The Encyclopedia Americana (1920)/Crustacea
CRUSTACEA, krŭs-tā'shĕ-a, a primary group (phylum) of animals represented by the barnacle, lobster, crayfish, shrimp and crab. Crustacea differ from other arthropod animals. The body consists of about 20 segments which in the more specialized forms are grouped into two regions, the head-thorax (cephalothorax) and hind-body or abdomen. The segments of the cephalothorax are fused together so that the limits between the segments are lost, and the whole mass is protected by the shield or carapace. The skin is thick and rendered solid by the deposition of lime (carbonate and phosphate), so that the integument forms a dense crust, hence the name Crustacea. They differ from trilobites and king crabs in having two pairs of antennæ, while they breathe by means of gills attached to the legs. Like the other marine arthropods named, they have legs which are divided into two divisions, an outer (exopodite) and an inner (endopodite). Crustacea differ also from the Palæopoda (trilobites, merostomes and arachnids) in the high degree of specialization of their appendages, there being three to six kinds, with corresponding functions, while in the trilobites, so far as we know, the single pair of antennæ are succeeded by numerous (over 20) pairs of legs, all of the same shape and functions. In the head-thorax, besides the antennæ there is on each side of the mouth a pair of mandibles, each with a palpus, two pairs of maxillæ or accessory jaws, which are flat, divided into lobes, and of unequal size; three pairs of foot-jaws (maxillipedes), which differ from the maxillæ in having gills like those on the five following pairs of legs. There are thus 13 pairs of cephalothoracic appendages, indicating that there are 13 corresponding segments; these, with the seven abdominal segments, indicate that there are 20 segments in a typical crustacean. There are six pairs of swimming legs (swimmerets), the last very broad in the lobster and shrimp, with the telson forming the “tail-fin.”
The Crustacea as a rule respire by gills. These, as in the lobster and crab, are composed of a series of little filaments into which the blood flows to be aerated. The filaments branch out from a common stalk which grows out of the basal joint of the five pairs of legs and the three pairs of foot-jaws. These gills are folded up toward the back, and are contained in a sort of chamber made in part by the carapace. In shrimps, lobsters and crabs the sea-water passing into the cavity between the body and the free edge of the carapace is afterward scooped out through an opening or passage on each side of the head by the movements of membranous flaps called “gill-bailers.” The digestive organs are well developed, especially the fore stomach in the hinder part of which are several very large calcareous teeth for crushing the food, serving, when closed together, as a strainer through which the partly digested food presses into the long slender straight intestine, which ends in the telson. The liver is very large, as in, all marine arthropods, or in such terrestrial types as the scorpions and spiders, which are derived from the king crabs. The brain of the higher Crustacea is very complex, corresponding with the complicated reflex movements of an animal composed of so many segments, and bearing such a complicated series of appendages devoted to so great a variety of functions. The eyes are usually compound or many-faceted, and are mounted on freely movable stalks. The ear is a sac in the basal joint of the smaller or second pair of antennæ. The organs of smell are usually well developed, as Crustacea mainly depend on this sense in finding their food. These consist of minute delicate sensory rods on the smaller antennæ. The hairs fringing the mouth-parts and legs are often delicate tactile organs. The green glands in the head function as kidneys, and open out at the base of the larger antennæ.
With only a single known exception (Squilla), Crustacea carry their eggs about attached to the swimming or other legs. The eggs of some crabs (Neptunus) are minute and excessively numerous, their number amounting to millions, while the lobster may produce from 20,000 to 80,000 eggs. Crustacea pass through a well-marked metamorphosis, nearly all (except the amphipods and isopods) hatching from the egg as a larva called “nauplius,” which has an oval non-segmented body, with three pairs of appendages, by which it swims about at the surface of the sea. After a series of molts, at each of which new segments with their appendages arise, it finally reaches maturity. The shrimps and crabs hatch in a more advanced larval stage called "zoëa," the nauplius stage being partly suppressed and thrust back into the embryo period. The zoëa has a head and abdomen, but no thorax: this, however, is developed later, and after a series of molts the parent form is attained.
The process of molting is a precarious one, not infrequently resulting in death. The crust being too solid to admit of a continuous growth, and increase in size being rapid, frequent sheddings of the skin are necessary. In the lobster, the old skin being detached from the under cellular layer by the secretion of the new cuticle beneath, ruptures between the thorax and abdomen, and the lobster draws itself out of the rent, shedding not only the entire skin and every hair, but also the lining of the mouth, throat and fore stomach, and likewise the outlet of the intestine. In about three weeks after the casting of the shell the new one becomes solid and hard. In the crayfish the old skin is loosened and pushed away from the cellular layer beneath by the growth of temporary, short stiff hairs, which disappear after the skin is shed.
The Crustacea are a very ancient type. The earliest remains are found in the Cambrian rocks, but are very scanty compared with the trilobites. They comprise traces of barnacles, Ostracoda or small shelled forms, Phyllocarida, and an obscure form supposed to be allied to the modern freshwater Apus. In the Devonian Period shelled phyllopods (Estheria) appeared, and in the Carboniferous arose an order (Symcardia), represented by an ancient form (Anaspides) still living in a lake in Tasmania. From this group the existing Schizopoda or opossum shrimps (Mysis), the Squilla, and the ordinary shrimps and crabs, are supposed to have descended. Isopoda also appeared as early as the Devonian. A shrimp-like Crustacean occurs in the Devonian, and true crabs date from the Jurassic.
The Crustacea are divided into 11 orders, the Branchiopoda, Phyllopoda, Ostracoda, Copepoda, Cirrepedia or barnacles, Arthrostraca, Cumacea, Phyllocarida, Syncarida, Schizopoda, Stomatopoda and Decapoda. More than 5,000 species are known. See Barnacle; Crab; Fish Lice; Hermit Crab; Shrimp.
Bibliography.— In addition to general textbooks, consult Huxley, ‘The Crayfish’ (London 1880); Kingsley, ‘Crustacea,’ in Standard Natural History (Vol. II, Boston 1884); Stebbing, ‘A History of Crustacea’ (London 1893); Smith and Weldon, ‘Crustacea,’ in Cambridge Natural History (Vol. IV, London 1909); Calman, ‘Life of Crustacea’ (New York 1911).