!4o THE ANIMAL KINGDOM. 



becomes so much a portion of their body that it has been 

 called an outer skeleton. These animals, like insects, are 

 formed in segments, sometimes twenty-one in number, of 

 which seven belong to the head, seven to the thorax, and 

 seven to the abdomen. But these are generally united into 

 masses. Thus, in the Lobster (Fig. 99), the head and thorax 

 are joined together, but the divisions can be traced in the 

 under side ; whilst in the Crab, the segments are still 

 further united, and form the large shell which, you know, 

 completely covers the thorax of the animal (Fig. 2). 



Some of the Crustaceans breathe by means of gills, 

 which are somewhat in the form of plates. In the lower 

 tribes of the class they project from the surface of the body ; 

 but in the higher, they are contained in a cavity through 

 which water passes. Almost all of these animals live in 

 water ; but some known as Land Crabs can leave it 

 for a time. These are provided with a kind of spongy 

 structure in the chamber containing the gills, by means of 

 which they are kept moist. 



The hard coating or shell of Crustaceans is shed at 

 regulaiv periods ; and they are enabled to renew it by 

 means of little lumps of limy matter contained in their 

 stomachs, known as " crabs' eyes." Directly the old shell 

 is shed, the lime mixes with the blood, and makes its way 

 to the surface of the skin, and so forms the new shell. 

 Of course the stock of lime is thus exhausted ; but the 

 crabs' eyes are formed in the stomach again by the next 

 time the animal requires a new covering. 



We find in Crustacea both simple and compound eyes, 

 as in insects. In some they are immovable ; but in 

 others, such as the Lobster, they are placed at the end of a 

 stalk, which can be projected from the orbit, and thus 

 enable the animal to see in all directions. These animals 

 are therefore termed stalk-eyed. In some, these footstalks 

 are short ; but in others as in the Sentinel Crab, of the 



