146 FIRST BOOK OF ZOOLOGY. 



microscope can the development of the young creature be 

 watched. The following figure represents a single egg of 

 the sowbug highly magnified : 



FIG. 136. EGG OP SOWBTTG, HIGHLY MAGNIFIED. The little dot, at one side, represents the 

 natural size of the egg. The head faces the left. 



Around the upper edge of the embryo (as a young animal 

 in the egg is called), from eighteen to twenty little blunt ap- 

 pendages may be seen; these represent the legs and other 

 appendages of the body the one longer than the rest is an 

 antenna. As the creature grows, these appendages become 

 jointed and variously modified to form the legs, mouth-parts, 

 antennae, and the appendages on the tail, which differ greatly 

 from each other, though at the outset they are all alike. 



133. Having studied a few of the many different kinds 

 of crustaceans, let the pupils examine them together to find 

 some points characteristic of them all. 



Their bodies, in common with the insects, are composed 

 of segments to which are attached jointed appendages of 

 various kinds. This body is divided into two regions, the 

 cephalo-thorax and the abdomen. In some the cephalo-thorax 

 is covered by a continuous shield, called the carapace, as in 

 the crawfish, crab, lobster, and shrimp. In others the seg- 

 ments of the cephalo-thorax are distinctly separate, and 

 movable upon each other, as in the sowbug and certain other 



