118 STUDIES IN EVOLUTION 



is to consider them as such. Furthermore, the free-cheeks 

 are distinctly separated from the cranidium by an open suture, 

 and may be wholly converted into eyes, as in ^glina armata 

 Barrande, or the unfaceted portion may be reduced to almost 

 nothing, as in Beiphon. In such cases the parallelism is 

 exact with true movable eyes. Bernard^ concludes from 

 his studies of Apus that the hypostoma is homologous with 

 the annelid prostomium. This would make the hypostoma 

 represent the first, and the free -cheeks the second of the 

 obsolete segments. Thus the trilobite cephalon would fulfil 

 the demand for additional evidences of primitive head seg- 

 ments, and account for the development of eyes separate from 

 the cephalothorax as commonly restricted. 



Supposed evidences of free-cheeks or of facial sutures have 

 been recognized in Limulus, Hemiaspis, and Bunodes, but 

 these seem really to correspond to the lines on the dorsal 

 surface of the cephalon of Harpes and some Trinucleus, run- 

 ning from the glabella to the eye-spots and to the margin, 

 and are not the sutures marking the limits of the free cephalic 

 elements, as in Asaphus and Proetus. Limulus, however, 

 has a suture comparable to that in Harpes and Trinucleus, 

 extending around the ventral border of the cephalothorax 

 nearly to the posterior angles, and partly separating the 

 ventral plate. In the process of moulting, this suture opens 

 and enables the animal to free itself from its former test. 



These interpretations may be employed to some advantage 

 in correlating the segmentation of the trilobite cephalon. 

 As previously stated, the recognized plan in the nervous 

 system of a generalized crustacean requires that there should 

 be a brain or supra-oesophageal ganglion innervating («) the 

 unpaired eye, (F) the frontal sensory organs and stalked 

 eyes, and (c) the anterior antennae; then a ventral nervous 

 cord consisting of a succession of double ganglia innervating, 

 respectively, the second pair of antennae, the mandibles, the 

 first pair of maxilla3, the second pair of maxillae, and lastly 

 each of the paired thoracic and abdominal appendages. 

 Altogether, there are seven neuromeres pertaining to the 



