Presence of Eyes in the Shells of the Chitonidic 1-13 



from the surface of the sliell-bcd into the shell, to give the 

 general network of soft tissue. In the anterior and posterior 

 shells there are usually a considerable number of such marginal 

 slits, each with a corresponding tubular tract and ramifying 

 strands of soft tissues. 



The network of soft tissues contained in the canals within 

 the tegmentum ramifies towards the shell-surface and termi- 

 nates there either in eyes or in peculiar elongate bodies, which, 

 apparently, are organs of touch. These latter are long, some- 

 what sausage-shaped bodies, which terminate at their free 

 extremity in dicebox-shaped plugs of transparent tissue, which 

 show a somewhat complicated structure. 



The tegmenta of the shells of most Chitonidai are perfo- 

 rated at tlie surface by circular apertures or pores of two sizes, 

 arranged in more or less definite patterns with regard to one 

 another and sometimes with regard to the eyes also. 



The end plugs of the sense-organs above described lie in 

 these larger pores. From the sides of the sausage-shaped 

 sense-organs are given off more or less numerous fine strings 

 of soft tissue, which, diverging, pass to the smaller pores 

 above described and there terminate in very small plugs, just 

 like those of the larger similar organs, but less complex in 

 structure. 



The eyes are evidently to be regarded as having arisen as 

 modifications of some of the organs of touch above described. 

 They are 'connected with the same network as terminal 

 organs of its ramifications in the same manner, and have 

 points of resemblance to them which are convincing as to the 

 lioraogeny of the two. The soft structures of each eye lie in a 

 more or less pear-shaped chamber excavated in the substance 

 of the tegmentum. The stalk of the pear, which forms the 

 canal for the passage of the optic nerve, is directed always 

 towards the free margin of the tegmentum, whence the nerve 

 reaches it. One side of the bulb of the pear is closely applied 

 to the outer surface of the tegmentum, and here its wall is 

 pierced by a circular aperture, which is covered by the cornea. 

 The cornea is calcareous ; it resists the action of strong 

 boiling caustic alkalies, but collapses at once when treated 

 with acid. Probably some soft tissue is present in its sub- 

 stance, but I have been unable as yet to find it. 



The cornea in sections shows itself to be formed of a series 

 of concentric lamellai ; its substance is continuous with the 

 general calcareous substance of the tegmentum at its margins. 



The pear-shaped cavity of the eye formed by the shell- 

 substance is lined by a dark brown pigmented choroid mem- 

 brane of a stiff and apparently somewJiat chitinous texture. 

 This membrane exactly follows the shape of the cavity, but 



