194 STUDIES IN EVOLUTION 



A common early larval form is recognized and called the 

 protaspis. 



The protaspis has a dorsal shield, a cephalic portion com- 

 posed of five fused segments and a pygidial portion consisting 

 of the anal segment with one or more fused segments. 



The simplest protaspis stage is found in the Cambrian 

 genera of trilobites. During later geological time it acquired 

 additional characters by earlier inheritance and became modi- 

 fied, though retaining its pentamerous glabella and small 

 abdominal portion. 



Some of these acquired characters of the dorsal shield are 

 the free-cheeks, the eyes, the eye-line, the genal angles, and 

 the ornaments of the test. The free-cheeks and eyes moved 

 to the dorsum from the ventrum. 



The history of the acquired characters is traced by means 

 of comparisons between larval and adult trilobites, through 

 Paleozoic time, and a progressive series of larval forms estab- 

 lished in exact correlation with adult forms, which them- 

 selves constitute a progressive series, chronologically and 

 structurally. 



The antiquity of trilobites is indicated by their remains in 

 the oldest Paleozoic rocks, and especially by the fact that 

 in the early Cambrian they are already much specialized and 

 differentiated in number of genera. The age of the trilobite 

 or crustacean phylum is further shown from the distinct 

 larval stages of trilobites and their having a cephalon and 

 pygidium of consolidated segments. 



Since the trilobites are among the oldest and most general- 

 ized of Crustacea, their ontogeny is of considerable impor- 

 tance in interpreting crustacean phylogeny. 



The protaspis in its segmentation shows that the cephalon 

 had five pairs of appendages as in the adult. 



The crustacean nauplius is shown to be homologous with 

 the protaspis and to have potentially five cephalic segments 

 bearing appendages, which should therefore be taken as char- 

 acteristic of a protonauplius. 



The nauplius is a modified crustacean larva. The pro- 



