146 On the Presence of Eyes in the Shells of the Chitonidaj. 



a moderate estimate, reckoning, as before, only the eyes in 

 tolerable condition, there must be at least 8500 eyes. 



In Tonicia marmorata the eyes have the peculiarity of being 

 sunk in little pit-like depressions of the shell-surface. This, 

 no doubt, is a contrivance for preventing them from being- 

 worn off, and the result is that they are all retained complete 

 in large old specimens. They are arranged in single, straight, 

 radiating rows on the anterior and posterior shell, disposed 

 with considerable symmetry. There are thirty-four such 

 lines on the anterior shell in one specimen, containing about 

 eighteen eyes each. On each lateral area of the intermediate 

 shells there are from two to four similar rows of eyes, with a 

 few eyes grouped irregularly also. In some forms placed in 

 the British-Museum collection as species of Tonicia, there are 

 no eyes present ; these possibly will be found to require to 

 be placed in a separate genus. 



In Ornithochiton the eyes are not sunk so deeply in pits, 

 but are disposed somewhat as in Tonicia, though the rows 

 are not so regular. In Ghitonellus there are no eyes and 

 but a scanty supply of organs of touch. 



I have been unable to trace the nerves supplying the shells 

 and eyes directly to their source, although I have no doubt 

 that they proceed from the parietal (branchial) nerve, from 

 which I have traced numerous offsets proceeding in the 

 required direction. 



I have searched in vain for any similar eyes in the shells 

 of Patella and allied genera. The tegmentary part of the 

 shell of the Chitonidai appears to be something siii generis, 

 entirely unrepresented in other Mollusca. Its principal func- 

 tion seems to be to act as a secure protection to a most exten- 

 sive and complicated sensory apparatus, which in the ChitonidjB 

 takes the place of the ordinary organs of vision and touch 

 present in other Odontophora, and fully accounts physiologi- 

 cally for the absence of these in the group. In some respects 

 the arrangement of the hard and soft parts curiously resembles 

 that existing in the Brachiopoda. 



It is most remarkable that these eyes should have been 

 missed hitherto by all writers on the shells of the Chitonidas. 

 The fact is due, no doubt, to their minuteness and to the fact 

 that they are not very easily seen with a powerful lens in the 

 dried condition of the shell in most instances. In order that 

 they may be made most conspicuous the dried shell should 

 be wetted with spirit, and a lens as powerful as Hartnack's 

 no. 4 objective be used. 



Dr. W. B. Carpenter^ observed the perforate structure of 

 the tegmentum in Chiton, but did not apparently investigate 

 * ' Cyclopoedia of Anatoiuy and Physiology/ article " Shell," p. 565. 



