SECT. XL EYES OF MOLLUSCA. 235 



Shell, and the beautiful porcellanous Cyprsese or Cowries, 

 are much valued by the artists who cut cameos, on ac- 

 count of the structure of their shells, which consists 

 of three strata, the same in composition, but differing in 

 arrangement, and sometimes in colour. Each stratum 

 of the shell is formed of many thin laminae, placed 

 side by side, perpendicular to the plane of the stratum, 

 and each lamina consists of a series of prismatic 

 spicules with their long sides in close approximation; 

 the laminae of the inner and outer strata have their 

 spicules parallel to one another, while the spicules of 

 the intermediate lamina are perpendicular to those on 

 each side. According to Dr. Bowerbank, who discovered 

 this complicated structure, the spicules are microscopic 

 tubes filled with carbonate of lime. 



The Spondylus gsedaropus has sixty ocelli constructed 

 for accurate vision. One can form no idea of the ef- 

 fect of so many eyes, unless they combine to form one 

 image as our eyes do. The common Pecten, or Scal- 

 lop, pretty both in form and colour, has a number of 

 minute brilliant eyes arranged along the inner edge of 

 the mantle, like two rows of diamond sparks. Some 

 families of mollusks are destitute of eyes, even of the 

 simplest kind ; and it has been observed that those 

 mollusks most liberally provided with eyes are also en- 

 dowed with the most active and vigorous motions. The 

 bivalves do not appear to have either taste or organs of 

 hearing, but they are exceedingly sensitive to touch. It 

 is singular that animals which have neither head nor 

 brain should have any senses at all. A nerve-collar 

 round the gullet with a trilobed nerve-centre on each side 

 supplies the place of a brain ; nerves extend from these ; 

 besides there are nerve-centres in various parts of the 

 unsymmetrical bodies of the acephalous mollusks. 



The Gastropoda, or crawling mollusks, have a head, 

 and are consequently animals of a higher organization 



