SYSTEMATIC POSITION OF THE TRILOBITES 165 



(4) Paired eyes are not present, or at least not visible in 

 the ijrotaspis stages of primitive trilobites. They may through 

 acceleration appear in the protaspis stages of later genera, as 

 they do in the nauplius embryos of certain modern decapods. 



I do not believe that the nauplius has any great phyloge- 

 netic significance, and have considered it " as a derived larva 

 modified by adaptation " (Z. c, p. 190), and as a " modified crus- 

 tacean larva " (ibid, p. 191). 



It does not seem necessary to correlate the post-oral second 

 pair of trilobite appendages with the mandibles of higher 

 Crustacea. The second pair in the nauplius is also post-oral 

 and manducatory, though they later develop into the antennae 

 and are pre-oral. 



As to the cephalon of a primitive crustacean, I have merely 

 accepted the conclusion approved by Claus, as stated by Lang, 

 in his reconstruction of the original crustacean, which is as 

 follows : " The head segment was fused with the four subse- 

 quent trunk segments to form a cephalic region" (Comparative 

 Anatomy, p. 406). 



Similarly in regard to the interpretation of the biramous 

 appendages, I have adopted the statements and conclusions 

 of a large number of zoologists who consider the most primi- 

 tive appendages as branched or consisting of a dorsal and a 

 ventral member, and I have followed them in thus interpret- 

 ing the trilobite appendages, which are clearly of this nature. 



