336 MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 



abundance of nerves. This network terminates at the shell 

 surface, either in eyes or in peculiar elongate bodies which are 

 probably organs of touch. The soft structures of each eye lie 

 in a pear-shaped chamber excavated in the substance of the 

 tegmentum, and one side of this chamber is pierced by a cir- 

 cular aperture, covered by the cornea which is calcareous. 



The cavity is lined by a dark brown pigmented choroid 

 membrane, which, by projecting round the margin of the cornea 

 all round, forms an iris. A perfectly transparent, hyaline, 

 strongly bi-convex lens is fitted in behind the iris aperture ; 

 this is composed of soft tissue, and slowly dissolves in strong 

 acetic acid. Some of the fibres of the nerve entering the eye 

 cavity proceed to the retina, while others, perforating the 

 choroid at its outer margin, end at the surface of the shell, all 

 round the area occupied by the cornea. 



In some cases the eyes are present in enormous numbers ; 

 in one specimen of CorepJiuwi aatleafi/m, which was densely 

 covered by a green alga, there were something like 12,000 

 eyes present in good condition, not counting the eyes destroyed 

 by the boring of the shell by a/g(E and animals on the rest of 

 the area. To see these eyes in dried specimens the shell should 

 be wetted with spirit, and examined with a lens as powerful as 

 No. 4 Hartnack. No eyes could be found in any of the fossil 

 specimens in the British Museum collection, although the 

 ancient forms of the group appear to be allied to Schizochito7i. 



The shell, then, is secreted or formed by the glands of the 

 mantle, and may be either calcareous or chitinous in texture. 

 It is first formed by the glands, near the free margin, but all 

 parts of the mantle have the power of secreting the shelly 

 matter, as may be frequently seen in specimens which have 

 been accidentally broken or damaged by the attacks of other 

 animals and afterwards repaired. The variation in the structure 

 of the shell itself is very great, and if I were to enter on the 

 subject it would in itself absorb all the time at my disposal. I 

 may say, however, that the late Dr. Carpenter made a special 

 study of the structure of shells. 



J.C., vii., Jan. 1894. 



