SUMMARY. I 5 I 



FINAL SUMMARY. 



It is generally believed that the Arthropoda constitute a natural, monophyletic group. 

 The data assembled in the preceding pages indicate that the other Arthropoda were 

 derived directly or indirectly from the Trilobita because : 



1 I ) the trilobites are the oldest known arthropods ; 



(2) the trilobites of all formations show great variation in the number of trunk seg- 

 ments, but with a tendency for the number to become fixed in each genus; 



(3) the trilobites have a constant number of segments in the head; 



(4) the position of the mouth is variable, so that either the Crustacea or the Arach- 

 nida could be derived from the trilobites; 



(5) the trilobite type of appendage is found, in vestigial form at least, throughout the 

 Arthropoda ; 



(6) the appendages of all other Arthropoda are of forms which could have been derived 

 from those of trilobites; 



(7) the appendages of trilobites are the simplest known among the Arthropoda; 



(8) the trilobites show practically all known kinds of sessile arthropodan eyes, simple, 

 compound, and aggregate; 



(9) the apparent specializations of trilobites, large pleural lobes and pygidia, are primi- 

 tive, and both suffer reduction within the group. 



The ancestor of the trilobite is believed to have been a soft-bodied, free-swimming, flat, 

 blind or nearly blind animal of few segments, because : 



(a) the form of both adult and embryo is of a type more adapted for floating than 

 crawling ; 



(b) the large pygidium is shown by ontogeny to be primitive, and the elongate worm- 

 like form secondary; 



(c) the history of the trilobites shows a considerable increase in the average number of 

 segments in successive periods from the Cambrian to the Permian; 



(d) the simplest trilobites are nearly or quite blind. 



