No. 634] PHYLOGENY OF THE ARTHROPOD A 401 



behind, and eventually a certain number are freed from 

 the anterior end of the pygidium to form the thorax. 

 Trilobites with an elongate worm-like form have numer- 

 ous thoracic segments and small pygidia, while many 

 others have few free segments and the pygidium nearly 

 as large as the cephalon. These latter have usually been 

 called more specialized than the former, but it is obvious 

 from the method of introduction of new thoracic seg- 

 ments that the reverse is the case. This opinion is con- 

 firmed by a study of the ontogeny, for it is found that in 

 the protaspis the pygidium of any species is proportion- 

 ally larger than at any later period in life, and that many 

 species pass through a stage in which they are isopygous. 



There is a certain amount of evidence that the pygi- 

 dium was used as a swimming fin, and some species seem 

 to have had sufficiently strong muscles to enable the ani- 

 mal to dart away suddenly when attacked. Trilobites 

 with large pygidia would thus have a certain advantage 

 over the others, and, as a matter of fact, it was this type 

 which persisted longest. The broad depressed body was 

 not as well adapted for a nectic mode of life as a com- 

 pressed fishlike one would have been, but it is the form 

 which could most easily be kept afloat and propelled with 

 the minimum of effort. The young of all species are cir- 

 cular or broadly oval in outline, and those adults with 

 subequal shields depart least from that form. A fair 

 inference from the above would be that the elongate 

 crawling trilobite was more specialized than the isopyg- 

 ous swimming one. 



Internal Anatomy 



Naturally knowledge of the internal anatomy is not as 

 full as could be desired, but what follows appears to be 

 based on reasonably clear evidence. 



The mouth is ventral and usually situated back of the 

 middle of the cephalon. Its position depends upon the 

 length of the "upper lip" or hypostoma, and in extreme 

 cases may be at the posterior end of the cephalon. There 



