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striking peculiarity is in the univalve shell, with numerous whorls, into which the animal may at any time withdraw completely. Ordinarily this is carried on the back of the spindle-shaped body, which is fashioned beneath into a great

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FIG. 54.-The slug (Ariolimax) and common snail (Helix). From life.

flat sole or creeping surface that bears on its forward border a wide opening through which mucus is continually issuing to enable the snail to slip along more readily. Slime also exudes on other points on the surface of the body and affords a valuable protection against excessive heat and drought.

Unlike the clams, the forward end of the body is developed into a well-marked head bearing the mouth and a complicated mechanism for gathering and masticating food, together with two pairs of tentacles, one of which carries the eyes. On the right side of the animal, some distance behind the head, is the opening of the little sac-like mantle cavity (Fig. 54) which contains the respiratory organs, and into which the alimentary canal and the kidneys pour their wastes. The relation of these organs to the mantle cavity is the same as in the clams, though the cavities differ much in size and position.

93. Other snails. The shell.-Extending our acquaintance to other species of snails, we find the same general plan of body, although somewhat obscured at times by

many modifications. A foot is generally present, also a more or less well-developed head, and the body is usually surrounded by a shell which varies widely in shape and size in different species. In the common limpets the early coiled shell is transformed into, an uncoiled cap-like one, and in the keyhole limpets is perforated at its summit. The

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56) common along our coasts, it may be entirely absent, or, as in the ordinary slugs, reduced to a small scale embedded in the skin.

94. Respiration.-A considerable quantity of oxygen is absorbed through the skin, as in all mollusks, but the chief part of the process is usually taken by the plume-like gills, one or two in number, which are located in the mantle cavity. In the chitons (Fig. 55) the number of gills is greater, amounting in some species to over a hundred, while in the Nudibranchs (Fig. 56) gills are absent, their places being taken by more or less feathery expansions of the skin on the dorsal surface.

Many of the gasteropods left exposed on the rocks by a retreating tide retain water in the mantle cavity, from which they extract the oxygen until submerged again.

Others breathe by means of gills while under water, and by the surface of the body and the moist walls of the mantle

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FIG. 56.-Three different species of naked marine snails or Nudibranchs. Natural size, from life.

cavity when exposed. In some of the small Littorinas attached so far from the sea as to be only occasionally washed by the surf this latter method may prevail for days together-in fact they live better out of water than in it. It is not difficult to imagine that such forms, keeping in moist places, might wander far from the sea, and, losing their gills, become adapted to a terrestrial life. It is believed that in past times this has actually occurred, and that our land forms trace their descent from aquatic ancestors. To-day they breathe by a lung-that is, they take oxygen through the walls of the mantle cavity, as the slug may be seen to do, though in some species traces of the old gill yet remain.

95. Food and digestive system.-Many mollusks live upon seaweeds, and the greater number of terrestrial forms are fond of garden vegetables or certain kinds of lichens, but, on the other hand, the latter, together with a large number of marine snails, are carnivorous. In all cases the food requires to be masticated, and, unlike the clams, the mouth is usually provided with horny jaws, and an additional

masticatory apparatus which consists of a kind of tongue with eight to forty thousand minute teeth in our land forms (Fig. 57), while in certain marine snails they are beyond computation. With the licking motion of the tongue this rasp tears the food into shreds before it is swallowed, and in the whelks or borers it serves to wear a circular hole through the shells of other mollusks, which

FIG. 57.-A small portion of the radula or tongue-rasp of a snail (Sycotypus).

are thus killed and devoured. This latter process is facilitated by the secretion of the salivary glands, which has a softening effect upon the shell. Ordinarily the saliva of snails exercises some digestive action.

In the stomach of some snails are teeth or horny ridges which also are instrumental in crushing the food, and in numerous minor respects peculiarities exist in different species according to the nature of the food; but in its general features the digestive tract is similar to that of the clams.

The processes of circulation and excretion are also carried on by means of systems which show a certain resemblance to those of the clams. As might be expected, certain differences exist, sometimes very great, but they are of too technical a nature to concern us further.

96. Sense-organs of lamellibranchs and gasteropods.— The eyes of mollusks differ widely in their structure and the position they occupy in the body. In our common land snails two pairs of tentacles are borne on the head, the lower acting as feelers, while each of the upper ones bears on its extremity the eye, appearing as a minute black dot (Fig. 54). In this same position the eyes of many marine snails occur, but there are numerous species in which there are other accessory eyes. In many of the

limpets, for instance, there are numbers of additional eyes carried on the mantle edge just under the eaves of the shell, and forming a row completely encircling the body. (In the scallops there are two rows of brilliantly colored eyes, set like jewels on the edges of the mantle just within the halves of the shell.) In the chitons the eyes of the head disappear by the time the animal attains maturity, and in some species at least their place appears to be taken by great numbers of eyes, sometimes thousands, which are embedded in the shells. On the other hand, eyes are completely absent in certain species of burrowing snails and in several living in the gloomy depths of the sea far from the surface; they appear to be absent also from fresh-water clams; but the fact that certain species close their shell when a shadow falls upon them, leads to the belief that while actual eyes are not present the skin is extremely sensitive to light. This is also the case with many snails.

97. Smell. Since the sense of sight is generally undeveloped in the mollusks, they rely chiefly upon touch and smell for recognizing the presence of enemies and food. Tentacles upon the head and other parts of the body, and a skin abundantly supplied with nerves, show them to possess a high degree of sensibility; but in the greater number of species the sense of smell is of chief importance. Many experiments show that tainted meat and strongly scented vegetables concealed from sight and several feet distant from many of our land and sea mollusks will attract them at once. In these forms the sense of smell appears to be located on the tentacles, but additional organs, possibly of smell, are located on various portions of the body, usually in the neighborhood of the gills.

98. Taste and hearing.-Several mollusks appear to be almost omnivorous, but others are decidedly particular in their choice of food, which leads us to suspect that they possess to some extent the sense of taste. Nerves supplying the base of the mouth have also been detected, which

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