Page:The New International Encyclopædia 1st ed. v. 05.djvu/726

CKUSTACEA. 630 CRUSTACEA. lower Crustacea lialcli as minute, oval, unseg- mented creatures with three pairs of appendages, and Ihis larva is called a nauplius. Although the liigher Crustacea hatch as a more highly or- ganized form vailed 'zoea,' and later pass through an intcrmeiliate larval form, the 'megalops,' all pass through a nauplius stage in the egg. See Barx.^cle for illustration of 'nauplius:' and see Ceab for illustrations of 'zoea' and 'megalops.' ilore tlian 10,000 species of living Crustacea are known, of which the greater number are marine; some inhabit fresh waters, running or stagnant ; comparatively few are terrestrial. -Many exhibit a liigli degree of intelligence. The Crustacea constitute, in an economic sense, per- liaps the most important group of invertebrates. The myriads of the smaller forms that drift aliout the ocean and the great lakes furnish the greater part of the fare of the important food- tishes, and are thus indirectly of value to man- kind. In the economy of the ocean itself Crus- tacea are also of great importance, for they act as the natural scavengers of the sea. Clas.sificatiox and Phylogext. Classifica- tion of the Crustacea is based upon the number and nuinner of consolidation of the segments of the body, and upon the number and character of the appendages. There are three large groups : (1) Trilobita, {2) Kntomostraca, and (3) Mala- costraea (qq.v. ), which may be considered to con- stitute subclasses. The principal characters of these subclasses and their component orders, to- gether with some notes of the geological history of the groups, are here briefly given. T. Trilobita. This is an extinct group of Crustacea that lived during Paleozoic time only. Xo near relatives are known, though they ex- hibit some affinities with the Phyllopoda. The body consists of three regions — head, thorax, and abdomen — and it is furtlier divided by a raised median dorsal ridge into tliree longitudinal lobes from which character the grou]) derives its name. The thorax has a variable number (2 to 10) of segments that are so articulated as to enable the animal to coil itself more or less closely after the manner of an armadillo. The trilobites consti- tute a very primitive group of Crustacea, and, as their remains are found in the oldest sedimen- tary rocks, they were among the earliest inhabi- tants of the earth. They are of great importance to the geologist, as the various species are very characteristic of the particular layers of rock in which they occur, and for this reason they will be more fully described in an article under their own name. II. Entomostraca. The members of this sub- class present ii great variety of form and habit of life, but they are alike in the variability of the degree of segmentation of the body. They are with few exceptions small, and all are aquatic. They are divided into four orders. (1) In the order Phi/Uopoda the segmentation is distinct, the anterior portion of the body is covered by a ce- phalic shield, and the thoracic appendages are leaf-like, this latter character giving thegroup its name. In some phyllopods. as Apus, and the brine-shrimps (Branchipus and Arteuiia), the body is elongated, well segmented, partly covered by a single dorsal shield and divided into three regions. In other genera, as Daphnia and Esthe- ria, the body is not well segmented and is in- closed in a bivalve shell articulated by a binge at the dorsal median line. The phyllopods are doubtfully represented in the Cambrian rocks. One genus, Estheria, has enjoyed a remarkably long life-period, since it has existed from De- vonian to recent times. During the Devonian and Carboniferous periods the Phyllopoda abounded in the brackish waters of the coastal swamps; in the rocks of the Tertiary era the members of this order are less common. A sin- gle doubtful representative of the suborder Cla- docera, the genus Lynceites, has been found in the Carboniferous rocks. (2) Ostriicothi. — In this order the animals are all small, mostly microscopic, and Avith the body inclosed in a liinged bivalve shell that can be tightl}' shut by a specially developed' adductor muscle. These animals occur by myriads in the modern ocean, seas, and lakes, mostly swinuning near the sur- face of the water, and they seem to have lived in equal abundance in the seas of past time, for their fossil shells are common in all the aqueous rocks from those of Cambrian to those of recent time. Their greatest expansion was in the Ordo- vician and Silurian, and again in the Carbonifer- ous and Cretaceous periods. Examples are Bey- richia, Leperditia, and Cypris. (3) C'opepoda. — Tlicse are all recent forms, with elongated well- segmented bodies in all except the degenerate ])arasitic members of the order. Tlie limbs of the free-swinnuing species are biramous: those of tile ]iarasitcs are greatly reduced in size or en- tirely wanting. Examples: The water-flea (Cy- clops), the fish-lice (Tjcrna'a), ami carp-lice (Ar- gulus). (4) Cirripedia. — In this order, the bar- nacles, we see perhaps the most aberrant of all crustaceans. Because of the usual sessile habiL of life, the body of the animal has suffered so great modification that only the study of the developmental stages shows that the peculiar forms of the adult barnacles arc acquired after passing through a series of larval stages exactly parallel to those of other Entomostraca. The body of the adult is attached by the head. It is inclosed in a leathery integiuncnt that develops articulated calcareous plates and is raised upon a peduncle as in the goose-barnacle (Lepas) : or it may be contained in a calcareous box made of a series of stony plates firmly joined to each other and to the foreign object that serves as a support, as in the acorn-shell ( Balanus ) . The eailiest members ' of the order, Turrilepas and Plunuilites, appeared in Cambrian and Ordo- vician time, and were ancestors of Le])as, while the oldest relative of Balanus, namely Palseo- creusia, occurs in the -lower Devonian rocks, and the genus Balanus itself has been found in the jNIesozoie and Tertiary deposits. The Cirripedia afTord the largest examples of Entomostraca in some of the goose-barnacles that attain a length of eight or nine inches, and they furnish also the most degenerate forms in those Ehizocephala that live as parasites in the bodies of crabs. See Barxaci£; Copepoda; Ostracod.; Phyllopo- da; Entomostraca; and Plate of Bar- nacles. III. Malacostrac. (crabs, lobsters, etc.). The most conunonly known, the largest, and the nmst highly organized Crustacea belong to this division. The body consists of a constant number of segments (20 or 21). distributed between the head (.■>). the thorax (S), and the alidomcn (7). Tn most of the orders the head and thorax are fused to form a cephalothorax of 1.'! segments, and each segment bears a pair of appendages, all