NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS IO9 



The antennal appendages of the trilobites, judging from their 

 filamentous form in contrast with the segmented structure of the 

 following appendages, evidently represent the first antennae (anten- 



FiG. 46. — Segmentation of the trilobite "head," or prosoma. 



A-D, four consecutive stages in the larval development of Blainia grcgaria 

 Walcott, showing gradual posterior extension of lateral wings (free cheeks) 

 of acron against sides of anterior somites, and division of glabella (C) into four 

 segmental areas behind frontal lobe of acron (from Lalicker, 1935). E, diagram 

 of typical trilobite head, dorsal surface. F, diagram of ventral surface of trilobite 

 head, showing labrum attached to margin of doublure. G, labrum of Pacdcumias 

 transitans Walcott, example of a stalked labrum (from Walcott, 1910). H, 

 head of Holotrachelus pwicHllosus, with segmentation obliterated in the large 

 swollen glabella (from Warburg, 1925). I, diagram of larval trilobite, with 

 head segmentation indicated. J, head of adult trilobite with probable segmenta- 

 tion deduced from the larval structure (I). 



Acr, acron; dbl, doublure; dO, dorsal ocellus; E, compound eye; fg, fixigene 

 (fixed cheek) ; fr/, frontal lobe (of acron) ; fsa, anterior part of facial suture; 

 fsp, posterior part of facial suture; gib, glabella; I-IV, cephalic somites; Ig, 

 libragene (free cheek); Lm, labrum; pi, palpebral lobe; IS-4S, intersegmental 

 sutures of head. 



nules) of other arthropods; if so, according to the theory here 

 followed, they should belong to the acron, and perhaps had their 

 muscle attachments on the frontal lobe. The position of the antennal 



