THE JOINTED-FOOT ANIMALS 169 



As in the insects, the hard exoskeleton necessitates fre- 

 quent molts to provide for growth, and also, as in that class, 

 growth is sometimes accompanied by marked metamorpho- 

 sis after hatching. Many of the Crustacea have direct 

 development from a rather simple larval stage (Fig. 93). 

 However, the lobsters (Fig. 86) and the crabs (Fig. 95) un- 

 dergo a more complicated development, in which the indi- 

 vidual passes through several different larval forms before 

 reaching the adult condition. In the crayfish the develop- 

 ment is similar to that of the lobster with the exception 

 that the various larval stages are passed before the egg is 

 hatched. Thus in the crayfish the young does not leave 

 the egg until it has passed through these stages and closely 

 resembles the adult. 



Animals related to Crustacea and to Hex apod a 



There are several groups of animals which in some 

 respects resemble the crustaceans and the insects. But 

 these differ so much from their nearest relatives in certain 

 characteristics that they are now considered as belonging to 

 several independent classes. A number of these forms are 

 inconspicuous and have value for the advanced student 

 only. The familiar forms commonly known as the centipeds 

 and the millipeds are the only members of these groups that 

 will be discussed here. 



Centipeds. The common centiped {Litho'bius, Fig. 96), 

 found under the bark of trees, is an elongate, flattened ani- 

 mal, with long antennae, many somites, and a pair of legs 

 on every somite. There is a poison gland in the base of the 

 first pair of legs, which is used to kill earthworms and in- 

 sects, upon which Lithobius feeds. The house centiped (Scu- 

 tig'era) feeds largely on flies, cockroaches, and other insects. 



Centipeds are widely distributed over the world. In the 

 tropics they grow to be over thirty centimeters (one foot) 



