Integument ofthelliollusca.—The soft surface of the body of the Mollusca in general is con stituted by an ordinary, commonly ciliated, cellular ecderon, which needs no special de scription. The hard or soft shells which so many of them possess, arise in two modes ; the calcareous and horny integumentary append ages being, I believe, invariably produced by excretion, while the Ascidian test, which con tains 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 imbedded 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 by 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 rugm of the "epiderm," while others are the free edges of epidermic laminm 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 arex 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 homogeneous; as we pass gradually inwards, however, dots appear in it, and the hard portion of the brown zone presents polygonal arem, precisely resem bling those under the epiderm on the outer sur face. Where the nacreous appearance com mences, these arem disappear, becoming ob scured by an opaque white substance, which is marked by elevations and depressions, corre sponding with, though less prominent than, the principal ones upon the external surface.
If, now, a section perpendicular to the surface and to the concentric lines be taken, and viewed in the same way by reflected light, the cause of the various appearances which have been described will become obvious.
It will be seen that the thick middle of' the shell is composed of three substances; of a very thin external brown layer, the " epi dermis," and of two other layers more or less equal in thickness ; an external, composed of minute polygonal prisms or columns set per pendicularly to the surface, and an internal, which looks structureless, with a fracture like loaf sugar. The outer prismatic layer pre serves its thickness as far as the " brown zone " above described, and then gradually thins out into the flexible marginal membrane. The inner nacreous layer, on the contrary, gradually thins out, and ceases at the corn. mencement of the brown zone. The ends of the prisms are, therefore, bare in the brown zone, whence the polygonal areolation ob served in it; while its colour arises partly from the brown epidermis shining through, partly from a slight tinge of the same kind which runs through the prismatic substance, and renders it distinguishable, even to the naked eye, from the intensely white nacreous layer.
Thin vertical sections of these shells pre sent the following appearances under a high magnifying power. The external edge is con stituted by a delicate brown band, the " epider mis," in which no structure of any kind can be detected. Within this is the pris matic layer, a dense transparent substance marked by strong parallel lines which run perpendicularly to the surface and either extend completely through the layer, or termi nate by joining some other within it. In the former case, the spaces which they enclose appear like the sections of prisms (of Th of an inch, more or less, in diameter): in the latter, they resemble longer or shorter cones whose bases are turned outwards. A number of such short cones are usually interposed between those ends of the prisms which are in contact with the epidermis.
Internally, or at the line of contact of the prismatic with the nacreous layer, the lines either remain parallel or converge.
The prisms are readily broken away from one another, and in this case, or in a sufficiently thin section of the whole layer, they are seen to be traversed by very closely set pa rallel transverse lines about To-f,b--0 in. apart. Each prism, however, does not possess a set of striw peculiar to itself; on the other hand, the parallel lines stretch without interruption through the whole length of the prismatic layer, as if the prisms were not there. A horizontal section of the prismatic layer pre sents, as has been said, a coarse polygonal reticulation corresponding with the lines of contact of the prisms. The substance of the latter appears granular, but without any other structure in fully formed portions (fig.313.