PERORAL LOBE. 267 



to the exterior by a pair of dorsally placed collar-pores, 

 and (3) the body-cavity proper surrounding the alimentary 

 canal. According to Fowler, who has recently described 

 them in Rhabdopleura, the nervous system and notochord 

 have essentially similar relations to those which obtain in 

 Cephalodiscus, but there are no proboscis-pores and no 

 gill-slits. 



THE PR^ORAL LOBE OF ECHINODERM LARV^. 



In the previous pages a good deal of stress has been 

 laid on the existence of a prasoral lobe in the various types 

 considered. We have recognised it in the snout of Am- 

 phioxus (prasoral coelom + praeoral pit), in the proboscis 

 of Balanoglossus, the fixing organ of the Ascidian tadpole, 

 and in the buccal shield of Cephalodiscus and Rhabdo- 

 pleura. 



From a morphological standpoint the praeoral lobe is 

 probably one of the most important, as it is certainly one 

 of the oldest, structures of the body of bilateral animals, 

 and it becomes, therefore, a matter of the first moment to 

 be able to trace the modifications which it has undergone 

 along the different lines of evolution which have culmi- 

 nated in the existing types of animal life. The subject is 

 a very large one, and can only be treated here in its 

 broadest outlines. 



It is now very generally admitted by zoologists that the 

 Echinoderms (star-fishes, sea-urchins, etc.) owe the radial 

 symmetry, which is one of the most obvious characteristics 

 of their organisation, to their having been derived from 

 bilaterally symmetrical ancestors, which became adapted 

 to a fixed or sessile existence. If this view is correct, 

 and there is good reason for supposing that it is, it follows 

 that the majority of living Echinoderms have secondarily 



