SUMMARY. I 5 ' 
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 : 
( 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. 
