116 



STUDIES IN EVOLUTION 



Sub-class I. Trilobita. 



Sub-class II. Entomostraca. Sub-class III. Malacostraca 



16. Abdominal limbs on 

 all segments except the 

 anal, phyllopodiform. 



17. Coxal elements of all 

 liml)S forming gnatho- 

 bases, wliicli become 

 manducatory organs on 

 the head. 



18. Respiration cuticular 

 and by fringes on exop- 

 odites. 



Abdominal limbs gener- 

 ally wanting. 



Coxal elements seldom 

 forming gnatliol)ases 

 except on the head. 



Respiration mainly cuticu- 

 lar and by the limbs and 

 gill appendages. 



Abdominal limbs often re- 

 duced except the last 

 pair, which with telson 

 frequently form a caudal 

 fin. Chiefly branchial 

 in some groups. 



Coxal elements seldom 

 forming gnathobases 

 except on the head ; 

 never on the abdomen. 



Respiration cuticular and 

 by the limbs and cpipo- 

 dites. 



The more primitive characters of the trilobites as drawn 

 from the foregoing table may be summarized as follows: 

 (1) They are all free marine animals; (2) the animal has a 

 definite configuration; (3) the larva is a protonauplius-like 

 form; (4) the body and abdomen are richly segmented, and 

 the number of segments is variable ; (5) the head corresponds 

 to the typical crustacean; (6) the thorax and abdomen are 

 always distinct, the number of segments in each being vari- 

 able; (7) all segments except the anal bear paired append- 

 ages; (8) all appendages are typically phyllopodiform; and 

 (9) the coxal elements of all limbs form gnathobases, which 

 become organs of manducation on the head. 



It may be questioned by some whether the present state 

 of knowledge of the ventral structure of trilobites warrants 

 such general assertions as to details of organization. In the 

 first place, it must be granted that there is a remarkable 

 uniformity in the features of the dorsal crust, which natu- 

 rally reflects to a degree the differentiation and variation of 

 the organs and appendages of the ventral side. Further- 

 more, the actual appendages have been observed in such 

 diverse and characteristic genera as Trmucleus, Triarthrus, 

 Asaphus, Ceraurus, and Oalymmene, and found to conform 

 closely to a single type, so that it seems safe to assume a like 

 agreement throughout. 



