BIVALVE CRUSTACEANS AND BARNACLES. 



149 



ming legs is discarded. The moulting process also takes 

 place with the bivalve shell, but, instead of its being dis- 

 carded, the inoult is held or cemented to the new shell which 

 forms underneath. Moult after moult of the shell is thus 

 retained, the increasing size of each moult showing as sep- 



FIG. 138. FRESH- WATER CRUSTACEANS; D, Species from Dubuque, Iowa; , Eye. The line 

 below indicates the natural length of the specimen ; Z, Species from Lynn, Mass. : this is 

 figured the natural size ; S presents a highly-magnified section of one of the shells, to 

 show the successive moults, these being numbered in their order of moulting ; B shows 

 the appearance of a portion of the edge of the shell along the back, representing the succes- 

 sive moults lapping one over the other. 



arate concentric lines of growth. If the shell is cut into 

 and the cut edge is examined with a microscope, the succes- 

 sive moults will be seen resting one upon the other, like the 

 leaves of a book. By reading carefully the description ac- 

 companying Fig. 138, the pupils will learn more about it. 



The very young stages of these creatures have some 

 resemblance to the young of the barnacle shown in Fig. 141. 



136. Another group of animals classed with the Crustacea 

 is still more unlike the forms already studied. These are 



