RELATION OF MEDIAN TO LATERAL EYES 



379 



the lateral eyes are absent or at least do not reach the surface of the head ; 

 the functional one being the parietal eye, which is of unusual size. 



1 6. " In the lampreys we see the same conditions, the parietal eye 

 being well developed in the larvae, while the lateral eyes are deeply 

 buried in the tissues of the head and are useless. During the trans- 

 formation the lateral eyes again become functional and the parietal eye 

 begins to atrophy, finally losing many of its structural details and its 

 function, although still retaining very nearly its original form." 



From this brief summary of some of the more important observations 

 and conclusions relating to the morphology of the parietal eye of verte- 

 brates and of the parietal eyes of the Entomostraca, it will be obvious that 



— v^-mec. 



Fig. 260. — Ventral Cord and Ocelli of Adult Cyclosalpa. Seen from 

 neural surface. (after patten.) 



A. : anterior end. n.e. 2 : nerve of posterior ocellus. 



e 1 , e- : anterior and posterior ocelli. P. : posterior end. 



me. c. : mesocephalon. pr. c. : procephalon. 



Patten believed not only that there was a close connection between the 

 arthropod stock and the Ostracodermi but that the vertebrate phylum 

 actually originated from arachnids (p. 378, para 14). Further, it is clear 

 that he believed the evidence afforded by his observations on the parietal 

 eyes in the two groups strongly supported this conclusion which he had 

 come to from his extensive study of eyes in general and of other 

 systems and organs in various classes of invertebrate animals, of which 

 we may specially mention that of the eyes of Acilius, Vespa, Pecten, and 

 Area. This material, taken along with that afforded by other pioneer 

 workers such as Reichenbach, Huxley, Grenacher, Bertkau, and Lowne, 

 and more recent writers on the Palaeozoic fishes and merostomata, provides 

 a basis for more accurate generalizations on the main principles which 

 are concerned in the evolution of the different types of eye, and of evolu- 



