The Soft Parts

Course of the blood: I. Heart to the system.

2. System to the kidney.

3. Kidney to gill.

4. Gill to heart.

The auricle is the receiver of the blood. The ventricle is the pump. The pericardium is the loose bag containing the heart. The arteries distribute pure blood throughout the living tissues; the veins collect it impure from these tissues. In the kidney the blood is relieved of its urea. In the gills it receives oxygen and gives out carbonic acid gas. The kidney and the gill are the two organs that relieve the blood of the impurities collected in the liv ing tissues. The blood of mollusks is cold and usually colourless.

The Alimentary Canal.

The mouth is at the tip of the long proboscis; behind it is the straight oesophagus, or gullet, which leads to the stomach. The odontophore has been mentioned 24 A Typical Univalve Mollusk before as the radula. This flexible ribbon is closely set with rows of horny teeth. It plays over the end of a central, stiff cartilage rod, by means of muscles attached at both ends, and contracted alternately. Thus holes are drilled in other shells by setting the tip of the proboscis on the surface and drawing the ribbon back and forth.

Two large salivary glands lie near the base of the cesophagus, with ducts to the mouth. The pancreas lies farther back on the right of the cesophagus. The liver is the third gland, furnishing juices that aid in the digestion of food. The stomach and intes tine complete the alimentary canal, which discharges its wastes into the mantle cavity.

The Nervous System. — A cluster of paired ganglia (little brains), brown in colour, encircle the cesophagus, three-fourths of an inch behind the base of the proboscis. One pair, the buccal ganglia, sends nerves to the mouth parts. Another pair, the pedal ganglia, supplies the foot. Both pairs are on the ventral side of the cesophagus. On the dorsal side the pleural and cere bral pairs are fused and bound by commissures of nerve fibre with the visceral ganglia; and the last are connected with the abdominal ganglion, a brown mass visible just below the opening of the kidney. The cerebral ganglia are the 'most centralised

"brains" of this mollusk, as they are not only joined, as a pair, but directly connected, by commissures or by contact, with the pedal, buccal and pleural pairs, and through the pleural with the other two. Muscles all over the body are controlled by nerves sent out from these ganglia. Sensations are brought to the nerve centres along nerves from the foot, head, and especially the sen sitive mantle border. By these the mollusk learns all it knows of what is going on outside its shell.

The Special Senses. — Snails have eyes, but generally of a low type. Sight is an unimportant sense. The sense of touch is well developed in the mantle margin; the tentacles are touch organs. The mouth has sensitive lips. The osphradium is an organ of doubtful use. It may be the seat of the sense of smell. It is believed to have a composite function of testing the water that passes over it and enters the mantle cavity. Smell is a well developed sense. The condition of the water and its fitness to furnish food and oxygen to the body are broader considerations probably determined by this generalised organ.

25 A Typical Univalve Mollusk Hearing is a very poorly developed sense in mollusks. Its special organ has been discovered in many genera of univalves and bivalves. A pair of sacs filled with liquid in which micro scopic pebbles float, have nerves connecting them with the cere bral ganglia. There is no denying the auditory function of these organs. As they are imbedded in the tissues, the vibrations re ceived are modified by the medium of the flesh through which they pass. Possibly such impressions ought not to be called sound, but touch, instead.

Taste as a special sense may exist, with nerve ends in the pharynx or back of the mouth.

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