Dissected so as to show its muscular and nervous systems, as well as certain other organs in relation with them.

Fresh water Mussel, Anodonta cygnea.

Plate VI.. Fresh-water Mussel, Anodonta cygnea.

The animal has been taken out of the shell; the gills and the mantle have been removed on the left side, together with the labial tentacles and parts of the pericardium, as well as of the organ of Bojanus or nephridium of the same side.

a. Right mantle lobe, free along its ventral edge.

a. Fimbriated portion of mantle corresponding to the inferior siphonal notch by which water is drawn into the branchial cavity. a". Dorsal raphe along which the two halves of the mantle meet.

b. Foot. The muscular portion is strongly contracted.

c. Gills of right side.

cr Union of external gill to the mantle between the inferior and superior siphonal notches.

d. Anterior adductor.

e. Posterior adductor.

f. Posterior retractor of the foot, inserted into either valve, anteriorly and superiorly to the posterior adductor, the scar or muscular impression of the two being more or less confluent. Its muscular expansion in the foot is especially well developed along the free or ventral edge of the foot, and it inter-digitates very freely with the protractor pedis, though it lies for the most part at a lower level than that muscle.

g. Protractor of the foot. This fan-shaped muscle spreads over the external surface of the foot, from an insertion into the shell, a little superiorly to the point where the pallial line joins the impression of the anterior adductor. It acts, consequently, as an antagonist to the preceding and succeeding muscles. Its impression is distinct in this animal from that of the adductor.

h. Anterior retractor of the foot. The fibres of this muscle take origin from a point in the shell, towards the dorsal aspect of the anterior adductor, though some way from its dorsal border. They spread thence into the foot especially along its anterior edge, and down as far as its anterior angle, for the most part at a deeper level than . the preceding muscle. Some of the fibres, however, spread over the visceral mass dorsally.

h' The line points to the position of the smaller retractor muscles with insertions just anteriorly to the umbones, whence they radiate over the regions of the stomach, and towards the pericardium.

i. Left cerebro-pleural ganglion lying in the angle between the anterior retractor and adductor, and the protractor pedis, above the entrance to the mouth.

j. Connective passing from the left cerebro-pleural ganglion to the left pedal. The pedal ganglion of each side gives off twelve nerves, six from its neural, and six, more slender, from its lateral surface. They are not figured in this plate.

j. Auditory vesicle appended to pedal ganglion. This vesicle is ordinarily said to be appended to a branch given off from the most backwardly placed but one of the posterior pedal nerves. But Simroth states that its nerve is derived from the cerebro-pedal connective and so figures it. See Z. W. Z. xxvi. 1876, Pl. XVI. Fig. 56, and p. 138, ante. It is not always symmetrically developed on both sides, and, when present on one side only, it has been found to contain two otoliths. It is situated in a part of the foot narrow from side to side, at the junction of its anterior two-thirds to its posterior third, and near to its purely muscular portion into which the viscera do not enter.

k. Commissure between the left cerebro-pleural and visceral ganglia. It passes between the fibres of the retractor pedis and the protractor through the upper part of the foot, internally to the generative orifice, t; then through the glandular portion of the nephridium, s; and across the tendon of the retractor pedis posterior where it bifurcates for insertion into either valve of the shell.

l. Left visceral ganglion. Two nerves are figured" in connection with it, one, a parietal nerve, going to the mantle, the other, a branchial nerve, going to the gill.

m. Rectum ending in the cloaca. A delicate nerve is figured by Duvernoy, op. cit. p. 138, ante, as passing to it from the visceral ganglion.

n. Heart; the letter pointing to the slit left by removal of the left auricle.

o. Pericardial space into which opens the glandular portion of the nephridium.

p. External opening of the nephridial duct.

q. Opening by which the glandular portion of the nephridium communicates with the duct.

r. Wide opening by which the ducts of the two nephridia communicate. This opening does not exist in Unio margaritifer.

s. Secretory or glandular portion of the nephridium, reaching from the level of the anterior end of the pericardial space to the under surface of the posterior adductor. It opens into the pericardium by a canal along which a bristle has been drawn as passing. The glandular portions of the two nephridia communicate freely with each other, as do also the excretory sacs in this species. t. Orifice of the duct of the generative gland. This orifice is concealed in Anodonta, though not in Unio, by the attachment of the inner gill-lamina to the visceral mass. See V. Baer, Meckel's Archiv. 1830, p. 318.