of which dwell on land, 

 others in fresh or salt water, and which are covered with a soft 

 shell or crust. They are oviparous, and divided into segments 

 or rings, articulated into each other, to the inside of which their 

 muscles are attached. The outer covering generally possesses 

 a considerable degree of hardness, containing no small propor- 

 tion of carbonate of lime. Its solidity varies; sometimes it is 

 membranous. 



The way in which the animals free themselves from the old 

 shell is quite singular ; they generally manage to get out of it 

 without occasioning the least change in its form. When the 

 shell is first stripped off, the surface of their bodies is extremely 

 soft; and it is some time before the substance which has been 

 exuded from the pores on the surface of their skin, acquires a 

 hard consistence. 



They generally have a distinct heart and a circulatory system 

 or blood vessels, but no internal skeleton, properly so called. 

 They breathe by means of gills or branchial plates, or else by 

 the skin. The breathing apparatus is adapted to aquatic rather 

 than aerial respiration. In those genera in which the head is 

 not separated from the thorax, the shield protects the whole of 

 the thorax. Other genera have the head distinct from the body, 

 which is divided into seven segments, to the lower sides of which 



