of the mantle are united near the posterior margin and pierced by two orifices; the upper one is called the anal opening (fig. 1, v), the lower one the branchial opening, b.

The digestive organs.-The mouth (o) is a horizontal opening situated anteriorly just above the foot, between two pairs of soft flattened lips (t);

it is rather difficult to find, but may be discovered by tracing the lips to their insertions, between which the mouth is situated. The mouth opens immediately into a stomach, which contains a cylindrical jelly-like body, termed the crystalline style, whose function is not known. The intestinal canal is a narrow tube wound around a large liver, and after passing through the heart, is continued along the back to the vent (v), which is situated near the anal orifice: the excremental matters are carried away by the water which had passed over the gills.

The circulatory apparatus consists of a heart composed of two auricles and a ventricle, and may be readily discovered, as it is situated in the middle portion of the back of the animal, under the hinge; its beatings, which are about six or eight in a minute, are easily seen under the large bag, or pericardium, containing the heart. The heart continues to beat for a long time after the valves have been opened. The blood of shellfish is white, or nearly colourless; and so essential is the red character of blood deemed by the vulgar, that it appears to them little less than an abuse of language to apply the term to the nearly colourless fluid of the mussel; but it possesses. all the essential properties of blood, flows in a similar circle of vessels, and answers the same purposes in the system. The blood is propelled by the contraction of the ventricle into the arteries, and after supplying the waste of the body, is collected as venous blood by the veins from the capillary extremities; and after becoming aërated in the gills, is received by the gill-veins and conveyed as arterial blood to the auricles; from thence it passes into the ventricle, to be again distributed through the whole system.

The respiratory organs are gills, consisting of two membranous plates, attached to each lobe of the mantle; each plate consists of two folds of its membrane, and is pierced by innumerable

holes, which are beset with vibratile cilia; by the action of these extremely delicate filaments, a current of water is produced, setting in by the branchial orifice (fig. 1, b), which is surrounded by a fringe of sensitive tentacular organs, and sweeping over the gills, it is propelled towards the mouth (0), and passes out by the anal orifice (v) by this agency, the layer of water in contact with the gills is thus renewed, and food is also brought to the mouth.

These currents may be rendered distinct by introducing some powdered indigo into a basin of water containing a fresh-water mussel.

The branchial currents are most conveniently exhibited by the Zebra mussel, possessing, as it does, two prominent siphons (fig. 2). The inhalent siphon is densely fringed, and if the processes be touched, the orifice closes, and the siphon is at once retracted. When unmolested, a current flows steadily into the one orifice, whilst another current rises up from the other, as indicated by the arrows.

The wonderful activity of the cilia is a most striking spectacle; a magnifying power of about 120 diameters is amply sufficient to obtain a general view of the movements. A small portion of the gill must be carefully spread out on a glass slide, with a little water, and covered by a glass disk; or the animalcule-cage may be con

veniently used. A much higher magnifying power is required to observe the movements of each cilium.

The nervous system consists of three pairs of white nervous centres, called ganglia, from which the nerves radiate. Two ganglionic masses (the cerebral) are situated on the sides of the mouth, and are united to form a ring around the gullet; they are connected with two others (pedal) in the foot, and with those (the branchial) placed a little below the posterior adductor muscle near the anal orifice. This distribution of the nervous centres is eminently characteristic of the subkingdom Mollusca.

Organs of Sense.-The organs of hearing are two little sacs, situated on a pair of nerves arising from the ganglia of the foot, and contain a fluid in which is suspended a small calcareous body or otolith. The organs of sight are in the form of black dots, or ocelli, along the margin of the mantle.

Muscular Organs.-The ventral portion of the body is prolonged to form a fleshy tongueshaped foot (f), and has some analogy with the Gasteropods this locomotive organ is large, flexible, and muscular, and capable of being protruded beyond the shell. Change of position is effected by the insertion of the foot beneath the shell, and, by being employed as a lever, the

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shell is jerked forward; or the extremity of the protruded foot is made a lever, and the animal draws itself forward. By means of the same organ, the animal can bury itself in the sand or mud, in which it lives. The pedal muscles (p, p') retract the foot.

The adductor muscles (a, a') are two in number, they extend from one valve to the other, and by their contraction the shell is closed; they are hence named adductors. On their relaxation, or if they be cut, the elasticity of the ligament comes into play, and the valves are opened. The term dimyary bivalves is applied to such, as the Unio, with two adductor muscles; the monomyary bivalves, as the oyster, have only one. The freshwater Conchifera are all dimyarian bivalves.

Reproduction.

Each individual is male or female. The impregnated eggs, of which 200,000 are computed to be produced by each individual in the year, pass into the external folds of the gills, which during the breeding season in early spring are broad and thick: here they are hatched, and the young remain for some time. The embryos are very unlike the parent, and have been described as parasites under the name of Glochidium. They have a triangular shell provided with serrated hooks, and have but one adductor muscle, and a long slender byssal filament. In this condition they have been found