THE TRILOBITA AND OTHER ARTHROPODA 205 



between (i) differential or static characters that keep 

 fairly constant through one line of descent; (2) progressive 

 characters that are likely to show similar sequences in 

 several lines of descent ; and (3) adaptative characters 

 that may occur over and over again in the same or 

 different branches. Let us first distinguish the progres- 

 sive characters. 



Arthropods are doubtless descended from the meta- 

 merically - segmented worms (annelids), and have 

 advanced beyond them by, among other things, the 

 coalescence of a number of anterior somites (six in 

 trilobites) to form a head : this process has been termed 

 cephalization. Among trilobites only, a similar process 

 takes place at the posterior end, somites fusing to form 

 the pygidium (caudaltzation), so that we have forms with 

 very small pygidia consisting of the unsegmented end of 

 the body alone or with addition of a very few somites 

 (micvopygons stage), forms with a pygidium including 

 more somites but still much smaller than the head 

 (heteropygous), and forms with a pygidium as large as the 

 head (isopygous). These stages were passed through 

 independently by various lines of descent, the number of 

 thoracic somites diminishing as the pygidium incorporated 

 more of them. 



These changes do not seem to bear any relation to 

 habitat or mode of life, but there are others that do 

 (adaptative characters). The ordinary trilobites seem to 

 have lived on the sea-bottom, but to have been able both 

 to crawl and to swim ; but particular genera or families 

 took, some to a mud-grubbing, others to a surface- 



