PROTOZOA, CCELENTERA, AND ECHINODERMA 107 



They are not born radially symmetrical, but become 

 so as they grow up. Moreover, we must bear in 

 mind that the radial symmetry of the adult only 

 obscures, it does not obliterate, the bilateral symmetry 

 of the larva. 



In the Holothurian, however, we can always dis- 

 cover a clear bilateral symmetry even in the adult. 

 That is to say, we can recognise an anterior and a 

 posterior end, a right and a left side of the body. It 

 is an organisation which emphasises, as it were, the 

 anterior and posterior ends, the right and left sides' 

 and the dorsal and ventral surfaces that characterise 

 this interesting deep-sea order, the Elasipoda. 



Here, then, we have an example of a character 

 common to all the larvae of the sub-kingdom and ex- 

 ceptionally well marked in the adults of a family 

 confined to deep-sea habitats. 



Now we know that there is a tendency for some 

 of the peculiar characters of the ancestors of animals 

 to be recapitulated in the course of their develop- 

 ment from the egg, and accordingly most naturalists 

 are agreed that all the Echinoderms have descended 

 from some form of bilaterally symmetrical ancestor. 

 Are we, then, to believe that the Elasipoda brought 

 from the depths of the sea are more closely related 



