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Fig. 254.-Clio Borealis.

with teeth, calculated to tear in pieces the minute animals on which it feeds. It has also a very effective apparatus for securing its prey, consisting of six tentacula of a reddish color. On examining one of these with a microscope, this color is found to be occasioned by red points arranged with great regularity. On magnifying these still farther, each point is seen to be a collection of about twenty suckers on the ends of as many stalks. Each collection is in a sort of sheath, and can be protruded from it. There are on all the tentacula about three hundred and sixty thousand of these suckers, constituting an apparatus for prehension more extensive, in proportion to the size of the animal, than any other to be found in the whole animal kingdom.

Questions.-What is the significance of the name of the third subkingdom of animals? What are naked Mollusks? What is said of the use of the covering which most of them have? Of what is it composed? What is said of the proportions of the constituents? How is the shell formed? What are the two kinds of shells? What is said of the changes which shells undergo in growing? What is said of the locomotion of Mollusks? What is said of the foot, and its various uses? What is the byssus? What provision for locomotion is there in most of the Mollusks that inhabit bivalve shells? What in those that are similar in structure, but have no shell? What is the special destiny of Mollusks? What is said of their breathing apparatus ? What of their blood, and its circulation? What are the two grand divisions of Mollusks? What are the groups in the first division, and their characteristics? Of the Cephalopods, what shelly species exist at the present time? What is said of the Ammonites? Describe the structure and habits of the Cuttle-fish, What is sepia? What

is Cuttle-fish bone? What is said of the Argonaut? What of the Pearly Nautilus? What is said of the Pteropod group? What of the Clio Borealis ?

CHAPTER XXXIII.

MOLLUSKS continued.

557. THE class of Gasteropoda is mostly composed of Mollusks that live in a univalve shell, which is usually of a spiral shape. You have two different forms of the spiral in Fig. 249, page 317. Some of the species, as the Slug, are naked or destitute of shell. There is, however, in these, sometimes a small shell, generally imbedded in the mantle, just over the cavity which contains the lungs. The body of the Gasteropods is terminated in front with more or less of a head, having fleshy tentacula, varying from two to six in number. The back is covered with a mantle which secretes the shell. On the under side of the animal is the fleshy mass called the foot. In those which have a shell, all the body remains in it except the head and the foot. These project beyond it when the animal expands them for walking, but they can be withdrawn into the first turns of the shell at pleasure. In most of the aquatic Gasteropods there is on the foot a plate of horny substance, which shuts over the opening

Fig. 255.-Limnæa Stagnalis.

in the shell after the head and the foot are drawn in. In Fig. 255 you see one of these animals with the head and the foot out of the shell.

558. Many of the Gasteropods are remarkable for an abundant supply of flinty teeth. Sometimes these are on the palate, and in some species even the stomach has teeth scattered over its inner surface. The tongue, in some, is remarkable for its length, and for the teeth which are all along on its upper surface. The tongue of the common Limpet, Fig. 256, is an example. It is from two

to three inches long, and this is longer than the whole animal. When not in use, it is turned backward down into the stomach. It is spoonshaped at the end. In its whole extent it is armed with rows of teeth, four in each row, and between each two rows there are two threepointed teeth. These two sets of teeth are represented in a magnified portion of the tongue in the figure. The part of the tongue toward its root generally has its edges turned over so as to meet, thus making a tube. The whole instrument is therefore an efficient rasper, and also a proboscis.

Fig. 256.-Limpet's Tongue.

559. Of the Gasteropoda, some are terrestrial and some live in fresh water, but most of them are found in the sea. The terrestrial Gasteropods are Snails and Slugs. In the common Slug there is a prominent head with four tentacula, which can be drawn inward by a process like the inversion of the finger of a glove. At the ends of the longer pair of the tentacula are the eyes. On the back there is a kind of shield formed by the mantle,

which usually incloses a small shell. This shield is over the breathing apparatus (§ 557), and the head can be so drawn in as to be under it. The Snails have very much the same shape and arrangement with the Slug, except that they have a shell into which they can withdraw the whole body. The common Snail, Fig. 257, lays eggs,

Fig. 257.-Snail.

which are very large in comparison with the size of the animal. They are of the size of a small pea, and are deposited in the ground about two inches below the surface. 560. A few of the

Gasteropods that, like the Snails and Slugs, breathe with lungs, are yet aquatic in their habits. But, like other aquatic animals that have lungs, as the Whales, they are obliged every now and then to come to the surface to get air. Among these are the Pond Snails, a species of which is represented in Fig. 255, page 324. These Mollusks, and those which are terrestrial, the Slugs and the Snails, are included in an order by themselves, as having lungs-the order Pulmonifera.

561. The second order of the Gasteropods includes all those which have gills instead of lungs, and also have a shell, usually of a spiral form. This order is much larger than the others, and presents a great variety of beautiful shell-coverings. Some of them have siphons to introduce water into the cavities where the gills are, so that the animal can breathe without putting its body out from the shell. There is a little notch always to be observed in the shell where this siphon passes out.

562. Of the many varieties of the shells of these Gasteropods I will notice but a few. In Fig. 249, page 317, on the left, is an example of the Turbinidæ, or Whorl fam

ily, called the Royal Staircase Wentletrap. This is found in the Chinese and Indian Seas. It is so costly-a fine specimen commanding, even now, four or five pounds sterling-that the specific name attached to it is pretiosa, precious. In the same figure is a specimen from the very extensive Cone family. In Fig. 258 the large shell is that

Fig. 258.

of a Whelk, belonging to a family which, from the shape of the shells, is called Buccinidæ, from buccinum, a trumpet. The famous Tyrian purple was obtained from one of this family. In the same figure is the little Cowry, which is a current coin among the natives of Bengal, Siam, and many parts of Africa. In Bengal, 3200 of these shells are reckoned equal to a rupee, or about two shillings of English money. In 1849 about three hundred tons of them were imported into Liverpool, designed to be used in the African trade. One of the most beautiful of the shells which are armed with spines is the Thorny Woodcock, Fig. 259, sometimes called Venus' Comb.

Fig. 259. Thorny Woodcock.

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