The New International Encyclopædia/Crustacea
CRUSTACEA (Neo-Lat. nom. pl., from Lat. crusta, crust). A class of gill-bearing aquatic arthropods, differing from other Branchiata in having five pairs of appendages on the head, the first two of which are antennæ, and in having all of the limbs except the first pair of antennæ biramose.
Structure. The Crustacea derive their name from the hard armor which in most of them covers the whole body, forming a complex exoskeleton, calcareous and intermediate between shell and bone in the higher forms, while in the lower and smaller kinds it consists principally of chitin, and corresponds more nearly in its nature with the integuments of insects. It is secreted by the epidermis on its outer side, and serves not merely for protection, but also for the attachment of the muscles. Its color is dependent upon the absence, or presence and amount, of pigment, which is also a product of the epidermis, and is formed during the growth of the skeleton. These pigment-cells are often very ornamental, especially under the microscope, as their shape is beautifully and very irregularly stellate. Usually, however, the pigment is so abundant as to make the body-wall opaque and the individual pigment-cells are no longer visible. The mineral matter of the skeleton is chiefly carbonate of lime. The smallest crustaceans have little or none in the skeleton, but in all of the larger forms it is present in greater or less abundance. In the so-called ‘stone-crabs’ so much is present that the skeleton is really as hard as a rock. In all cases, however, where the skeleton covers a joint it is thin and free from mineral matter, thus permitting perfect freedom of movement. The bristles and hairs often found on the outside of the skeleton are of the same essential structure as it, and have a central core of epidermis. In barnacles (q.v.), ostracods, water-fleas (q.v.), and a few other crustaceans, there is, in addition to the external skeleton, and formed as an outgrowth of it, a true shell, whose only function is protection. This shell may consist of several pieces, as in barnacles, or be strictly bivalve, as in water-fleas, and it may be wholly chitinous or very firmly calcified. Sometimes it is capable of inclosing the entire animal, but often it only seems to protect certain parts. It is usually hinged along the dorsal side of the animal, where it is attached to the skeleton, and is supplied with muscles for closing it.
The crustacean body is composed of segments, some very distinct, while others coalesce or are consolidated, of which the thorax of a crab affords an excellent example. The first five rings are regarded as forming the head, the next eight the thorax, when that part of the body is separable from what follows.
Epidermis and Molting.—The crustaceous covering has beneath it a true skin, and, like the epidermis, is cast off from time to time and supplied anew, as the growth of the animal requires more room for the internal parts. In this molting or casting of the shell, the animal divests itself of its covering not in separate parts, but in one piece, including the coverings of the limbs and even of the antennæ, although the membranes which connect the hard plates are split and torn. A period of apparent sickness precedes and agitation accompanies the process; and the thick muscular parts of the limbs of crabs and lobsters become soft and flaccid, so as to be more easily extricated from their hard covering. The loss of a limb, which sometimes takes place in this process, and is otherwise a frequent occurrence, is easily repaired, for a new one grows in its stead; but it is a curious circumstance that in order to this reproduction the limb must be broken off at a particular joint. the second from the body, thus leaving only a short stump; and when a limb is broken elsewhere, the animal itself exercises the remarkable power of throwing it off by this joint.
Locomotive and Other Organs.— The principal organ of locomotion in many Crustacea, as in the lobster, shrimp, etc., is the abdomen, terminating in fan-like appendages; by bending the abdomen suddenly down under the body, the animal darts backward in the water. The limbs, which are connected with the thoracic rings, are, in some, organs of swimming; those of others are used for walking at the bottom of the water or on dry ground. Some have ‘swimming-feet’ or pleopods attached to the abdomen, often very different from the thoracic legs. The legs of some are fitted for burrowing. The first pair of legs is not infrequently transformed into a pair of powerful pincers or ‘claws,’ the last joint but one being prolonged so as to oppose the last joint, which becomes attached as to the side of it; and these are used for seizing and tearing food. The first pair of appendages are organs of touch called antennules; following these are the antennæ, also sense-organs; then come a pair of powerful jaws, the mandibles; and back of these are two pairs of accessory jaws, the maxillæ. The thoracic feet, which follow in regular order, are sometimes modified to function as jaws, and are then called maxillipeds. In some forms the mouth-parts are greatly modified to form sucking instead of masticating organs. The digestive organs are very simple; there is a short but capacious gullet, a large stomach, and a straight and simple intestinal tube. A well-developed digestive gland called the liver is often present. The pyloric region of the stomach is sometimes furnished with a remarkable apparatus of hard tubercles or sharp teeth for grinding or tearing food. Many of the Crustacea feed on animal food, and are very voracious; many, however, feed on vegetable food. The nervous system of crustaceans agrees generally with that of other arthropods, and exhibits many gradations of concentration. The eyes are either simple (ocelli), aggregate (consisting of several ocelli under a common cornea), or compound; and the compound eyes are often elevated on stalks. Besides eyes some crustaceans have ciliated pits or cavities that seem to be sense-organs, perhaps for smell or taste. Others have otocysts or positional organs, but it is doubtful whether any have the sense of hearing. In some Schizopoda there are also ‘accessory eyes’ on the basal points of certain thoracic feet and in the middle line of the abdomen. The gills are variously placed, on the sides of the body, or on the thoracic limbs, on the abdominal legs, etc. The heart is always in the middle line of the body on the dorsal side, is of variable form, and distributes the blood by a number of trunks through the system; but the blood returns to venous sinuses, from which, and not from the heart, it is sent into the gills, and it is not until after its aëration in the gills that it comes to the heart again.
Reproduction and Ecology. The sexes are distinct in most Crustacea, and all are oviparous. The eggs are almost invariably hatched in water, even those of terrestrial forms, and a sort of incubation often takes place, as the eggs are carried about under the abdomen or thorax of the female, attached to the pleopods or other appendages. Except in a few cases the development takes place by metamorphosis, through a very remarkable series of larval stages. All the 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 food-fishes, 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.
Classification and Phylogeny. Classification 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 microscopic, 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 surface 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 well-segmented 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 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. The 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); 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 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 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 Rhizocephala that live as parasites in the bodies of crabs. See Barnacle; Copepoda; Ostracoda; Phyllopoda; Entomostraca; and Plate of Barnacles.
III. Malacostraca (crabs, lobsters, etc.). The most commonly known, the largest, and the most highly organized Crustacea belong to this division. The body consists of a constant number of segments (20 or 21), distributed between the head (5), the thorax (8), and the abdomen (7). In most of the orders the head and thorax are fused to form a cephalothorax of 13 segments, and each segment bears a pair of appendages, all 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 Archæeocaris. Crangopsis, and Pygocephalus from the Carboniferous, and possibly also Palæocaris and Gampsonyx, the latter from the Permian, are the earliest 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 chelæ 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 (Palæmon), 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 quickswimming 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 Palæopalæmon, 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 Willemœsia, 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 seas. 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.
(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, and with a disproportionately large abdomen. The live 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 sandy 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) Cumacea.—Another small order of prawn-like 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, especially the more primitive, played more important roles in the early ages than they do at present, and that they have been superseded by the more specialized types. The letters at the heads of the vertical columns indicate the geological systems as follows: C, Cambrian; O, Ordovician; 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 different 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 Zoölogy, 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 Commission (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, vol. v. (Leipzig, 1866-94). For description of the fossil forms, consult the various manuals of paleontology, such as those by Zittel, Zittel-Eastman, Bernard, Nicholson, and Etheridge. A good synopsis of the class, dwelling more especially on the fossil forms, is to he found in Zittel-Eastman, Text-Book of Paleontology (New York and London, 1900), where are given very useful bibliographic lists of works on both recent and fossil forms.