MOLLUSCA. 



15 



skin caused by the cuticle being somewhat thickened. 

 Beneath these the epithelial cells are larger than usual ; 

 and under them, again, lies a cellular mass, the minute 

 structure of which he was not able to determine, but 

 which is connected with a nerve. 



On the mantle of the Chitons are also certain 

 well-defined organs, probably of touch. They occupy 

 pores in the shells, and resemble obconical or somewhat 

 dice-box shaped plugs of transparent, highly refracting 



Fig. 17. — Diagram of the structure of the soft and some of the hard parts in the teg- 

 mentum of a shell of a Chiton {Acanthopleura spiniger), as seen in a section vertical 

 to the surface and with, the margin of the shell bordering on the girdle lying in the 

 direction of the left side of the drawing. /, Calcareous cornea ; h, iris ; g, lens ; 

 k, pigmented capsule of eye ; n, optic nerve ; r, rods of retina ; n', branches of 

 the optic nerve, perforating the capsule wall, and terminating in b', b', b', ocular 

 sense-organs ; p, p, nerves to sense-organ ; m, body of sense-organ cut across; 

 a. p, fusiform body of sense-organ entire ; a, obconical termination of sense- 

 organ ; e, nerve given off by one sense-organ to another, b". 



tissue. The terminal knobs end in flat discs, which 

 show a series of concentric rings, as if composed of a 

 series of concentric layers or inverted cones fitted one 

 within the other.* Each one terminates in a nerve- 



* Moseley, Quarterly Journal of Microscopical Sociefij, 1885. 



