Lobsters, Shrimps and Crabs (Decapoda).—Technical Note.—Teachers living near the sea-shore can get specimens of live and dead lobsters, shrimps, and crabs in the markets. Schools in the interior should have a few preserved specimens for examination. These specimens should be compared with the crayfish; although differences in shape of body are evident, the character and arrangement of body parts will be found to be very similar.

The largest and most familiar Crustaceans, as the crayfishes, lobsters, shrimps, prawns and crabs, all belong to the order Decapoda, or ten-legged Crustacea. The members of this order have, including the large claws, ten walking feet; they all have eyes on movable stalks, and the front portion of the body is covered by a horny fold of the body-wall called the carapace.

The lobsters are large ocean-inhabiting crustaceans which are very like the fresh-water crayfish in all structural characters. They live on the rocky or sandy ocean-bottom at shallow depths. They feed largely on decaying animal matter. They are caught in great numbers in so-called "lobster pots," a kind of wooden trap baited with refuse. "The number thus taken upon the shores of New England and Canada amounts to between twenty and thirty million annually." Live lobsters are brownish or greenish with bluish mottling; they turn red when boiled. A single female will lay several thousand eggs. The eggs are greenish and are carried about by the mother until the young hatch. The young are free-swimming larvæ, until they reach a length of half an inch.

The shrimps and prawns are mostly marine, though some species live in fresh water. They are, like the lobsters, used for food. Some of the species are gregarious in habit, occurring in great "schools" of individuals. Like the lobsters they crawl about on the sea-bottom feeding on decaying animal matter. Shrimps are very abundant near San Francisco, where extensive "shrimp fishing" is done by the Chinese.

Fig. 37.—Some crabs and barnacles of the Pacific coast; the short sessile acorn barnacles in the upper left-hand corner belong to the genus Balanus; the stalked barnacles in the upper right-hand corner are of the species Pollicipes polymenus; the largest crab (upper left-hand) is Brachynotus nudus; the one in left-hand lower corner is a young rock-crab, Cancer productus; the crab in the sea-weed at the right is a kelp-crab, Epialtus productus, while the two in snail-shells in lower corner are hermit-crabs, Pagurus samuelis. (From living specimens in a tide-pool on the Bay of Monterey, California.)

The crabs (fig. [37]) differ from the lobsters and crayfishes and shrimps in having the body short and broad, instead of elongate. This is due to the special widening of the carapace and the marked shortening of the abdomen. The abdomen, moreover, is permanently bent underneath the body, so that but little of it is visible from the dorsal aspect. The number of abdominal legs or appendages is reduced. When the tide is out the rocks and tide-pools of the ocean shore are alive with crabs. They "scuttle" about noisily over the rocks, withdrawing into crevices or sinking to the bottom of the pools when disturbed. They move as readily backward or sidewise, "crab-fashion," as forward. They are of various colors and markings, often so patterned as to harmonize very perfectly with the general color and appearance of the rocks and sea-weeds among which they live. The spider-crabs are especially strange-looking creatures with unusually long and slender legs and a comparatively small body-trunk. They include the Macrocheira of Japan, the largest of the crustaceans. Specimens of this crab are known measuring twelve to sixteen feet from tip to tip of extended legs; the carapace is only as many inches in width or length. The soft-shelled crab is a species common along our Atlantic coast. It is "soft-shelled" only at the time of molting, and has to be caught in the few days intervening between the shedding of the old hard shell and the hardening of the new body-wall. The little oyster-crabs (Pinnotheres) which live with the live oyster in the cavity enclosed by the oyster shell are well-known and interesting crabs. They are not parasites preying on the body of the oyster, but are simply messmates feeding on particles of food brought into the shell by the currents of water created by the oysters.

Among the most interesting crabs are the hermit crabs (fig. [37]), familiar to all who know the seashore. There are numerous species of these crabs, all of which have the habit of carrying about with them, as a protective covering into which to withdraw, the spiral shell of some gastropod mollusc. The abdomen of the crab remains always in the cavity of the shell; the head and thorax and legs project from the opening of the shell, to be withdrawn into it when the animal is alarmed or at rest. The abdomen being always in the shell and thus protected loses the hard body-wall, and is soft, often curiously shaped and twisted to correspond to the cavity of the shell. It has on it no legs or appendages except a pair for the hindmost segment which are modified into hooks for holding fast to the interior of the shell. As the hermit crab grows it takes up its abode in larger and larger shells, sometimes killing and removing piece-meal the original inhabitant. Some hermit crabs always have attached to the shell certain kinds of sea-anemones. It is believed that both crab and sea-anemone derive advantage from this arrangement. The sea-anemone, which otherwise cannot move, is carried from place to place by the crab and so may get a larger supply of food, while the crab is protected from its enemies, the predaceous fishes, by the stinging threads of the sea-anemone, and also perhaps by the concealment of the shell its presence affords. This living together by two kinds of animals to their mutual advantage is called commensalism or symbiosis (see Chapter [XXX]). The hermit crabs are not true crabs, but are more nearly related to the crayfishes and shrimps than to the true broad-bodied, short-tailed crabs.

Barnacles.—Technical Note.—Specimens of barnacles may be got readily from the tide rocks or from piles in a harbor. Interior schools should have, if possible, specimens preserved in alcohol or formalin for examination. The "shells" of acorn (sessile) barnacles may often be found on oyster shells (get at restaurants).