ANATOMY AND LIFE HISTORY OF MOLLUSCA. 135 



sometimes on the surface of the mantle, or immersed to some 

 extent in its tissue. 4. Eyes and sense-organs on the opercula, 

 generally stalked or on raised tubercles. A short description of 

 each of these kinds is necessary before describing particular 

 instances. 



Shell Eyes of the Tegmentum. — If a section be made of any 

 of the following common littoral shells of Australia, certain 

 appearances, to be mentioned presently, will be noticed. The 

 species I now particularly refer to are Patella tramoserica, Martyn, 

 Acmrea septiformis, Q. & G., Siphonaria diemenensis, Q. <fc GL, 

 Cerithium ebeninum, Brug., Turbo (Senectus) gruneri, Philippi, T. 

 ( Lunella) undulatus, Chem., Malleus vulgaris, Lam., (the hammer- 

 headed oyster), Mercenaria paucilamellata, Dunker, Trigonia 

 lamarckii, Gray, T. margaritacea, Lamarck, Anatina tas7nanica, 

 Reeve, Ostrea mordax, Gould, and Area trapezia, Desh. It will 

 be observed that I do not select these species as being exceptional 

 or extraordinary illustrations (except in the genus Trigonia). 

 They are in truth taken indiscriminately from the many shells 

 which I have examined, only I happen to have these specimens 

 before me while I am writing the present remarks. A section 

 through any of these shells shows on the outer surface a thin 

 stratum of partly refractive structure. Under the microscope 

 this layer is seen to consist of transparent capsules with a 

 hyaline covering outwardly, and frequently a distinct lens and 

 pupil within. These capsules are supplied with nerves from 

 below, from a large nerve-ganglion in the shell in the case of 

 the bivalves, and from a spirally shaped trunk in the columella, 

 in the case of univalves. All the capsules cannot be regarded as 

 visual organs, or at any rate they are too minute to ascertain this 

 satisfactorily ; but they are all supplied with nerves abundantly. 

 For the most part they are so close together as to form a 

 pavement, but occasionally they are scattered. This layer has 

 been of course observed by every naturalist, and has generally 

 been confounded with the fibrous layer of prismatic shell-structure, 

 but that it subserves a far more interesting and important purpose 

 I think I shall be able to show. 



If instead of taking a tranverse section, a portion of the shell 

 is ground flat and thin from below, the eyes can be seen and the 

 sense-organs (through the shell) with great distinctness, sometimes 

 with the aid of a lens, but sometimes requiring to be magnified 

 considerably. They occur in the form of minute pellucid, raised, 

 circular, or oval points, transparent, refracting the light brightly 

 and with a minute dark dot in the centre. They are so thickly 

 scattered over the shell-surface as in some cases to leave scarcely 

 a point which is without them. In some others they are sunk in 

 little pits and depressions, or in the valleys between the rugae of 



