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Mollusca.

XXI. MOLLUSKS

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The name Mollusca (Latin mollis soft) gives the character which chiefly aids us in identifying a mollusk. The body is soft and unsegmented. It is usually covered with a limy shell, formed by the agency of a delicate envelope called the mantle. The animal usually possesses a single muscular foot, by means of which locomotion takes place. There are several groups of mollusks which are, as we shall see, quite unlike in appearance and in habits.

Notice that the

The Shell of the Fresh-water Mussel (Unio species).1 shell is made up of two parts or valves. Such a shell is called a bivalve. Notice that the valves are joined together by a structure, somewhat elastic, called the hinge ligament. Close the two shells; why do the shells spring open again? The lines which run more or less parallel to the edge of the shell are called lines of growth. If a line of growth once represented the outer edge of the shell, then

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find the oldest part of the shell. This raised area is called the umbo. It is always possible to locate the anterior end of the shell because the umbo points toward that end. The hinge ligament marks the dorsal side of the animal.

The shell is covered on the outside by a thin layer of horny material. This is called the periostracum. Can you explain why it does not cover the entire shell? The shell proper, if tested with acid, will be found to contain considerable lime.

Shell of fresh-water clam, the left half polished to show the prismatic layer.

Draw, natural size, a single valve, and locate the hinge ligament, umbo, and lines of growth. Place the dorsal surface upmost in the drawing.

Mussels may be opened by first placing the living animal in hot water until the shell gapes. Then insert a knife, keeping the blade close to the inner surface of one valve, cut through the tough muscles which hold the shells together, and the shells will open. Notice the mother of pearl covering the inner surface. Notice in a freshly opened clam that a delicate membrane, the mantle, adheres to the shell.

Structure of Shell.-The shell, if examined in cross section with a good lens, is seen to be made up of three layers: the outer periostracum, made

1 See Hunter and Valentine, Manual, page 138, for exercise on Venus Mercenaria.

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P

L

up of material much like that which forms a cow's horn, or chitin; a middle layer composed of tiny prisms of lime held in by the horny material (this S layer is called prismatic layer); and an inner layer (the laminated layer), made up of layers of lime and horn alternating parallel to the surface of the shell. The inner layer is formed by the action of the whole surface of the mantle. The two outer layers are made by the edge of the mantle only. So a shell grows in thickness largely from the inner surface of the mantle, while it grows in diameter from the edge of the mantle only.

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Vertical section of shell and mantle of a mollusk; C, periostracum; P, prismatic layer; L, laminated layer; S, shell; M, mantle. (After Claus.)

The Open Shell. - Pull the shells completely open. Find on the dorsal side projections and grooves which fit into each other when the shell is closed. These are the hinge teeth. Compare the number in each shell. How might they be of use to the animal? Find the marks on the shell where the adductor muscles were fastened. What was the use of the adductor muscles? Why do dead mussels always have the shell partly open?

Label the anterior and

Draw one opened valve showing all above parts. posterior adductor muscle scars, according to position.

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Body and Mantle Cavity. In one valve lies the body of the clam. If we remove the mantle, we shall find under a roundish soft mass, the body, or visceral mass. Surrounding the visceral mass but ventral to it is a cavity bounded on the outside by the inner surface of the mantle. the mantle cavity. In life this cavity is full of water. See if you can discover how and where water gets in. In a living mussel the posterior edge of the mantle on the right side is folded so as to fit with the adjoining edge of the mantle on the left side. The funnel-like openings thus formed are called siphons.

Siphons.-The siphons can best be seen in living mussels which have been left quiet for some time in an open trough or tank. If a little powdered carmine is allowed to drop from a medicine dropper close to the siphons (the fringed edges of which may be seen extending from the shell), a current of water will be seen to draw in and expel the carmine grains. Where is the incurrent siphon with reference to the excurrent? (In the "long-necked" or "soft" clam the siphons are greatly developed and are made of

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Cross section of a mollusk: A, mantle cavity; a, shell; b, gills; B, cloacal cavity; k, body.

tough muscles. If they are cut lengthwise, the two tubes, incurrent and excurrent, can be easily seen.) The siphon permits water, bearing food and oxygen, to get into the mantle cavity. Here are found the gills.

The Gills.-The gills are striated platelike structures lying on each side of the visceral mass. How many gills on each side? Any difference in size of those on one side? When the clam is in a natural position, the gills hang freely in the mantle cavity. In structure each gill is a long, narrow bag open on the dorsal side. This baglike opening leads into a second cavity, dorsal to the mantle cavity. This space, called the cloacal cavity, is in communication with the outside through the excurrent siphon. A mussel when viewed from one end or in cross section somewhat resembles a book. The shell has the position of the board cover, the mantle the paper pasted to its inner surface, the gills the fly leaves, and the body the printed pages in the book.

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A fresh-water mussel with the right valve, mantle, and gills and some tissue at the base of the foot removed; a, anterior adductor muscle; au, auricle of the heart; ft, foot; g, reproductive gland; gl, gill; i, intestine; k, kidney; m, mantle; n, nerve ganglia connected by nerve cords o; p, posterior adductor; pl, labial palps; si, siphon; v, ventricle. The dark lines on the mantle and foot are blood vessels. Davison, Zoology.

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Circulation of Water over Gills. We have already observed that a more or less constant circulation of water takes place; carmine entering through the incurrent siphon passes out through the excurrent siphon. How is this circulation explained? If a small piece of the gill of a clam or oyster is placed in a drop of the fluid found in the mantle cavity and examined under the compound microscope, the explanation is found. The surface of the gill is seen to be pierced by numerous holes. These holes,

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.

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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.

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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. This results in an asymmetry in the shell of the oyster.

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.

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Shell of oyster, showing asymmetry.

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.

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