Vlll POLYPLACOPHORA. 



main canals which ramify horizontally in this plane, and find open- 

 ing from the valves at the eaves, or outer bases of the teeth and 

 anterior sinus. From the sides of the megalopore chambers or from 

 the deeper plexus of canals, are given off fine canals which perforate 

 the tegmentum vertically and join the bases of the micropore cavities. 

 All of these canals are occupied by fibrous and nerve tissues con- 

 tinued in from the girdle and interior. From these ramifications are 

 given off branches to each megalopore canal, in the superficial 

 chamber of which they expand into obconic knobs of highly refract- 

 ing tissue, sensory in function, and in all probability tactile. These 

 knobs are called by Moseley, megalcesthetes ; they are capable of being 

 somewhat protruded from the mouths of the pores. The micro- 

 pores hold exactly similar but smaller sense-organs, the micrcesthetes. 

 These structures are found in probably all Chitons ; but in certain 

 genera they are subobsolete. In others some of the megalsesthetes 

 have become transformed into eyes. These are connected with the 

 network of soft tissues, and occupy pear-shaped cavities like the true 

 megabesthetes. On one side the bulb of the pear, more or less near its 

 extremity, is closely applied to the outer surface of the tegmentum, 

 and here its wall is pierced by a circular aperture, the pupil-like 

 opening. This opening covered by the cornea, the periphery of 

 which extends to a considerable distance beyond its margin all 

 round. 



The cornea is a concavo-convex, watch glass-shaped lamina, trans-' 

 parent, and calcareous in structure, being continuous all around with 

 the superficial calcareous layer of the tegmentum. "The pear- 

 shaped cavity of the eye in the tegmentum is lined by a dark brown 

 pigmented membrane of a stiff and apparently somewhat chitinous 

 texture, which forms the eye capsule. This capsular membrane 

 exactly follows the shape of the eye cavity, except near the surface 

 of the tegmentum, where its margin curves inward beneath the 

 cornea, forming a sort of iris and bounding the circular pupil, 

 which is of less diameter than the cornea. The aperture of the pupil 

 is occupied by the front surface of the lens. The lens is perfectly 

 transparent and hyaline, arid strongly biconvex. It is filled in 

 behind the iris aperture. It is composed of soft tissue and dissolves 

 in strong acetic acid gradually and completely, showing a fibrous dis- 

 tinct structure in the process. There is a space between the front 

 surface of the lens and the cornea. 



" Within the pigmented tubular prolongation of tfce eye capsule 



