156 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



many observers as being the cause of the nacreous iridescence as 

 already stated. The whole surface of the inner lining is perforated 

 by exceedingly minute pores, apparently at regular intervals, 

 sometimes perforating the fibres themselves. They are about jfa 

 of an inch in diameter. The fibres and the lines of darkened cells 

 already referred to give a striated appearance to the nacre which 

 is different from any other tissue. There are little oblong lacunae 

 occurring at intervals, and some of these appear to be associated 

 with the transverse bands of colour already referred to. Some of 

 these transverse bands are no doubt strands of nerve-tissue which 

 have been partly destroyed in making the section. 



If the species of Patella with which we are dealing be treated 

 with acid, so as to decalcify some of the plates, the thin 

 membranaceous tissue can be detached and placed under the 

 microscope, where the folds are still visible and the fine nerve- 

 fibres that formerly passed along its surface can be detected, though 

 of course much broken, twisted, and out of position. If the 

 interior of the shell be closely examined it will be seen that the 

 acid has eaten into some of the eye-chambers and destroyed every 

 thing except the lining of pigment-cells. This coating however, 

 is not universal in all the eye-chambers. It should be mentioned 

 that the membrane and membranous fibres seen in section are best 

 visible with the aid of the polariscope. 



A fourth layer is found in limpets in what is called the spathula, 

 which consists of a porcellanous structure very much like the 

 second outer layer, but which differs apparently in this, that its 

 lower surface is highly polished, and under the microscope has a 

 somewhat papillated appearance, consisting of minute pits 

 somewhat like the surface of a thimble, in amongst larger rugosities, 

 yet in no case rising higher than just sufficient to give a somewhat 

 dead appearance to the enamel. This latter cracks like old china 

 in the older shells and those which have had even a trifling amount 

 of exposure. Under a moderate power of the microscope, distinct 

 pores can be perceived. 



A peculiarity with this spathula is the mode in which it varies 

 in its colour ; being brown, yellow, orange, olive, pale bluish-green, 

 and mottled brown, sometimes opaque and sometimes transparent. 

 Round the spathula, as already stated, there is a distinct horseshoe 

 shaped depression which limits it, and at the outer edge of which 

 I believe is the principal exit for the nerves. The edge of this 

 spathu la-plate is gradually bevelled off, and here it is, that with 

 microscopic aid, pores can be seen. In nearly all the shells that 

 I have examined, I notice a peculiarity in the spathula which is 

 that the colouring matter seems to be associated with a kind of 

 granular structure, or as if flecks and fragments of a lighter colour 

 were floating about in a transparent medium. In very many shells 



