

ANATOMY AND PHYSIOLOGY OF PATELLA VULGATA. 625 



other parts of the inside of the shell. The entire shell is translucent, though 

 the portion round the apex tends to become opaque. 



The microscopic structure of the shell is extremely difficult to determine 

 accurately. If it be soaked in strong nitric acid for some time, a series of 

 laminae may be peeled off, each lamina being apparently made up of a net- 

 work, or meshwork, of very fine fibres. The superficial laminae have much 

 wider meshes than the deeper laminae. 



A vertical section through the entire shell presents three layers, the middle 

 layer above extending from edge to edge (PL CLII. fig. 46). The inner layer 

 is found only beneath the dome, and, under the low power, presents itself as a 

 granular, more or less structureless film, occupying mainly that portion of the 

 shell not touched by the derm of the visceral mass. The outer layer, which also 

 appears granular, carious, and pigmented, extends over the entire surface, save 

 the extreme borders. This layer, when examined under a higher power, is found 

 to be perforated in every direction by minute canals (PL CLII. fig. 49). These 

 canals are not more than xrRny °f an mcn m diameter, and branch and anasto- 

 mose to such an extent as to give that part of the shell the appearance of yellow 

 elastic cartilage, or the framework of a sponge. The canals are much more 

 abundant towards the surface. The canals break into one another, and thus 

 form larger canals varying in size according to the number of small canals which 

 have gone to form the larger. The small canals are long, unbranched, and 

 straight, and much fewer in number as the middle layer is approached. Indeed, 

 there can scarcely be said to exist three distinct layers ; the inner and outer 

 layers being simply the middle layer under different conditions of growth or 

 decay. The middle layer itself is composed of long, branching, polygonal " cells " 

 or rods, whose long axes lie at right angles to the outer surface of the shell. 

 Each rod is made up of a large number of febrils, lying parallel to the long axis 

 of the rod, and the rod has in consequence a striated appearance. In section 

 the rods are polygonal, round, oval, or irregular ; they are separated from each 

 other by a cement substance, which forms the reticulum left after the removal 

 of the mineral matter by the nitric acid. This substance, which is fibrillated, is 

 stained yellow by the acid, and is therefore probably animal in its nature. These 

 rods are crossed, especially towards the border of the shell, by lines indicating 

 the successive laminae of deposition. The arrangement of the various elements 

 will be best understood by reference to the figures (PL CLII. figs. 46-49). 



The inner and middle layers are perfectly colourless ; the outer layer has 

 yellowish-green and brown pigment granules deposited in the canals by which 

 it is perforated. The bands of bluish-grey, usually seen on the inner surface of 

 the shell, are therefore not due to pigment, but probably to the peculiar 

 arrangement of the rods, and the effect of light upon them. On the inner 



VOL. XXXII. PART III. 5 L 



