TEGUMENTARY ORGANS 395 



Crustacean and MolluScan shells {vide infra), but to the development 

 •of the teeth, strongly confirming, I think, the view which I have taken 

 ■of that process. 



Integument of the Mollusca. — The soft surface of the body of the 

 Mollusca in general is constituted by an ordinary, commonly ciliated, 

 •cellular ecderon, which needs no special description. The hard or 

 soft shells which so many of them possess, arise in two modes ; the 

 calcareous and horny integumentary appendages being, I believe, 

 invariably produced by excretion, \^\iA& the Ascidian test, which contains 

 •cellulose, is formed by conversion. It will be advisable to treat of the 

 structure and histological development of these two forms separately ; 

 and, first, of the 



Excretionary integument of the Mollusca. — This is to be met with 

 in its simplest form in the Polyzoa, in which the integument (ectocyst 

 ■of Allman) is formed by a structureless membrane containing im- 

 ■bedded calcareous or silicious particles.^ 



An admirable example of the calcareous integument formed by 

 ■excretion is to be found in the shell of Unio and Anodon. The outer 

 surface of the shell in these Lamellibranchs is, as is well known, covered 

 hy a brownish or greenish irregular membranous substance, the so-called 

 ■" epidermis '' of the shell. This substance, however, by no means 

 ■constitutes a single membrane ; on the other hand, the surface of the 

 shell is marked by an immense number of closely set, more or less 

 parallel, concentric lines, some of which appear to be formed by rugse 

 •of the " epiderm," while others are the free edges of epidermic laminae 

 •cropping out under those of older date. Viewing this surface of the 

 shell by transmitted light with a low power, a number of polygonal 

 closely set areae come into view on depressing the focus through the 

 thickness of the epiderm. 



The inner surface of the shell has, for the greater part of its 

 •extent, a pearly or nacreous lustre ; but along the gape of the shell, 

 at a distance of from less than one line, to as much as two or 

 three lines, from the free edge, the nacreous appearance ceases, and 

 we find, instead, a brownish hue similar to that of the epiderm, and 

 becoming gradually more intense till the very margin is constituted 

 by a flexible brown membrane continuous and identical with the 

 epiderm on the exterior. If the surface of the flexible zone be 

 examined as before, its outermost portion appears quite homo- 

 geneous ; as we pass gradually inwards, however, dots appear in it, and 

 the hard portion of the brown zone presents polygonal areae, precisely 



' I am indebted to Mr. Busk, whose extensive researclies on these animals are well 

 known, for the information on which this statement is based. 



