mussel, it is necessary to pass a sharp blade between the valves, and cut through the muscles, before the valves will open. These muscles are called the adductor muscles, and the scars or impressions on the valves are called the adductor muscular impressions. Very close to the adductor muscular impressions are seen smaller impressions, and these indicate where the muscles are attached which move the foot. These muscles are called the pedal muscles, and the impressions are called the pedal muscular impressions. One occurs just behind the anterior adductor impression; the other will be found just above, and in front of the posterior adductor impression.

FIG. 39. THE Right Valve of a FRESH-WATER MUSSEL.-C, Cardinal Teeth; 7, Lateral Tooth; li, Ligament; aa, Anterior Adductor Impression; pa, Posterior Adductor Impression; ap, Anterior Pedal Muscular Impression; pp, Posterior Pedal Muscular Impression; P, Pallial Line.

34. Besides these marks, the pupil will see a delicate and slightly irregular line running from the anterior to the

posterior muscular impression, just inside, and nearly parallel with the lower margin of the shell. This line is called the pallial line, and indicates where the mantle is attached to the shell. It will be observed that, when the soft parts are removed from the shell, the mantle adheres along this line.

The pupil may mark with a pen the names of all the parts upon the inside of a fresh-water mussel.

35. When the mussel is opened by separating the adductor muscles with a knife, the valves slowly open, and after the animal is removed the valves still remain partly open, and, to preserve them closed, a string has to be tied around them, and in this condition, if the ligament is allowed to dry, the valves will then remain closed. From this it is evident that the ligament acts upon the valves to draw them apart. To keep them closed, then, the animal must continually exert itself by contracting the adductor muscles; and it will be found that, when these creatures are left in the water, undisturbed for a while, the muscles relax, and the valves partly open. The ligament is elastic, and is stretched as it were from one valve to the other, over the back. A possible imitation of the action might be represented by partly opening the lids of a book, and then gluing across the back, from one lid to the other, a sheet of elastic rubber. If, now, the lids are tightly closed, the rubber is drawn out, or stretched across the back, and, if allowed to regain its elasticity, the lids are pulled apart. This experiment illustrates the way in which the ligament acts in those shells which have the liga ment external.

CHAPTER VI.

CLAMS, MUSSELS, AND OYSTERS.

36. CLASSES having access to salt-water may now collect some bivalves, as the clam, mussel, razor-shell, oyster, scallop, and whatever species they can find belonging to this group. A much greater variety of forms will be found in salt-water than in fresh-water.

Among some of the common species met with will be the following:

FIG. 40.-SALT-WATER BIVALVES.

In these the pupil may trace out the muscular impressions within the shell, and make out their relations to the impressions already described in the fresh-water mussels.

Many differences will be observed in the muscular impressions, as well as in the teeth and the position of the ligament.

37. As the common soft-shelled clam can be readily procured in the fish-markets, it will be well to study this first. A live specimen must be selected, and, as the clam lives a long time after it has been removed from the water, there will be no difficulty in getting the proper specimen. Upon pressing the valves together, or touching the soft parts which partly protrude from between the valves, the creature will show signs of life, by drawing the shells closer together, and this will assure the pupil that the specimen is alive.

A large shallow dish may now be filled with pure seawater, and in this the clam may be placed. After it has remained there for some time, the black end of the animal, which is incorrectly called the head, will slowly stretch out from between the shells, and the end, unfolding, will display two openings fringed with little feelers (see Fig. 42). Into one of these openings the water will be seen flowing, while from the other a current of water will be seen issuing. And these openings are called the incurrent and excurrent orifices, and correspond to similar parts previously described in the fresh-water mussels. In the latter creature, the openings just protruded beyond the edge of the shell. In some very small species of fresh-water bivalves, one of which was shown in Fig. 37, these openings were at the end of separate tubes. In the clam the tubes are inclosed in one sheath.

The clam can protrude this apparatus to a length equaling that of the shell two or three times. As the clam lives

buried at some depth below the level of the sand or mud in which it occurs, it requires this extension of the openings to reach the sea-water above.

38. It may be stated here, that the current of water passing into the general cavity of the shell not only carries the particle of food upon which the animal subsists, but conveys the pure sea-water to the gills by which it breathes, the gills performing the same function for animals living immersed in water as the lungs perform for creatures which breathe air. All bivalves depend upon currents of water to convey their food to them.

While, in the snails, the creatures could go in quest of food, having the power of protruding the head from the shell, and mouths furnished with means to bite or rasp their food, in the bivalves there is really no

head, they having anterior adductor

only a little opening directly under the muscle, which is the mouth, and into which the particles of food are swept.

39. If, now, the clam is opened, the edges of the mantle will be found much thickened and united, except a small slit near the front edge, through which can be protruded a small, tongue-shaped foot. Powerful muscles will be found at the base of the united siphons or tubes, which move the siphons in and out, and an examination of the inside of the shell will show where these muscles are attached. The pallial line, instead of running directly from the anterior adductor impression to the posterior one, is abruptly curved back, and forms a sharp bend, as it turns again to the posterior ad