and most of the surface of the gill, are lined with ciliated cells, the cilia of which, beating more strongly toward the cloacal cavity, cause a current of water to flow over the gills and through the holes into the cloacal cavity. Cilia are also found lining the inner surface of the mantle and on the labial palps, which we shall take up later.

Structure of Gills. The internal structure of the gills is such that blood slowly circulates through a network of thin-walled spaces, which, in the margin of the gill, are separated from the water by only a single layer of thin cells. Through this layer oxygen is taken by osmosis from the water, and carbon dioxide given up. The latter gas is passed off in the water through the excurrent siphon.

Food Getting. The cilia of the gills (and of the mantle cavity in general) play an important part in food getting. The clam, because of its sedentary life, must receive its food in the water which enters the mantle cavity. Food consists principally of onecelled animals and plants. This food is collected by the cilia surrounding the ostia, or holes in the gills, and is passed to the labial palps, four little flaps which surround the mouth. The mouth may be found at the anterior end of the visceral mass. The ciliated palps act as lips and pass the food on into the mouth.

FOOD TUBE. The food tube and its digestive gland (the latter a greenish mass easily seen through the body wall) occupies part of the visceral mass. It is a thin-walled tube which makes several turns before leaving the body. The stomach is a slight enlargement surrounded by the dark-colored digestive gland. This gland has the same function as the pancreas of higher animals.

Circulation of Blood. The circulation of the blood in the clam is of chief interest to us because of the curious heart, which is well developed, and has somewhat the same structure found in higher animals. The heart may easily be found in a living clam and the rate of beating counted. It is located near the surface of the dorsal side of the body close to the hinge ligament. It consists of two chambers, an auricle, which receives the blood, and a ventricle, which by muscular contraction pumps the blood on its course. The heart is surrounded by a thin-walled

pericardium or bag. The intestine passes directly through the heart, a condition found in no other group of animals.

Locomotion.1 Locomotion may be observed if mussels are kept in an aquarium. The fleshy foot is thrust down into the mud or sand and then contracted. This action pulls the clam forward for a short distance. Locomotion is thus very slow.

THE NERVOUS SYSTEM. Although the mussel appears to have no organs of sight or hearing, yet it is provided with a complicated nervous system which appears to have much to do with muscular activity. Three large collections of nerve cells (called ganglia) are found, one near each adductor muscle and one near the foot (see diagram).

EARLY DEVELOPMENT.-The early life history of most mollusks includes a free-swimming stage before the young possess a shell. At this time the tiny larva swims by means of cilia, near the surface of the water. The fresh-water mussel at an early stage attaches itself to the gills of a fish, thus living for a time as a parasite. Eventually all bivalve mollusks come to live near the bottom, where they are near a source of food supply.

The Oyster.

The chief difference between the oyster and the clam lies in the fact that the oyster fastens itself by one valve to some solid object, while the clam or mussel moves about. results in an asymmetry in the shell of the oyster.

This

Oysters are never found in muddy localities, for in such places

they would be quickly smothered

by the sediment in the water. They are found in nature clinging to stones or on shells or other objects which project a little from the bottom. Here food is abundant and oxygen is obtained from the water surrounding them. Hence oyster raisers throw oyster shells into the water to hold the young off the muddy bottom.

In some parts of Europe and this country where oysters are raised artificially, stakes or brush are sunk in shallow water so that the young oyster, which is at first free-swimming, may escape the danger of smothering on the muddy bottom.

After the oysters are a year or two old they are taken up and put down in deeper water as seed oysters. At the age of three

1 See Hunter and Valentine, Manual, page 142.

and four years they are ready for the market. Sometimes oysters are artificially fattened by placing them on beds near the mouths of fresh water streams. Too often these streams are the bearers of much sewage, and the oyster, which lives on microscopic organisms, takes in a number of bacteria with other food. Thus a person might become infected with the typhoid bacillus by eating raw oysters.

The oyster industry is one of the most profitable of our fisheries. Nearly $30,000,000 a year has been derived during the last decade from such sources. Hundreds of boats and thousands of men are engaged in dredging for oysters. Some of the most important of our oyster grounds are Long Island Sound and Chesapeake Bay.

Clams. Other bivalve mollusks used for food are clams and scallops. Two species of the former are known to New Yorkers,

one as the "round,” another as the "long" or "soft-shelled clams. The former (Venus Merceneria) was called by the Indians quahog, and is still so called in the Eastern States. The blue area of its shell was used by the Indians as wampum, or money. The quahog is now extensively used as food. The "long" clam (Mya arenaria) is considered better eating by the inhabitants of Massachusetts

and Rhode Island. This clam was highly prized as food by the Indians. The long siphon, incorrectly called neck, enables these clams to burrow deep into the mud and yet take food and

oxygen from the water above. The clam industries of the eastern coast aggregate nearly $1,000,000 a year.

Scallop. The scallop, another highly esteemed mollusk, forms an important fishery. The scallop rests on one valve on the bottom in shallow water and if disturbed swims away by clapping the valves rapidly together. The single adductor muscle is eaten, whereas in the clam the soft parts of the body are used as food.

Fulgur, a univalve mollusk common in Long Island Sound, which does much harm by boring into the shells of edible mollusks.

When

lines of growth run parallel, as in the clam, to the edge of the shell. Hold the opening toward you. The opening is known as the aperture. the animal is alive, part of the body is protruded through this. Draw the shell twice natural size, showing all above parts.

Living Snail.-(Use the pond snail Limnea or Physa.) Watch the movement of living snails in the aquarium. The large fleshy mass which protrudes from the shell is called the foot. Try to decide how locomotion takes place as the animal moves along the side of the aquarium.

Watch the animal as it feeds. What kind of food does this snail eat? Notice the position of the mouth. Is there any distinct head? These animals are called gastropods (stomachfooted). Do you see any reason for this name? Find the two tentacles or horns. Touch them with a pencil. What happens? Look for the dark eye-spots at the base of the tentacles. Make any experiments you can to see Strophias from different localities at Anif the snail can distinguish between dros and New Providence, Bahamas. light and darkness. (Cover part of From photograph loaned by the Amerithe dish and leave it for some minutes can Museum of Natural History. undisturbed.) Do you find any other structures protruding from the edge of the foot? Two siphons, one for taking in water, the other for sending it out, may be found.

GASTROPODS. - Snails, whelks, slugs, and the like are called gastropods because the foot occupies so much space that most of the organs of the

1 See Hunter and Valentine, Manual, pages 143 and 145.

body, including the stomach, are covered by it. In most gastropods the body is spirally twisted in the shell. In the garden slug, the mantle does not secrete a shell and the naked body is symmetrical. The twisting of the body is not seen in very young snails, so that this peculiar state is believed to be something secondary which has appeared as a consequence of the animal's bearing a shell.

VARIABILITY OF SNAIL SHELLS.-Snail shells are very variable in shape and color markings, as may be seen if several of the same species be examined carefully. Varieties of snails in mountain valleys in certain of the Hawaiian Islands are found to be quite distinct, each in its own valley. Yet it is quite certain that all the snails of these several varieties were at one time alike. Helix nemoralis, a European snail, has been introduced into this country and has multiplied so rapidly and varied so greatly that in an area one thousand feet in diameter three hundred and eighty-five varieties have been collected, each slightly differing from the other either in color or form of shell.1

FEEDING HABITS.-The mouth of the snail is easily found on the under side of the foot. Just within

the mouth is the lingual ribbon. The ribbon consists of a flap of membrane bearing many sharp, filelike teeth, microscopic in size. This structure, which is moved by muscles, passes over a pad of cartilage and rubs, filelike, against the surface to which it is applied. In this manner some snails can bore circular holes in shells of other mollusks, in order to get the soft part, which they use as food. An example of a univalve mollusk which thus obtains food is the oyster drill, which annually does thousands of dollars' worth of damage to the oysters. In Europe and this

country slugs and some snails do considerable damage to gardens by eatThe snail Physa may be observed feeding on tiny

ing young plants.

1 See Howe, Amer. Nat., December, 1898.