ANATOMY AND LIFE HISTORY OP MOLLUSCA. 153 



action of the sea, that very few shells can be found in which they 

 are easily visible, and rarely any in which they are arranged in 

 their full symmetry as in the plate. A thousand different causes 

 may interfere to destroy or remove the structures, so that the 

 symmetry of the whole arrangement becomes interfered with and 

 rendered, in most shells, quite unrecognizable. It should also be 

 remembered that in every case, or nearly every case, the eyes are 

 hidden in a little depression or pit which conceals them very 

 effectually when the shell is dry. Various methods must be 

 adopted so as to view the organs under different conditions and to 

 familiarize ourselves with their aspects. If the shell surface be 

 cleaned with acid the eyes are destroyed effectually, but the 

 position they occupy becomes easily visible. The shell-surface 

 then can be cleaned of foreign matter and the plan of the 

 arrangement becomes apparent. Prolonged maceration of the 

 shell in caustic potash cleanses it from impurities and though it 

 does not destroy the eyes like the acid, yet it effects them to some 

 extent, 'and then they are no longer visible except in a few cases. 

 Prolonged maceration in dilute acid, so as to lead to partial 

 decalcification, reveals many other portions of the structure which 

 are invisible otherwise. Observers should also be warned that 

 other things may be mistaken for the eyes, especially the peculiar 

 appearance on the surface of the shell which a reflection of light 

 from little asperities causes. I have found also that very hard 

 brushing of the surface has a tendency to remove the eyes. The 

 best way probably is to cleanse and macerate a number of shells 

 by all the different methods and subject them to minute examination 

 side by side. 



Before passing on to other species I must try to show how it is 

 possible to reconcile my observations with what other observers 

 have stated. Every one knows what has been regarded hitherto 

 •as the characteristic structure of the ordinary bivalve. The 

 following quotation on the subject is taken from Dr. Carpenter's 

 work on the microscope, who takes as a type the group of 

 Margaritacese. He says, "In all these shells we readily distinguish 

 the existence of two distinct layers ; an external, of a brownish- 

 yellow colour ; and an internal which has a pearly or nacreous 

 aspect, and is commonly of a lighter hue. The structure of the 

 outer layer may be conveniently studied in the shell of Pinna, in 

 which it commonly projects beyond the inner, and there often 

 forms laminae sufficiently thin and transparent to exhibit the 

 general nature of its organisation without any artificial reduction. 

 If a small portion of such a lamina be examined with a low 

 magnifying power, even without any preparation by transmitted 

 light, each of its surfaces will present very much the appearance 

 of a honey-comb ; whilst its broken edge exhibits an aspect which 



