The shells which have been noticed in the preceding chapter belonged literally to the stay-at-homes of the family. They rarely wander far, and many, as we have seen, never leave the place which the young shell first selected as its home. What are known as the univalves, the mollusks with one shell, or perhaps no shell at all, are the reverse of this, being in many instances travelers, wandering here and there. This suggests that they have more highly organized locomotive organs. Those shown on the upper part of Figure 94 are univalves, and if we make a section of a univalve (Fig. 95), we see that the shell is much more complicated than in the previous forms. The univalve has a shell-secreting mantle and organs resembling those of the bivalves, only differently placed. This marvelous mantle performs some singular feats, judging from the spines found on many shells. To make these, the mantle must have been thrown outward and upward, forming a tube in which the spine was secreted.

In the univalves a distinct head is seen (Fig. 98) with tentacles and prominent eyes. The foot is now elaborated into a huge sucking, clinging, disklike organ. In the whelk it is as long as the shell, the latter being perched high above it, presenting a remarkable spectacle as it moves along the sandy floor of the ocean. On the head are two tentacles, feelers or sense organs, and sometimes the eyes are mounted on tall stalks, that the shell may have a wide range of vision. A siphon, such as we have seen in the clam, is present and extended upward and forward. It protrudes from a canal formed in the shell for the purpose, and is often very long. If the whelk (Fig. 99) is disturbed, it suddenly withdraws its body, including the enormous colored foot; and if the shell is picked up, the entrance is found securely closed by a horny door called the operculum, which is attached to the foot (Fig. 99). This door takes many shapes. In the beautiful conch it is saber-shaped, and is used to dig into the sand, or, as a lever, to force the conch along by a series of jerks. In other shells it is apparently made of porcelain, hard and highly polished. It is well known as the "eye stone" of popular fancy.

Many of the univalves are flesh eaters, preying upon others of their kind. They have a remarkable tongue (Fig. 96) for the purpose, in fact, the teeth are upon the tongue in sawlike rows. The tongue, which is called the lingual ribbon, is ribbon shaped, long and slender, and is really a soft, pliable saw with which the animal bores into the hardest shells of the helpless clams. In strolling alongshore a large majority of the "dead" clam shells found bleaching in the sun, where they have been washed by the sea, will be seen to contain a circular hole of perfect symmetry (Fig. 97). This has been made by the boring, sawlike tongue of a univalve, which, after gaining an entrance into the tightly locked shell, deliberately sucked it out. It is interesting to note the location of this hole, which is invariably over the softest and plumpest part of the victim, near the lungs, showing that the cannibalistic univalve is very clever in its mode of attack.

While the oyster deposits vast quantities of eggs, which float out into the water to be destroyed by other animals, many of the univalves protect their eggs in remarkable cases. I have often found on the Florida Reef strings (Fig. 98) of singular objects which resembled sections of a yellowish cylinder connected by a little cord. Each section is an egg case, or capsule, and contains many shells, the entire chain being two or three feet in length. This becomes tangled in the coral or seaweed, and holds the young shells, all of which escape through a little door in each section.

Other shells, as the whelk (Fig. 99), deposit their egg cases in heaps or mounds. They are soft and spongelike, and are often mistaken for sponges when divested of their shells and cast ashore. Perhaps the best-known egg case is that of the common Natica, which forms a singular object called the "sand collar" (Fig. 100). The animal molds this collar out of fine sand with its foot, and deposits its eggs in the interior, all being cemented or glued together in a solid mass. We shall find that certain birds deposit their eggs in the nests of others, so saving the wearying process of hatching. A certain shell, called Nassa, has a similar habit. At times it deposits its eggs on the collar nest of the Natica.

Among the myriads of shells which we may select to illustrate the various interesting types, shapes, and kinds, are the Chitons (Fig. 101). Their shells are made up of many plates resembling the plates of a hawkbill turtle. Many live in holes in the rocks, and all have a very large, sucking, disklike foot which clings to the rocks with great tenacity. Resembling them somewhat are the limpets. These are interesting and beautiful shells, especially when polished, forming attractive domes marvelously tinted and colored. Some are called keyhole limpets (Fig. 102), from the fact that they have a keyhole-like opening in the top. They range in size from very minute forms to giants a foot in length.

Among the most beautiful of all shells, and at the same time the most common in tropic and semitropic seas, are the abalones. They are also called ear shells. They have an enormous foot that covers the entire lower surface, being a remarkably powerful organ. Instances have been known where Chinese abalone hunters have tried to pry off the shell from a rock with their hands, and have had their fingers caught and held as though by a vice.

The Haliotis is very common on the shores of the southern Californian islands. In some localities every rock is covered with them, and in places where the black abalone is common, I have found them piled one upon the other. There are two hundred species living. Every tint, color, or tone known in the scale of color, or its combinations, is flashed from these marvelous shells, which, if rare, would be counted among the most beautiful of all natural productions. On the Californian coast they are collected in large numbers, and when polished are converted into buttons and a thousand and one other objects. The meat, which is of excellent quality, is sold in large quantities to the Chinese. Thousands of the shells are bought by tourists, the outside being richly polished. From the ancient graves or Indian mounds of the Californian islands I have taken quantities, especially the large kind known as the red abalone, showing that they were used by the ancient inhabitants. In all of these islands heaps and piles of abalones are found far from the water. By stopping up the holes in the shell with asphaltum, which drifts ashore here, the natives had an excellent dish, or bailer. They cut the shell into earrings and ornaments of many kinds, and most of their fishhooks were evolved from this beautiful shell, which also supplied a large proportion of their food.

On the Florida Reef the great conchs (Fig. 103) are very common. They live on the sandy floor of the lagoons, hitching themselves slowly along by their long-pointed, saberlike operculums. This is the conch of commerce, in which appears the most delicate of all pink colors, and which is the source of the rare pink pearls. In the same locality, but in deeper water, is found the Queen conch (Fig. 104), which is cut into medallions and cameos. The beautiful CyprÆa (Fig. 105), of which many varieties are known, are called micramocks in Florida, and cowries elsewhere. Their luster and natural polish often excite wonder, for they commonly live concealed in the rough portions of dead coral branches, where they would easily become scratched. The cowry, however, is protected by a remarkable mantle which covers the entire shell, thus keeping its pianolike surface as smooth as a mirror. Many cowries are beautifully striped; some are spotted with dark spots on a white background; some are yellow; others are red or old gold, every tint and color seemingly being employed by nature in painting these gems of the sea. Few other shells have been so universally esteemed by all nations. Among certain African tribes they are used as money, and not many years ago collections of cowries were made with all the ardor that actuated the tulip collectors, thousands of dollars being paid for single shells, as the orange cowry.

The cone shells (Fig. 106) represent a beautiful group, spotted like leopards, striped like the tiger, black, red, yellow. Some shells are very pointed, like the augur shell (Fig. 107). Some have an extraordinarily long projection for the siphon, as the spindle shell (Fig. 108). In some the opening is very small, as the cone shells, while in others it is immense, and protected by a large, doorlike operculum. The Bulimas is a famous nest builder. Bulla is interesting from the lightness and delicacy of the shell and its rich neutral browns.

Some of the snails of Africa are six inches across, and the eggs are an inch in length. Semper found a little snail in the Philippines, which when caught by the foot or "tail" throws it off as a lizard jerks off its tail. This is not a great hardship, as the tail is soon renewed. In a collection of shells which came from France some years ago I found several snails of different colors which were joined one to the other. The collector had cut the top from an empty brown snail and placed a living snail with a yellow shell upon it, tying the two together. The snail, supposing that its shell had been broken, immediately began to repair the wound, and closed up the breach with its shell-secreting mantle, so that the two shells became one.

One of the most interesting slugs is the Onchidium (Fig. 112). According to the naturalist Semper, it has upon its back numerous eyes, which enable it to see from above. It is a mud-loving form, common in our new provinces, the Philippines, where the heat is intense and the water warm. One of the land slugs, Limax noctiluca, emits light; and the eggs of another, Arion, have been noticed to be luminous for nearly two weeks after being deposited.

Nearly all these shells are slow-moving animals, but there are others, the pteropods (Fig. 113), which are swimmers. The veritable fairy craft of the sea, they are housed in shells of dainty structure and moving by singular winglike fins, which give them the name of ocean butterflies. They have the property of phosphorescence to a remarkable degree. Cleodora (Fig. 113, D) emits a soft light which gleams through the delicate shell like a light in a lantern. In swimming it moves its fins up and down very much like a butterfly, so that they touch at the top. As delicate and dainty as this little creature is, it has a marvelous arrangement for seizing prey, each tentacle having about three thousand transparent cylinders, each of which contains twenty stalked suckers. As there are six tentacles on each sucker, Cleodora can grasp its prey with three hundred thousand hands. Equally dainty in its way is the sea snail, Ianthina, a violet shell of great delicacy, whose foot develops a raft which resembles a mass of soap bubbles, so the violet snail floats upon the surface of tropical and semi-tropical seas. I have seen the shores of the keys of the Florida Reef lined with an undulating ribbon of these shells after a storm. When touched they emit a rich violet ink which lasts a long time as a stain. A small species of Ianthina is found in the winter on the southern Californian shores, and beneath the attractive float will be found the eggs.