ARTHROPODS. CLASS CRUSTACEA 



99 



form of minute larvae, provided with three pairs of append- 

 ages, a median eye (Fig. 56), and a firm external skeleton 

 or cuticle. This latter prevents the continuous growth of 

 the larvae or nauplius, and every few days it is thrown off, 

 and while the new one is forming the body enlarges. Dur- 

 ing this time new appendages are developed, so that after 

 each moult the young crusta- 

 cean emerges less like its 

 former self and more and more 

 like its parents. In the bar- 

 nacles, after several moults 

 have taken place, the young 

 become permanently attached 

 by means of their first anten- 

 nae, their thoracic feet change 

 into feathery appendages, and 

 several other changes occur. 

 In some of the parasitic bar- 

 nacles (Sacculina) the larva 

 attaches itself to a crab, throws 

 off its various appendages, and, 

 after other great degenerative 

 changes, enters its host. For 

 a time, therefore, their development is toward greater com- 

 plexity, but the later stages constitute a retrograde meta- 

 morphosis. 



100. More complex types. The larger, more useful, and 

 usually more familiar Crustacea belong to the second divi- 

 sion (subclass Malacostraca). It comprises such animals as 

 the shrimps, crayfish, lobsters, crabs, and a number of other 

 forms which are at once distinguished from the preceding 

 by the constant number of segments composing the body. 

 Of these, five constitute the head, eight the thorax, and 

 seven the abdomen. The head segments are always fused 

 together, and with them one or more thoracic segments 

 unite to form a more or less complete cephalothorax. Also, 



FIG. 56. Development of a barnacle 

 (Lepas). a, larva ; b, adult. 



