is open along one side, and allows the feather-like feet to project and produce currents in the surrounding water which brings food within reach. In the acorn-barnacles (Fig. 61) the stalk is absent, and the body, though possess

ing the same general character as the goose-barnacles, is shorter, and enclosed in a strong palisade consisting of six calcareous plates.

The larger number of barnacles attach themselves to the supports of wharves, the hulls of ships, floating timbers, the rocks from the shore-line down to considerable depth, and a few species occur on the skin of sharks and whales. On the other hand, there are several species which are parasitic, and in accordance with this mode of life exhibit various degrees of degeneration. In the most extreme

cases (Sacculina) the sac-like body, attached to the abdomen of crabs, is entirely devoid of appendages and any signs of segmentation. A root-like system of delicate filaments extends from the exposed part of the animal into the host and absorbs the necessary nutriment. The mouth and alimentary canal are accordingly absent-in fact, the body contains little but the reproductive organs and a very simple nervous system.

108. Structure. In the internal organization of these smaller crustaceans many differences may be noted, though they are usually less profound than the external. Ordinarily the alimentary canal is a straight tube passing through the body, and is provided with a pouch-like stomach, and a more or less clearly defined liver. In all, except the parasitic species, the external mouth-appendages masticate the food, and in a very few of the above-described groups it may be further ground between the horny ridges on the stomach-walls. After this preliminary treatment it is subjected to the action of the digestive juices, and when liquefied is absorbed into the body. Here it is circulated by a blood-system of widely different character. In many cases definite arteries and veins are absent. The blood courses through the body in the spaces between the different organs propelled by the beating of the heart, which it is made to traverse. In Cyclops (Fig. 60) even the heart is absent, and the blood is made to circulate by contractions of the intestine. In most of these smaller Crustacea considerable oxygen is absorbed through the body-wall; but in several species, for example, the fairy-shrimp (Fig. 59), special gills are developed on the appendages of the body.

109. Multiplication.-Among the Crustacea thus far considered the males are usually readily recognized owing to their small size. The females also are usually provided with brood-pouches in which the developing eggs are protected. In almost every case the young are born in the

form of minute larvæ, provided with three pairs of appendages, a median eye (Fig. 62), and a firm external skeleton or cuticle. This latter prevents the continuous growth of the larvæ or nauplius, and every few days it is thrown off, and while the new one is forming the body enlarges. During this time new appendages are developed, so that after cach moult the young crusta

cean emerges less like its former self and more and more like its parents. In the barnacles, after several moults have taken place, the young become permanently attached by means of their first antennæ, their thoracic feet change into feathery appendages, and several other changes occur. In some of the parasitic barnacles (Sacculina) the larva attaches itself to a crab, throws off its various appendages, and, after other great degenerative changes, enters its host. For a time, therefore, their development is toward greater complexity, but the later stages constitute a retrograde metamorphosis.

FIG. 62.-Development of a barnacle (Lepas). a, larva; b, adult.

110. More complex types.-The larger, more useful, and usually more familiar Crustacea belong to the second division (subclass Malacostraca). It comprises such animals as the shrimps, crayfish, lobsters, crabs, and a number of other forms which are at once distinguished from the preceding by the constant number of segments composing the body. Of these, five constitute the head, eight the thorax, and seven the abdomen. The head segments are always fused together, and with them one or more thoracic segments unite to form a more or less complete cephalothorax. Also,

some of the head segments give rise to a great fold of the body-wall, the carapace, which extends backward and covers all or a part of the thorax, with which it may firmly unite, as in the crayfish. The appendages are usually highly specialized, and are made to perform a variety of functions.

111. The shrimps.-Among the simplest of these are the opossum-shrimps (Fig. 63) and their relatives, small trans

FIG. 63.-The opossum-shrimp (Mysis americana).

parent creatures often seen swimming in great numbers at the surface of the sea or hiding among the seaweeds along the shore. In general appearance they resemble crayfishes or prawns, but are readily distinguished by the two-branched thoracic feet. This "split-foot" character also occurs among many of the preceding Crustacea, and is generally a badge of low organization, tending to disappear in the more highly organized forms. In this and other respects the shrimps are especially interesting in their relation to the preceding Crustacea, and in the fact that they may closely resemble the ancestors of the modern prawns (Fig. 64), lobsters, crayfishes, and crabs.

112. Crayfishes and lobsters.-The last-mentioned species and their allies, usually large and familiar forms, constitute a group known as the decapods (meaning ten feet), referring to the number of thoracic feet. Among the members of this division probably none are more familiar than the crayfishes, which occur in most of the larger rivers and their tributaries throughout the United States and Europe. It is their habit to remain concealed in crevices of rocks

or in the mouths of the burrows which they excavate, and from which they rush upon the small fish, the larvæ of

FIG. 64.-Prawn (Heptacarpus brevirostris).

many animals, and other equally defenseless creatures which constitute their bill of fare.

In turn they are

eagerly sought by certain birds and four-footed animals, and, especially in France,

are extensively used for food by man.

Closely related to the crayfishes and differing but little from them structurally are the lobsters. In this country they are confined to the rocky coasts from New Jersey to Labrador, living upon fish, fresh or otherwise, various invertebrates, and occasionally seaweeds. Far more than the crayfish, the lobster is in demand as an article of food. By the aid of nets or various traps