XXI. THE MOLLUSKS

Problem XXXIII (Optional). A study of mollusks and their enemies with reference to their economic importance. (Laboratory Manual, Prob. XXXIII.)

To the average high school pupil a clam or oyster on the "half shell" is a familiar object. The soft "body" of the animal lying between the two protecting" valves" of the shell gives the name to this group (Latin mollis-soft). Most mollusks have a limy shell, either bivalve (two-valved), as the oyster, clam, mussel, and scallop, or univalve, as in the snail. Usually the univalve shell is spiral in form, some of nature's most beautiful objects being the spiral shells of some marine forms. Still other mollusks, for example, the garden slug, have no external shell whatever.

This limy shell envelope when present, is formed from the outer edge and surface of a delicate body covering called the mantle. The mantle may be found in the opened oyster or clam

sticking close to the inside of the valve of the shell in which the body rests. Between the mantle and the body of the clam or oyster is a space, the mantle cavity. In the space hang the gills, platelike striated structures. By means of cilia on the inner surface of the mantle and on the gills a constant current of water is maintained through the mantle cavity bearing oxygen to the gills and carbon dioxide away. This current of water passes, in most mollusks, into and out from the mantle cavity through the siphons, the muscular tubes forming the "neck" of the "soft clam" being an example of such an organ.

The food of clams or oysters consists of tiny organisms, plant and animal, which are carried in the current of water to the mouth of the animal, this water current being maintained in part by the action of cilia on the palps or liplike flaps (p. 269) surrounding the mouth. A single muscular foot aids in locomotion when the animal moves about. Many mollusks, as the oyster, are fixed when adult.

The shallow water of bays and other quiet bodies of salt water where clams and oysters live, literally swarm with tiny plants. The conditions for the growth of such plants is ideal. Water from the rivers containing organic waste and depositing daily its load of mud on the bottom

gives one basis for the support of these plants. The carbon dioxide from the thousands of species of fish, mollusks, crustaceans, worms, and other forms of animal life gives another source of raw food material for the plant. The sunlight penetrating through the shallow waters supplies the energy for making the food. Thus conditions are ideal for rapid multiplication; hence the water becomes alive with all kinds of plant life, especially the lower forms. Among these plants are always found bacteria, both harmless and harmful. Mollusks feed upon these plants, including the bacteria; man feeds on the mollusks, and, if he eats them raw, may eat living bacteria as well. Thus disease might result, and, as a matter of fact, epidemics of typhoid fever have been traced to such a source. Some Common Mollusks. The fresh-water clam, a common resident in shallow water in inland ponds and rivers, although not useful

for food to man, has become the source of a very important industry. The making of pearl buttons has so depleted the number of adult clams in our Middle West that the state and United States governments have undertaken the study of the life habits of these animals with a view to restocking the rivers. The development of the fresh-water clam or The egg develops into a free-swimming larval form which fastens to the gills of a fish and there lives as a parasite until almost mature. Then it drops off and begins life in the sand of the river or lake where it lives.

Shell of fresh-water clam, the left half polished to show the prismatic layer from which buttons are made.

mussel is complicated.

The Oyster.

The chief difference between the oyster and the clam lies in the fact that the oyster is fastened by one valve to some solid object, while the clam or fresh-water 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 Shell of oyster, showing asymto stones or on shells or other objects which project a little above the bottom.

Here

metry.

food is abundant and oxygen is obtained from the water surrounding them. Hence oyster raisers throw oyster shells into the water and the young oysters attach themselves.

In some parts of Europe and this country where oysters are raised ar

tificially, 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 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 and four years they are ready for the market.

The oyster industry is one of the most profitable of our fisheries. Nearly $65,000,000 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. Three of the most important of our oyster grounds are Long Island Sound, Narragansett Bay, and Chesapeake Bay.

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. It is evident that state and city supervision ought to be exercised with reference not only to the sale of shellfish which comes from contaminated localities, but also to prevent the growth of oysters or other mollusks in the neighborhood of the openings of sewers or sewage-bearing rivers. Clams. Other bi

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valve 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 softshelled" 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

Round clam (Venus merceneria): AAM, anterior adductor muscle; ARM, anterior retractor muscle; PAM, posterior adductor muscle; PRM, posterior retractor muscle; F, foot; C, cloacal chamber; IS, incurrent siphon; FS., excurrent siphon; EO, heart; G, gills; M, mantle; DGL, digestive glands; S, stomach; I, intestine; P, palp; R, posterior end of digestive tract.

Island. This clam was highly prized as food by the Indians. The clam industries of the eastern coast aggregate nearly $1,000,000 a year.

Scallop.

- The scallop, another molluscan delicacy, forms an important fishery. Only the single adductor muscle is eaten, whereas in the clam the soft parts of the body are used as food.

Gastropods.

Pearls and Pearl Formation. Pearls are prized the world over. It is a well-known fact that even in this country pearls of some value are sometimes found within the shells of such common bivalves as the freshwater mussel and the oyster. Most of the finest, however, come from the waters around Ceylon. If a pearl is cut open and examined carefully, it is found to be a deposit of the mother-of-pearl layer of the shell around some central structure. It has been believed that any foreign substance, as a grain of sand, might irritate the mantle at a given point, thus stimulating it to secrete around the substance. It now seems likely that most perfect pearls are due to the growth within the mantle of the clam or oyster of certain parasites, stages in the development of a flukeworm. The irritation thus set up in the tissue causes mother-of-pearl to be deposited around the source of irritation, with the subsequent formation of a pearl. Snails, whelks, slugs, and the like are called gastropods, because the foot occupies so much space that most of the organs of the body, including the stomach, are covered by it. Such animals are partially covered by a more or less spirally formed shell which has but one valve. In most gastropods the body is spirally twisted in the shell. In the garden slug, the mantle does not secrete an external shell, and the naked body is symmetrical.

Forest snail, showing the two tentacles with an eye on the end of each. From photograph by Davison.

Gastropods of various species do considerable damage, some in the garden, where they feed upon young plants, others in the sea, where they bore into the shells of other living mollusks in order to get out the soft part of the body which they use as food.

Cephalopods. Another class of mollusks are those known as cephalopods. The name "cephalopod" means head-footed. As the Figure shows, the mouth is surrounded with a circle of tentacles. The shell is internal or lacking, the so-called pen of the cuttlefish being all that remains of the shell in that form. A cuttlefish is strangely modified for the life it leads. It moves rapidly through the water by squirting water from the siphon. It can seize its prey with the suckers on the long tentacles

It is pro

and tear it in pieces by means of its horny, parrotlike beak. tected from its enemies and is enabled to catch its prey because of its ability to change color quickly. In this way the animal simulates its surroundings. The cuttlefish has an ink bag near the siphon which contains the black sepia. A few drops of this ink squirted into the water may effectually hide the animal from its enemy.

To this group of animals belongs also the octopus, or devilfish, a cephalopod known to have tentacles over thirty feet in length, the paper nautilus and the pearly nautilus, the latter made famous by our poet Holmes.

Habitat of the Mollusks. the earth and sea. They are more abundant in temperate localities than elsewhere, but are found in tropical and arctic countries. They are found in all depths of water, but by far the greatest number of species live in shallow water near the shore. The cephalopods live near the surface of the ocean, where they prey upon small fish. The food supply evidently determines to a large extent where the animal shall live. Some mollusks are scavengers; others feed on living plants.

Mollusks are found in almost all parts of

We have found in the forms of mollusks studied that almost all mollusks live in the water. There is one great group which forms a general exception to this, certain of the snails and slugs called pulmonates. But even these animals are found in damp localities, and at the approach of drought they become inactive, remaining within the shell. The European snail (Helix pomatia) imported to this country as a table delicacy exists for months by plugging up the aperture to the shell with a mass of slimy material which later hardens, thus protecting the soft body within. Economic Importance. In general the mollusks are of much cconomic importance. The bivalves especially form an important source of our food supply. Many of the

mollusks also make up an important part of the food supply of bottomfeeding fishes. On the other hand, some mollusks, as natica, bore into other mollusk shells and eat the animal thus attached. Some boring mollusks, for example the shipworm (Teredo navalis), do much damage, especially to wharves, as they make

their home in piles. Still others bore holes in soft rock and live there. The shells of mollusks are used to a large extent in manufactures and