No. 634] PHYLOGENY OF THE ARTHROPOD A 413 



the mouth, as indicated by the increase in length of the 

 hypostoma, and an increase in size of the anterior part of 

 the glabella. There is nothing mysterious about the 

 process, as it is probably due to the increase in the size 

 of the anterior (digestive) portion of the " stomach." 

 This indicates that in the ancestral form the mouth and 

 eyes were close to the anterior margin, and does away 

 with the necessity of the bent annelid to explain their 

 migration. Many of the simpler trilobites are of course 

 blind, as is Naraoia. 



It is true that much of the argument involves the use 

 of principles drawn from the study of ontogeny, but 

 where the ontogeny points to animals which actually exist 

 and helps to explain observed facts, its use seems to be 

 justified. Swinnerton has recently suggested that stu- 

 dents of the ontogeny of trilobites have been led into an 

 entirely wrong interpretation because they have not real- 

 ized that the protaspis, like the nauplius, is a specialized 

 larva adapted to a nektonic mode of life. Since Swinner- 

 ton believes that the trilobites are descended from ben- 

 thonic annelids, one can not but wonder why the nektonic 

 trochophore was not carried over instead of requiring 

 the development of a new and totally different free-swim- 

 ming larva by the trilobites. All indications derived 

 from the present study are that the primitive trilobites 

 were floating and swimming animals, that their adoption 

 of a crawling habit was a specialization, that the protas- 

 pis was nektonic because the adults were, and that the 

 nauplius of recent Crustacea is a similar free-swimming 

 larva because it harks back to ancestral conditions. 



