POLYPLACOPHORA. IX 



the numerous fine fibres composing the optic nerve become separated 

 from one another and loose. Immediately underneath the retina the 

 fibres become still more widely separated, forming an expansion of 

 fibres. The retina is formed on the type of that of Helix, and not, 

 as might have been expected, on that of the dorsal eyes of Onchid- 

 ium or the eyes of Pecten. The fibres of the optic nerve do not pass 

 in front of the layer of rods to be distributed to them from in front, 

 but are directed to the rods directly from behind. The retina pre- 

 sents a single layer of short but extremely well defined rods, the 

 extremities of which are directed toward the light. The rods when 

 viewed from the surface of the layer they compose are seen to be 

 hexagonal or pentagonal in .outline, and each contains a nucleus. 

 They form a layer which is concave toward the lens, there being a 

 space between the hind surface of the lens and the concave face of 

 the layer." 



Two eyes from the shell of Enoplochiton (x 40) are shown on pi. 

 52, fig. 27. On pi. 51, fig. 3 is seen an eye of Schizochiton magnified 

 200 diameters, showing outside the pigmented eye-capsule, seen partly 

 through the superficial layers of the tegmentum, inside of which is 

 the cornea, and the lens, seen through the cornea. 



The arrangement of the eyes varies in different genera, as will be 

 seen by reference to the systematic descriptions. Eyes are present 

 in but one family of Chitons, the Chitonldce; and in this group they 

 are found in all but two genera. In these two the megalsesthetes are 

 larger and more regularly developed than in any Chitons of other 

 families. This high development of sense organs is associated with 

 the most complex system of insertion-plates, the completest form of 

 girdle-armor and of gills to be found in the Polyplacophora. The 

 family Chitonidce therefore, easily ranks as the consummation of 

 Chiton development. Tonicia or Acanthopleura represent the utmost 

 height yet attained by the great Amphineurous branch of mollusk 

 life. 



Theory of the origin of valve-structures. Any rational theory of the 

 development of a structure must rest upon a knowledge of the rela- 

 tions of that structure' to the organism possessing it and to the 

 external world. In discussing the genesis of the insertion-plates of 

 Chitons we must therefore bear in mind (1) that the function of 

 these plates is to bind the valves firmly to the girdle, and (2) that 

 the entire force of all impacts on the valves is transmitted to these 

 plates, which are separated from the solid surface supporting the 



