276 OUTLINES OF EVOLtJTIONAKY BIOLOGY 



metamorphosis, during which the long arms are completely lost 

 and the form of the body entirely changed. The echinoids or 

 sea-urchins pass through a very similar larval stage. 



The free-swimming " Zoaea " larva of the crab (Fig. 128) is also 

 provided with conspicuous defensive spines which are lost in the 



FIG. 12G. 



Fj4. 127. 



FIG. 126. A Brittle Star (Ophiura aliaris), X . 

 FIG. 127. The Plutetfs Larva of a Brittle Star, : 



x 62. 



adult condition (Fig. 102) and which have doubtless been developed 

 in relation to the special conditions of the larval life. 



In many cases the development of larval organs causes the 

 young animal to differ so widely from the adult that the relation- 

 ship of the two for a long time remained unsuspected, and hence 

 the special names which many larvae have received. In other 

 cases, however, it is the adult animal which has become so pro- 

 foundly modified in relation to special conditions of life as to 



