lower Crustacea hatch as minute, oval, unsegmented creatures with three pairs of appendages, and this larva is called a nauplius. Although the higher Crustacea hatch as a more highly organized form called 'zoea,' and later pass through an intermediate larval form, the 'megalops,' all pass through a nauplius stage in the egg. See BARNACLE for illustration of 'nauplius;' and see CRAB for illustrations of 'zoea' and 'megalops.' More than 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 high degree of intelligence. The Crustacea constitute, in an economic sense, perhaps the most important group of invertebrates. The myriads of the smaller forms that drift about the ocean and the great lakes furnish the greater part of the fare of the important foodfishes, and are thus indirectly of value to mankind. In the economy of the ocean itself Crustacea are also of great importance, for they act as the natural scavengers of the sea.

Classifica

CLASSIFICATION AND PHYLOGENY. tion of the Crustacea is based upon the number and manner 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) Entomostraca, and (3) Malacostraca (qq.v.), which may be considered to constitute subclasses. The principal characters of these subclasses and their component orders, together with some notes of the geological history of the groups, are here briefly given.

I. TRILOBITA. This is an extinct group of Crustacea that lived during Paleozoic time only. No near relatives are known, though they exhibit some affinities with the Phyllopoda. The body consists of three regions--head, thorax, and abdomen-and it is further divided by a raised median dorsal ridge into three longitudinal lobes from which character the group derives its name. The thorax has a variable number (2 to 19) 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 constitute a very primitive group of Crustacea, and, as their remains are found in the oldest sedimentary rocks, they were among the earliest inhabitants 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 subclass present a 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 Phyllopoda the segmentation is distinct, the anterior portion of the body is covered by a cephalic shield, and the thoracic appendages are leaf-like, this latter character giving the group its name. In some phyllopods, as Apus, and the brine-shrimps (Branchipus and Artemia), the body is elongated, well segmented, partly covered by a single dorsal shield and divided into three regions. In other genera, as Daphnia and Estheria, the body is not well segmented and is inclosed in a bivalve shell articulated by a hinge 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 Devonian 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 single doubtful representative of the suborder Cladocera, the genus Lynceites, has been found in the Carboniferous rocks. (2) Ostracoda.-In this order the animals are all small, mostly microscopie, and with the body inclosed in a hinged bivalve shell that can be tightly shut by a specially developed adductor muscle. These animals occur by myriads in the modern ocean. seas, and lakes, mostly swimming 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 Ordovician and Silurian, and again in the Carboniferous and Cretaceous periods. Examples are Beyrichia, Leperditia, and Cypris. (3) Copepoda.— These are all recent forms, with elongated wellsegmented bodies in all except the degenerate parasitic members of the order. The limbs of the free-swimming species are biramous; those of the parasites are greatly reduced in size or entirely wanting. Examples: The water-flea (Cyclops), the fish-lice (Lernæa), and carp-lice (Argulus). (4) Cirripedia.—In this order, the barnacles, we see perhaps the most aberrant of all crustaceans. Because of the usual sessile habit

The

or

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 are acquired after passing through a series of larval stages exactly parallel to those of other Entomostraca. body of the adult is attached by the head. It is inclosed in a leathery integument that develops articulated calcareous plates and is raised upon a peduncle as in the goose-barnacle (Lepas); 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 earliest members of the order, Turrilepas and Plumulites, appeared in Cambrian and Ordovician time, and were ancestors of Lepas, while the oldest relative of Balanus, namely Palæocreusia, occurs in the lower Devonian rocks, and the genus Balanus itself has been found in the Mesozoic and Tertiary deposits. The Cirripedia afford the largest examples of Entomostra ea 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 Rhizocephala that live as parasites in the bodies of crabs. See BARNACLE; COPEPODA; OSTRACODA: PHYLLOPO

DA;

ENTOMOSTRACA; and Plate of BAB

of which are usually well differentiated to perform special functions. The compound eyes are stalked in all orders except the Cumacea and Arthrostraca. (1) The order Phyllocarida was much more fully represented in Paleozoic times than it is now by such genera as Ceratiocaris, Echinocaris, Pephricaris, etc., in which the cephalothorax was covered by a hinged bivalve shell provided with an adductor muscle like that in the bivalve phyllopods and the ostracods. The order enjoyed a long life-period, as it first appeared in the upper Cambrian, and it lasted to the end of the Paleozoic. Through the Mesozoic and Tertiary eras the order could not have been very abundant, as no fossil remains referable to it have been found in the rocks of those ages, and the modern genera, three in number, show considerable modifications from their early ancestors. This group presents synthetic types which may have given rise on the one hand to the Phyllopoda, and on the other to the higher Crustacea represented by the Schizopoda and Decapoda. (See PHYLLOCARIDA.) (2) Schizopoda. The animals of this order, the opossum shrimps (Mysis), resemble in their general form the shrimps and prawns among the decapods. They are, however, more loosely built and the thoracic limbs, of which there are eight pairs, are biramous (whence the name), and all alike. These characters mark the group as of primitive rank. Its relation, as one of lower phylogenetic rank, to the Decapoda, is shown by the fact that certain decapods, such as the prawns, pass in their ontogeny, that is, in their development from egg to adult, through a 'mysis stage,' in which schizopod characteristics are strongly marked. The fossil genera Archæocaris, Crangopsis, and Pygocephalus from the Carboniferous, and possibly also Palæocaris and Gamp sonyx, the latter from the Permian, are the earli est ancestors of the Schizopoda. These all present very primitive characters and are separated from the modern descendants by the interval of Mesozoic and Tertiary time, from which we have no traces of the group.

(3) Decapoda.-The character upon which the order (shrimps, lobsters, and crabs) is based is the prominence of the five posterior pairs of thoracic limbs developed as walking feet; the three anterior pairs being adapted as maxillipedes or accessory mouth-parts. The first pair of walking feet is usually provided with strong chela or nipping claws. The cephalothorax is covered by a single firm carapace, which in many crabs is so shaped as to afford a most efficient protective covering for the entire animal, including the legs. The frontal portion of this carapace is produced into an anterior rostrum that is often of considerable length, and the lateral portions are extended downward to cover the gills, which are borne on the bases of the thoracic limbs. The order is divided into two suborders: the Macrura, with long-tailed bodies, as the shrimp, prawn (Palemon), and lobster (Homarus); and the Brachyura, or crabs, with the abdomen reduced and closely applied to the under surface of the cephalothorax. A number of species allied to the hermit-crab (Pagurus), often distinguished under the subordinal designation Anomura, are either Macrura or Brachyura which, through adaptation to their peculiar habits of life, in occupying the discarded shells of gastropods, have lost the regularity of their

thoracic and abdominal segments. The compound eyes of all decapods are raised on stalks, which in many crabs are developed to extraordinary lengths. Some of the largest among Crustacea are found among the decapods. As a rule the quick-swimming decapods (prawns) may be recognized by the lateral compression of the body, while the bottom crawlers have a more or less flattened form. The crabs are the most highly specialized forms of all Crustacea, and they exhibit many interesting and peculiar adaptations to habit of life. (See CRAB; LOBSTER; PRAWN; etc.) The early ancestry of the decapods is not well known for those periods previous to Mesozoic time. Some doubtful Paleozoic forms that seem to be transitional between the schizopods and decapods are Palæopalamon, from the Devonian of Ohio, and Anthrapalæmon, from the coal-measures of Illinois, but during this era the order was greatly subordinated to other crustaceans like the trilobites and ostracods, and to the merostomates, represented by Eurypterus. In the Mesozoic of Europe the group attained a great development, and this continued through the early Tertiary period. Fossil decapod remains are peculiarly sparse in the American deposits of these periods. The Jurassic lithographic slates of Bavaria have afforded large numbers of finely preserved specimens of fossil decapods of many genera and species. Some of these, as Eryon, have modern representatives, as Willemoesia, still living in the abysmal depths of the ocean. Other genera, among them the prawn (Penæus), began in the Jurassic and Cretaceous periods and have survived without changes of greater than specific rank throughout the succeeding ages, and are now abundant in. the Mediterranean Sea and the Atlantic and Indian oceans. The Macrura declined toward the end of Cretaceous time and gave way to the Brachyura, which attained great expansion during the Eocene or early Tertiary period. In fact, the crabs were the dominant types of invertebrate life in many parts of the Tertiary seas of Europe, and their fossil remains are now found in abundance in the rocks that are relics of those The Eocene deposits of the London Clay of England, those of Bavaria and Hungary, and especially those of Vicenza, in northeastern Italy, have afforded the finest specimens, nearly all of which belong to modern families.

seas.

(4) Stomatopoda.-This is a small order comprising nine marine genera, of which the best known is the mantis-shrimp (Squilla). They have the body loosely built, elongated and flattened, with an incomplete carapace that leaves the three posterior thoracic segments uncovered,

EXPLANATION OF PLATE OF CRUSTACEA, FOSSIL.

1. Dromiopsis, a round crab from the Upper Cretaceous of Denmark. 2. Leperditia, an ostracod: Silurian; island of Gotland. 3. Eger, a prawn; Jurassic lithographic limestones; Bavaria. 4. Xanthopsis, a cyclometopan crab; Eocene; Bavaria. 5. Turrilepas, a primitive cirriped, allied to the goose-barnacle; Silurian; Dudley, England. 6. Seulda pennata, a stomatopod; Upper Jura of Bavaria; A, outline of the fossil; B, dorsal view of head; C, ventral view of entire body, showing thoracic and abdominal appendages. 7. Ceratiocaris, a phyllocarid; Ordovician; Lanarkshire, England; showing the bivalve shell, some of the head appendages, and the segmented long abdomen with terminal spines. 8. Palæoerensia, a barnacle, allied to the acorn-shell, imbedded in a fossil coral of Lower Devonian age: New York State. 9. Pephricaris, a phyllocarid with spinose carapace; Chemung; New York State. 10. Eryon, a macruran; lithographic limestones of Bavaria; A, ventral aspect; B, dorsal aspect.

and with a disproportionately large abdomen. The five anterior pairs of thoracic limbs function as maxillipedes, the second pair being much the largest and provided with a spiny terminal segment that folds back upon the penultimate segment, like a knife-blade into its handle, to form a very efficient clasping organ. A powerful burrowing organ, developed by the union of the sixth pair of abdominal appendages with the terminal plate or telson, enables the animal to bury itself in the andy bottoms near shore. Fossil forms of this group are rare. The earliest, Necroscylla, from the coal-measures of England, resembles Squilla. Another allied genus, Sculda, is found in fine preservation in the lithographic slates of the Bavarian Jurassic, and Squilla is found in the same horizon and in succeeding horizons of the Cretaceous and Tertiary.

(5) Cumaceu.-Another small order of prawnlike Crustacea, in which the compound eyes are sessile and sometimes fused together, or are entirely absent. The carapace is further reduced than in the Stomatopoda, for it leaves the five posterior pairs of thoracic segments uncovered. Only two pairs of thoracic limbs function as maxillipedes, and six pairs are legs, of which two or three anterior pairs are biramous. This group is related to the Schizopoda and Isopoda; no fossil representatives of it are known.

(6) Arthrostraca.-In this large order the fusion of the segments is still further reduced, only one or two of the anterior being united with the head to form a rudimentary cephalothorax. These fused segments bear maxillipedes, while the remaining free segments bear legs that end in claws or are built for swimming. The two suborders are quite different. in outward appearances. The Amphipoda have the body laterally compressed and the legs adapted for swimming and jumping, as in the sand-fleas (Gammarus and Orchestia). The Isopoda have the body dorsoventrally flattened, the legs adapted for crawling, and the abdominal segments are fused to form a single terminal plate. The amphipods (see AMPHIPODA) are wholly aquatic, in both marine and fresh water, and are often also found living in the damp flotsam and jetsam of the beach between low and high tides. The isopods, while generally aquatic, afford some fine examples of adaptation to terrestrial conditions in the wood-louse (Oniscus) and pill-bug (Armadillidium), which are commonly found under the bark of dead trees and in other like situations. Paleontologically the amphipods are of little importance. The earliest indisputable ancestor is Acanthotelson, of the Carboniferous, and the genus Gammarus itself is found in the Tertiary rocks. The isopods are better known in a fossil state, and the ancestors seem to have been as a rule of larger size than the recent forms. Arthrapleura of the Carboniferous attained a length of about twenty inches. Præarcturus, also a large form, from the Old Red Sandstone of the English Devonian, is the earliest isopod known. Archæoniscus. Cyclosphæroma, and Eosphæroma are other genera from the Mesozoic and Tertiary rocks that are closely allied to modern forms.

GEOLOGICAL DISTRIBUTION. The accompanying table indicates in a rough way the larger classification of the Crustacea and the distribution and expansion of the various subdivisions in past geologic times. It shows that certain types, es

HISTORICAL VIEW OF CRUSTACEA.

cian; S. Silurian; D, Devonian; Cb. Carboniferous; P. Permian: T, Triassic; J, Jurassic; Cr. Cretaceous; E, Eocene; M, Miocene; Pl, Pliocene; R, Recent. An interrogation-point indicates the doubtful presence of a member of the group, and the width of the black line indicates in a very imperfect way the relative amount of expansion of the group. The articles on the dif ferent orders and suborders should be consulted for further information.

BIBLIOGRAPHY. The best general introduction to the study of the Crustacea is T. H. Huxley, The Crayfish: An Introduction to the Study of Zoology, in International Scientific Series (New York, 1884). Other works of a general nature on the morphology and classification are: J. S. Kingsley, "The Classification of the Arthropoda," in American Naturalist, vol. xxviii. (Philadelphia, 1894); K. Groben, "The Genealogy and Classification of the Crustacea," in Annals and Magazine of Natural History, ser. 6, vol. xi. (London, 1893); T. R. R. Stebbins, History of the Crustacea (New York, 1893); R. Rathbun, "Natural History of the Economic Crustaceans," in Bulletin U. S. Fish Commis sion (Washington, 1889); H. Milne-Edwards, Histoire naturelle des crustacés (3 vols., Paris, 1834-40); H. Gerstaecker, "Gliederfüssler," in Bronn's Klassen und Ordnungen des Thierreichs,