SURFACE-SWIMMING FAUNA (INVERTEBRATES). 113 



Brachiolaria, and on account of the manner in 

 which these arms are inclined towards the apex, 

 the larva has a rough resemblance to the form 

 of a painter's easel. This type of larva is called 

 the Pluteus. The main point of interest about 

 the Pluteus, however, is that the arms are sup- 

 ported by delicate bars of carbonate of lime which 

 are connected together at the apex and form a 

 very definite larval skeleton. 



This larval skeleton is cast off with the skin 

 when the metamorphosis takes place, and it is 

 consequently of great interest to scientists in the 

 fact that it is one of those structures which are 

 formed to meet the exigencies 

 of larval life only, and is per- 

 fectly useless for the adult. In 

 considering the manifold ques- 

 tions which arise in the study 

 of the relation of animals to 

 their surroundings we are often 

 inclined to fix our attention too 

 exclusively upon the adapta- 

 tions that are manifested in FIG. 32. 

 the adult form. In the case 

 of some classes in which the immature stages of 

 life are passed through very rapidly and under 

 the protection of the parents, this is not to be 

 deprecated ; but in most cases it is important to 

 remember that in the struggle for existence there 

 is such danger of extermination that each stage 

 of life may have acquired special characters for 

 adaptation to its particular mode of existence. 

 The peculiar markings and colours of the Cater- 

 pillars is a familiar example of the special 

 H 



