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will here be fully explained and illustrated. The characters of the families and genera will be first defined, giving their resemblances and differences, and then the species contained in each genus will receive the lion's share of attention. Here we purpose to give a short description of the animal and its shell, so that we may be able to distinguish the species from others, followed by an account of its habits, where it may be sought for, and how the prize may be secured'; in fact, all the incidents of its short but interesting life history will be given. Analytical tables, however, based on artificial characters, to facilitate a ready determination of the species and genera, will doubtless be found useful.
1.-STRUCTURE OF THE PAINTERS' MUSSEL
THE most common of the fresh-water bivalves
is the swan mussel, an ordinary tenant of our rivers, streams, and ponds, lying halfburied in the mud. We select, however, the less widely distributed Painters' Mussel, as a type of the bivalves for study; because the characters of the class are better exhibited by it than by the swan mussel.
Let us first glance at the characters afforded by the shelly covering. The shell is at once seen to be composed of two pieces or valves; one is applied to the left side of the body of the mussel, and the other to the right: the valves are equal, and the shell is therefore said to be equivalve. The line along which the two valves are joined is the hinge, and that part in its vicinity, because it covers the back of the animal, is called the dorsal region ; that opposite to it, the ventral region. The rounded margin is the anterior
margin (fig. 1); and the pointed, the posterior. The prominent part of each valve near the hinge is the umbo, u, which, when it is in the middle, the shell is said to be equilateral ; but in the painters' mussel, we observe that the portion of the shell lying in the front of the umbo is shorter than that behind it, and the shell in this case is said to be inequilateral. Behind the umbo is a ridge, composed of a horny elastic substance, called the ligament, l, which is a mechanical contrivance by which the valves are opened. The depressed space in front of the umbo is the lunule. The length of the shell is measured from the anterior to the posterior side ; its breadth is the perpendicular distance from the umbo to the front; its thickness is the diameter through the closed valves. Externally the valves are marked by concentric lines of growth, which diverge from the umbo, which is the point from which the growth of the valve commences.
The shell is composed of layers of animal matter impregnated with carbonate of lime, and consists of three structures :-an outer horny layer, called the epidermis, which does not contain calcareous salts; and which may be removed by steeping the shell in an acid solution, when the epidermis alone remains. Under the microscope, it exhibits a cellular structure in some parts, and a granular in others. Beneath the horny layer is a stratum
consisting of delicate prismatic cells of calcareous matter, and an internal layer, which is shining and pearly, and makes up nearly the whole thickness of the shell. This latter, or nacreous layer, consists of folded plates of carbonate of lime, which, by refracting the light, give rise to that characteristic pearly lustre of the interior: this portion, when polished, forms “mother-of-pearl.” For the microscopic examination of the shell structure, sections are necessary; but in place of these, thin edges of broken portions of the shell may be employed. The shell grows partly by addition to the margin and partly to the interior.
In the interior of the shell, the following markings and parts are to be distinguished :
The umbonal cavity corresponding to the umbo.
The hinge-line of the right valve preşents a prominence or tooth (Plate II., fig. 2) towards the front, which fits into a depression between two teeth in the left valve. On the posterior part of the hinge-line of each valve is developed an. elongated tooth: these are said to be lateral; but when they are situated beneath the umbo, as in the fresh-water cyclas, they are termed cardinal. On the inner surface of the valves are seen two impressions or distinct pits, one near the posterior (a, a'), and one near the anterior sides in each valve, made by two strong muscles extending internally from one valve to the other.
These adductor muscular scars, as they are called, are connected by a faintly impressed line following the curvature, and near the front of the valve: this is the impression left by the attachment of the muscles of the mantle. Near to the adductor muscular scars, but a little further from the edge of the shell, are situated the impressions of the muscles that move the foot.
Figure 1 represents the right valve of the shell removed so as to disclose the animal. It is necessary
for all dissections to remove one of the valves. This may be readily accomplished by first killing the animal by hot water, when the valves will gape open. Forcing them now wider apart, the muscles which close the valves may be then readily seen, as white cords, which must be cut with a pair of scissors. The whole body of the animal is covered by a thin fleshy envelope, termed the mantle, which consists of two lobes, joined at the back, but free in front. The disposition of the lobes has been well compared to the "covers of a book when it is placed on its edge with the back uppermost.”
Each lobe of the mantle corresponds with a valve of the shell, and is attached to it in front by a series of muscles, which produce the mantle-line of impressions. The mantle extends as a free portion beyond the muscles to the edge of the shell, which portion is much thickened, and secretes the shell. The lobes