ON THE MICROSCOPIC STRUCTURE OF SHELLS. 9 



Other nor polygonal in form, but separated by a greater or less amount of 

 intercellular substance, and presenting a rounded instead of an angular 

 border (fig. 1 2 c). Upon looking still nearer the margin, the cells are seen 

 to be yet smaller, and more separated by intercellular substance (fig. 12 b.); 

 and not unfrequently we lose all trace of distinct cells, the intercellular sub- 

 stance presenting itself alone, but containing cytoblasts scattered through it 

 (fig. 12 a.). This appearance has been noticed by myself in Perna and 

 Unio, and by Mr. Bowerbank in Ostrea ; so that I have no doubt that it is 

 general in this situation. We may, I think, conclude from it, that the cells 

 of the prismatic cellular substance are developed, like those of cartilage, in 

 the midst of an intercellular substance, which at first separates them from 

 each other ; that as they grow and draw into themselves the carbonate of 

 lime poured out from the subjacent surface, they approach each other more 

 and more nearly ; and that as they attain their full development, their sides 

 press against each other, so that the cells acquire a polygonal form, and the 

 intercellular substance disappears. 



V. Membranous Shell-substance. 



16. Under this appellation I describe the substance, of which (under va- 

 rious forms) all those shells consist, that do not present the prismatic cellular 

 tissue just described. In this substance no trace of cells can for the most 

 part be discovered ; and when they do present themselves, they are usually 

 scattered through it with little or no regularity, and do not form a continuous 

 stratum, when the calcareous matter has been i-emoved by acid. In no shell, 

 even those most decidedly porcellanous, have I failed in detecting some 

 membranous basis, although the film is often of extreme tenuity. I believe 

 that there is no shell, in which this kind of structure does not exist under 

 some form ; for even where almost the entire thickness is made up of the 

 prismatic substance, as in Pinna and its allies, there is still a thin lining of 

 nacre, which I shall presently show to be but a simple modification of the 

 ordinary membranous structure. 



17. Although I cannot yet speak positively on the subject, still lam much 

 disposed to believe, that in every distinct formation of shell-substance there is a 

 single layer of membrane; and I am further of opinion that this membrane was 

 at one time a constituentpart of the mantle of the mollusc. The late researches 

 of Mr. Bowman upon mucous membrane, have shown that the essential consti- 

 tuent of this tissue is a delicate, transparent and homogeneous expansion, the 

 free surface of which is usually covered with epithelium-cells, whilst the attached 

 side is in contact with that complex tissue (composed of areolar structure, 

 blood-vessels, lymphatics, &c.) to which the name of " mucous membrane" 

 is commonly applied. This expansion is termed by Mr. B. the " basement 

 membrane;" and it is found, not merely on the raucous membranes, but also 

 on the external surface of the tnie shin, lying beneath the epidermic cells. 

 Now the mantle of the MoUusca, being essentially analogous to the true skin 

 of higher animals, may be inferred to possess this element ; and if it be pe- 

 riodically thrown off and renewed, we have a case strongly analogous to the 

 formation of the " decidua" in the human uterus. Whether this be or be 

 not the origin of the membranous residuum, which is found after the decal- 

 cification of shell, the correspondence between this tissue and the basement- 

 membrane of Mr. Bowman is extremely close. In its simplest condition, the 

 former, like the latter, is a pellucid structureless pellicle of extreme delicacy 

 and transparency, exhibiting no trace either of cells, granules or fibres (fig. 19). 

 I have occasionally found it, however, of a somewhdX granular appearance, as 

 if formed by the solidification of a thin stratum of fluid, including an immense 



