1911 Encyclopædia Britannica/Entomostraca

ENTOMOSTRACA. This zoological term, as now restricted, includes the Branchiopoda, Ostracoda and Copepoda. The Ostracoda have the body enclosed in a bivalve shell-covering, and normally unsegmented. The Branchiopoda have a very variable number of body-segments, with or without a shield, simple or bivalved, and some of the postoral appendages normally branchial. The Copepoda have normally a segmented body, not enclosed in a bivalved shell-covering, the segments not exceeding eleven, the limbs not branchial.

Under the heading Crustacea the Entomostraca have already been distinguished not only from the Thyrostraca or Cirripedes, but also from the Malacostraca, and an intermediate group of which the true position is still disputed. The choice is open to maintain the last as an independent subclass, and to follow Claus in calling it the Leptostraca, or to introduce it among the Malacostraca as the Nebaliacea, or with Packard and Sars to make it an entomostracan subdivision under the title Phyllocarida. At present it comprises the single family Nebaliidae. The bivalved carapace has a jointed rostrum, and covers only the front part of the body, to which it is only attached quite in front, the valve-like sides being under control of an adductor muscle. The eyes are stalked and movable. The first antennae have a lamellar appendage at the end of the peduncle, a decidedly non-entomostracan feature. The second antennae, mandibles and two pairs of maxillae may also be claimed as of malacostracan type. To these succeed eight pairs of foliaceous branchial appendages on the front division of the body, followed on the hind division by four pairs of powerful bifurcate swimming feet and two rudimentary pairs, the number, though not the nature, of these appendages being malacostracan. On the other hand, the two limbless segments that precede the caudal furca are decidedly non-malacostracan. The family was long limited to the single genus Nebalia (Leach), and the single species N. bipes (O. Fabricius). Recently Sars has added a Norwegian species, N. typhlops, not blind but weak-eyed. There are also now two more genera, Paranebalia (Claus, 1880), in which the branchial feet are much longer than in Nebalia, and Nebaliopsis (Sars, 1887), in which they are much shorter. All the species are marine.

Branchiopoda.—In this order, exclusion of the Phyllocarida will leave three suborders of very unequal extent, the Phyllopoda, Cladocera, Branchiura. The constituents of the last have often been classed as Copepoda, and among the Branchiopods must be regarded as aberrant, since the “branchial tail” implied in the name has no feet, and the actual feet are by no means obviously branchial.

Phyllopoda.—This “leaf-footed” suborder has the appendages which follow the second maxillae variable in number, but all foliaceous and branchial. The development begins with a free nauplius stage. In the outward appearance of the adults there is great want of uniformity, one set having their limbs sheltered by no carapace, another having a broad shield over most of them, and a third having a bivalved shell-cover within which the whole body can be enclosed. In accord with these differences the sections may be named Gymnophylla, Notophylla, Conchophylla. The equivalent terms applied by Sars are Anostraca, Notostraca, Conchostraca, involving a termination already appropriated to higher divisions of the Crustacean class, for which it ought to be reserved.

1. Gymnophylla.—These singular crustaceans have long soft flexible bodies, the eyes stalked and movable, the first antennae small and filiform, the second lamellar in the female, in the male prehensile; this last character gives rise to some very fanciful developments. There are three families, two of which form companies rather severely limited. Thus the Polyartemiidae, which compensate themselves for their stumpy little tails by having nineteen instead of the normal eleven pairs of branchial feet, consist exclusively of Polyartemia forcipata (Fischer, 1851). This species from the high north of Europe and Asia carries green eggs, and above them a bright pattern in ultramarine (Sars, 1896, 1897). The Thamnocephalidae have likewise but a single species, Thamnocephalus platyurus (Packard, 1877), which justifies its title “bushy-head of the broad tail” by a singularity at each end. Forward from the head extends a long ramified appendage described as the “frontal shrub,” backward from the fourth abdominal segment of the male spreads a fin-like expansion which is unique. In the ravines of Kansas, pools supplied by torrential rains give birth to these and many other phyllopods, and in turn “millions of them perish by the drying up of the pools in July” (Packard). The remaining family, the Branchipodidae, includes eight genera. In the long familiar Branchipus, Chirocephalus and Streptocephalus the males have frontal appendages, but these are wanting in the “brine-shrimp” Artemia, and the same want helps to distinguish Branchinecta (Verrill, 1869) from the old genus Branchipus. Of Branchiopsyllus (Sars, 1897) the male is not yet known, but in his genera of the same date, the Siberian Artemiopsis and the South African Branchipodopsis (1898), there is no such appendage. Of the last genus the type species B. hodgsoni belongs to Cape Colony, but the specimens described were born and bred and observed in Norway. For the study of fresh-water Entomostraca large possibilities are now opened to the naturalist. A parcel of dried mud, coming for example from Palestine or Queensland, and after an indefinite interval of time put into water in England or elsewhere, may yield him living forms, both new and old, in the most agreeable variety. Some caution should be used against confounding accidentally introduced indigenous species with those reared from the imported eggs. Those, too, who send or bring the foreign soil should exercise a little thought in the choice of it, since dry earth that has never had any Entomostraca near it at home will not become fertile in them by the mere fact of exportation.

2. Notophylla.—In this division the body is partly covered by a broad shield, united in front with the head; the eyes are sessile, the first antennae are small, the second rudimentary or wanting; of the numerous feet, sometimes sixty-three pairs, exceeding the number of segments to which they are attached, the first pair are more or less unlike the rest, and in the female the eleventh have the epipod and exopod (flabellum and sub-apical lobe of Lankester) modified to form an ovisac. Development begins with a nauplius stage. Males are very rare. The single family Apodidae contains only two genera, Apus and its very near neighbour Lepidurus. Apus australiensis (Spencer and Hall, 1896) may rank as the largest of the Entomostraca, reaching in the male, from front of shield to end of telson, a length of 70 mm., in the female of 64 mm. In a few days, or at most a fortnight, after a rainfall numberless specimens of these sizes were found swimming about, “and as not a single one was to be found in the water-pools prior to the rain, these must have been developed from the egg.” Similarly, in Northern India Apus himalayanus was “collected from a stagnant pool in a jungle four days after a shower of rain had fallen,” following a drought of four months (Packard).

3. Conchophylla.—Though concealed within the bivalved shell-cover, the mouth-parts are nearly as in the Gymnophylla, but the flexing of the caudal part is in contrast, and the biramous second antennae correspond with what is only a larval character in the other phyllopods. In the male the first one or two pairs of feet are modified into grasping organs. The small ova are crowded beneath the dorsal part of the valves. The development usually begins with a nauplius stage (Sars, 1896, 1900). There are four families: (a) The Limnadiidae, with feet from 18 to 32 pairs, comprise four (or five) genera. Of these Limnadella (Girard, 1855) has a single eye. It remains rather obscure, though the type species originally “was discovered in great abundance in a roadside puddle subject to desiccation.” Limnadia (Brongniart, 1820) is supposed to consist of species exclusively parthenogenetic. But when asked to believe that males never occur among these amazons, one cannot but remember how hard it is to prove a negative. (b) The Lynceidae, with not more than twelve pairs of feet. This family is limited to the species, widely distributed, of the single genus Lynceus, established by O. F. Müller in 1776 and 1781, and first restricted by Leach in 1816 in the Encyclopaedia Britannica (art. “Annulosa,” of that edition). Leach there assigns to it the single species L. brachyurus (Müller), and as this is included in the genus Limnetis (Lovén, 1846), that genus must be a synonym of Lynceus as restricted. (c) Leptestheriidae. Estheria (Rüppell, 1837) was instituted for the species dahalacensis, which Sars includes in his genus Leptestheria (1898); but Estheria was already appropriated, and of its synonyms Cyzicus (Audouin, 1837) is lost for vagueness, while Isaura (Joly, 1842) is also appropriated, so that Leptestheria becomes the name of the typical genus, and determines the name of the family. (d) Cyclestheriidae. This family consists of the single species Cyclestheria hislopi (Baird), reported from India, Ceylon, Celebes, Australia, East Africa and Brazil. Sars (1887) having had the opportunity of raising it from dried Australian mud, found that, unlike other phyllopods, but like the Cladocera, the parent keeps its brood within the shell until their full development.

Cladocera.—In this suborder the head is more or less distinct, the rest of the body being in general laterally compressed and covered by a bivalved test. The title “branching horns” alludes to the second antennae, which are two-branched except in the females of Holopedium, with each branch setiferous, composed of only two to four joints. The mandibles are without palp. The pairs of feet are four to six. The eye is single, and in addition to the eye there is often an “eye-spot,” Monospilus being unique in having the eye-spot alone and no eye, while Leydigiopsis (Sars, 1901) has an eye with an eye-spot equal to it or larger. The heart has a pair of venous ostia, often blending into one, and an anterior arterial aorta. Respiration is conducted by the general surface, by the branchial lamina (external branch) of the feet, and the vesicular appendage (when present) at the base of this branch. The “abdomen,” behind the limbs, is usually very short, occasionally very long. The “postabdomen,” marked off by the two postabdominal setae, usually has teeth or spines, and ends in two denticulate or ciliate claws, or it may be rudimentary, as in Polyphemus. Many species have a special glandular organ at the back of the head, which Sida crystallina uses for attaching itself to various objects. The Leydigian or nuchal organ is supposed to be auditory and to contain an otolith. The female lays two kinds of eggs—“summer-eggs,” which develop without fertilization, and “winter-eggs” or resting eggs, which require to be fertilized. The latter in the Daphniidae are enclosed in a modified part of the mother’s shell, called the ephippium from its resemblance to a saddle in shape and position. In other families a less elaborate case has been observed, for which Scourfield has proposed the term protoephippium. In Leydigia he has recently found a structure almost as complex as that of the Daphniidae. In some families the resting eggs escape into the water without special covering. Only the embryos of Leptodora are known to hatch out in the nauplius stage. Penilia (Dana, 1849) is perhaps the only exclusively marine genus. The great majority of the Cladocera belong to fresh water, but their adaptability is large, since Moina rectirostris (O. F. Müller) can equally enjoy a pond at Blackheath, and near Odessa live in water twice as salt as that of the ocean. In point of size a Cladoceran of 5 mm. is spoken of as colossal.

Dr Jules Richard in his revision (1895) retains the sections proposed by Sars in 1865, Calyptomera and Gymnomera. The former, with the feet for the most part concealed by the carapace, is subdivided into two tribes, the Ctenopoda, or “comb-feet,” in which the six pairs of similar feet, all branchial and nonprehensile, are furnished with setae arranged like the teeth of a comb, and the Anomopoda, or “variety-feet,” in which the front feet differ from the rest by being more or less prehensile, without branchial laminae.

The Ctenopoda comprise two families: (a) the Holopediidae, with a solitary species, Holopedium gibberum (Zaddach), queerly clothed in a large gelatinous involucre, and found in mountain tarns all over Europe, in large lakes of N. America, and also in shallow ponds and waters at sea-level; (b) the Sididae, with no such involucre, but with seven genera, and rather more than twice as many species. Of Diaphanosoma modiglianii Richard says that at different points of Lake Toba in Sumatra millions of specimens were obtained, among which he had not met with a single male.

The Anomopoda are arranged in four families, all but one very extensive. (a) Daphniidae. Of the seven genera, the cosmopolitan Daphnia contains about 100 species and varieties, of which Thomas Scott (1899) observes that “scarcely any of the several characters that have at one time or another been selected as affording a means for discriminating between the different forms can be relied on as satisfactory.” Though this may dishearten the systematist, Scourfield (1900) reminds us that “It was in a water-flea that Metschnikoff first saw the leucocytes (or phagocytes) trying to get rid of disease germs by swallowing them, and was so led to his epoch-making discovery of the part played by these minute amoeboid corpuscles in the animal body.” For Scapholeberis mucronata (O. F. Müller), Scourfield has shown how it is adapted for movement back downwards in the water along the underside of the surface film, which to many small crustaceans is a dangerously disabling trap. (b) Bosminidae. To Bosmina (Baird, 1845) Richard added Bosminopsis in 1895. (c) Macrotrichidae. In this family Macrothrix (Baird, 1843) is the earliest genus, among the latest being Grimaldina (Richard, 1892) and Jheringula (Sars, 1900). Dried mud and vegetable débris from S. Paulo in Brazil supplied Sars with representatives of all the three in his Norwegian aquaria, in some of which the little Macrothrix elegans “multiplied to such an extraordinary extent as at last to fill up the water with immense shoals of individuals.” “The appearance of male specimens was always contemporary with the first ephippial formation in the females.” For Streblocerus pygmaeus, grown under the same conditions, Sars observes: “This is perhaps the smallest of the Cladocera known, and is hardly more than visible to the naked eye,” the adult female scarcely exceeding 0.25 mm. Yet in the next family Alonella nana (Baird) disputes the palm and claims to be the smallest of all known Arthropoda. (d) Chydoridae. This family, so commonly called Lynceidae, contains a large number of genera, among which one may usually search in vain, and rightly so, for the genus Lynceus. The key to the riddle is to be found in the Encyclopaedia Britannica for 1816. There, as above explained, Leach began the subdivision of Müller’s too comprehensive genus, the result being that Lynceus belongs to the Phyllopoda, and Chydorus (Leach, 1816) properly gives its name to the present family, in which the doubly convoluted intestine is so remarkable. Of its many genera, Leydigia, Leydigiopsis, Monospilus have been already mentioned. Dadaya macrops (Sars, 1901), from South America and Ceylon, has a very large eye and an eye-spot fully as large, but it is a very small creature, odd in its behaviour, moving by jumps at the very surface of the water. “To the naked eye it looked like a little black atom darting about in a most wonderful manner.”

Fig. 1.Dolops ranarum

The Gymnomera, with a carapace too small to cover the feet, which are all prehensile, are divided also into two tribes, the Onychopoda, in which the four pairs of feet have a toothed maxillary process at the base, and the Haplopoda, in which there are six pairs of feet, without such a process. To the Polyphemidae, the well-known family of the former tribe, Sars in 1897 added two remarkable genera, Cercopagis, meaning “tail with a sling,” and Apagis, “without a sling,” for seven species from the Sea of Azov. The Haplopoda likewise have but a single family, the Leptodoridae, and this has but the single genus Leptodora (Lilljeborg, 1861). Dr Richard (1895, 1896) gives a Cladoceran bibliography of 601 references.

Branchiura.—This term was introduced by Thorell in 1864 for the Argulidae, a family which had been transferred to the Branchiopoda by Zenker in 1854, though sometimes before and since united with the parasitic Copepoda. Though the animals have an oral siphon, they do not carry ovisacs like the siphonostomous copepods, but glue their eggs in rows to extraneous objects. Their lateral, compound, feebly movable eyes agree with those of the Phyllopoda. The family are described by Claus as “intermittent parasites,” because when gorged they leave their hosts, fishes or frogs, and swim about in freedom for a considerable period. The long-known Argulus (O. F. Müller) has the second maxillae transformed into suckers, but in Dolops (Audouin, 1837) (fig. 1), the name of which supersedes the more familiar Gyropeltis (Heller, 1857), these effect attachment by ending in strong hooks (Bouvier, 1897). A third genus, Chonopeltis (Thiele, 1900), has suckers, but has lost its first antennae, at least in the female.

Ostracoda.—The body, seldom in any way segmented, is wholly encased in a bivalved shell, the caudal part strongly inflexed, and almost always ending in a furca. The limbs, including antennae and mouth organs, never exceed seven definite pairs. The first antennae never have more than eight joints. The young usually pass through several stages of development after leaving the egg, and this commonly after, even long after, the egg has left the maternal shell. Parthenogenesis is frequent.

The four tribes instituted by Sars in 1865 were reduced to two by G. W. Müller in 1894, the Myodocopa, which almost always have a heart, and the Podocopa, which have none.

Myodocopa.—These have the furcal branches broad, lamellar, with at least three pairs of strong spines or ungues. Almost always the shell has a rostral sinus. Müller divides the tribe into three families, Cypridinidae, Halocypridae, and the heartless Polycopidae, which constituted the tribe Cladocopa of Sars. From the first of these Brady and Norman distinguish the Asteropidae (fig. 3), remarkable for seven pairs of long branchial leaves which fold over the hinder extremity of the animal, and the Sarsiellidae, still somewhat obscure, besides adding the Rutidermatidae, knowledge of which is based on skilful maceration of minute and long-dried specimens. The Halocypridae are destitute of compound lateral eyes, and have the sexual orifice unsymmetrically placed.

Podocopa.—In these the furcal branches are linear or rudimentary, the shell is without rostral sinus, and, besides distinguishing characters of the second antennae, they have always a branchial plate well developed on the first maxillae, which is inconstant in the other tribe. There are five families: (a) Cyprididae (? including Cypridopsidae of Brady and Norman). In some of the genera parthenogenetic propagation is carried to such an extent that of the familiar Cypris it is said, “until quite lately males in this genus were unknown; and up to the present time no male has been found in the British Islands” (Brady and Norman, 1896). On the other hand, the ejaculatory duct with its verticillate sac in the male of Cypris and other genera is a feature scarcely less remarkable. (b) Bairdiidae, which have the valves smooth, with the hinge untoothed. (c) Cytheridae (? including Paradoxostomatidae of Brady and Norman), in which the valves are usually sculptured, with toothed hinge. Of this family the members are almost exclusively marine, but Limnicythere is found in fresh water, and Xestoleberis bromeliarum (Fritz Müller) lives in the water that collects among the leaves of Bromelias, plants allied to the pine-apples. (d) Darwinulidae, including the single species Darwinula stevensoni, Brady and Robertson, described as “perhaps the most characteristic Entomostracan of the East Anglian Fen District.” (e) Cytherellidae, which, unlike the Ostracoda in general, have the hinder part of the body segmented, at least ten segments being distinguishable in the female. They have the valves broad at both ends, and were placed by Sars in a separate tribe, called Platycopa.

The range in time of the Ostracoda is so extended that, in G. W. Müller’s opinion, their separation into the families now living may have already taken place in the Cambrian period. Their range in space, including carriage by birds, may be coextensive with the distribution of water, but it is not known what height of temperature or how much chemical adulteration of the water they can sustain, how far they can penetrate underground, nor what are the limits of their activity between the floor and the surface of aquatic expanses, fresh or saline. In individual size they have never been important, and of living forms the largest is one of recent discovery, Crossophorus africanus, a Cypridinid about three-fifths of an inch (15.5 mm.) long; but a length of one or two millimetres is more common, and it may descend to the seventy-fifth of an inch. By multitude they have been, and still are, extremely important.

Fig. 2.—Cythereis ornata (G. W. Müller). One eye-space is shown above on the left.
Fig. 3.—Asterope arthuri.
Left valve removed.
M,  End of adductor muscle.
OC,  Eye.
AI,  Second antenna.
MX. 1, First maxilla.
MX. 2, Second maxilla.
P. 1, First foot.
V.O, Vermiform organ.
BR, Seven branchial leaves.
F,  Projecting ungues of the furca.

Though the exterior is more uniform than in most groups of Crustacea, the bivalved shell or carapace may be strongly calcified and diversely sculptured (fig. 2), or membranaceous and polished, hairy or smooth, oval or round or bean-shaped, or of some less simple pattern; the valves may fit neatly, or one overlap the other, their hinge may have teeth or be edentulous, and their front part may be excavated for the protrusion of the antennae or have no such “rostral sinus.” By various modifications of their valves and appendages the creatures have become adapted for swimming, creeping, burrowing, or climbing, some of them combining two or more of these activities, for which their structure seems at the first glance little adapted. Considering the imprisonment of the ostracod body within the valves, it is more surprising that the Asteropidae and Cypridinidae should have a pair of compound and sometimes large eyes, in addition to the median organ at the base of the “frontal tentacle,” than that other members of the group should be limited to that median organ of sight, or have no eyes at all. The median eye when present may have or not have a lens, and its three pigment-cups may be close together or wide apart and the middle one rudimentary. As might be expected, in thickened and highly embossed valves thin spaces occur over the visual organ. The frontal organ varies in form and apparently in function, and is sometimes absent. The first antennae, according to the family, may assist in walking, swimming, burrowing, climbing, grasping, and besides they carry sensory setae, and sometimes they have suckers on their setae (see Brady and Norman on Cypridina norvegica). The second antennae are usually the chief motor-organs for swimming, walking and climbing. The mandibles are normally five-jointed, with remnants of an outer branch on the second joint, the biting edge varying from strong development to evanescence, the terminal joints or “palp” giving the organ a leg-like appearance and function, which disappears in suctorial genera such as Paracytherois. The variable first maxillae are seldom pediform, their function being concerned chiefly with nutrition, sensation and respiration. The variability in form and function of the second maxillae is sufficiently shown by the fact that G. W. Müller, our leading authority, adopts the confusing plan of calling them second maxillae in the Cypridinidae (including Asteropidae), maxillipeds in the Halocypridae and Cyprididae, and first legs in the Bairdiidae, Cytheridae, Polycopidae and Cytherellidae, so that in his fine monograph he uses the term first leg in two quite different senses. The first legs, meaning thereby the sixth pair of appendages, are generally pediform and locomotive, but sometimes unjointed, acting as a kind of brushes to cleanse the furca, while in the Polycopidae they are entirely wanting. The second legs are sometimes wanting, sometimes pediform and locomotive, sometimes strangely metamorphosed into the “vermiform organ,” generally long, many-jointed, and distally armed with retroverted spines, its function being that of an extremely mobile cleansing foot, which can insert itself among the eggs in the brood-space, between the branchial leaves of Asterope (fig. 3), and even range over the external surface of the valves. The “brush-formed” organs of the Podocopa are medially placed, and, in spite of their sometimes forward situation, Müller believes among other possibilities that they and the penis in the Cypridinidae may be alike remnants of a third pair of legs, not homologous with the penis of other Ostracoda (Podocopa included). The furca is, as a rule, a powerful motor-organ, and has its laminae edged with strong teeth (ungues) or setae or both. The young, though born with valves, have at first a nauplian body, and pass through various stages to maturity.

Brady and Norman, in their Monograph of the Ostracoda of the North Atlantic and North-Western Europe (1889), give a bibliography of 125 titles, and in the second part (1896) they give 55 more. The lists are not meant to be exhaustive, any more than G. W. Müller’s literature list of 125 titles in 1894. They do not refer to Latreille, 1802, with whom the term Ostracoda originates.

Copepoda.—The body is not encased in a bivalved shell; its articulated segments are at most eleven, those behind the genital segment being without trace of limbs, but the last almost always carrying a furca. Sexes separate, fertilization by spermatophores. Ova in single or double or rarely several packets, attached as ovisacs or egg-strings to the genital openings, or enclosed in a dorsal marsupium, or deposited singly or occasionally in bundles. The youngest larvae are typical nauplii. The next, the copepodid or cyclopid, stage is characterized by a cylindrical segmented body, with fore- and hind-body distinct, and by having at most six cephalic limbs and two pairs of swimming feet.

The order thus defined (see Giesbrecht and Schmeil, Das Tierreich, 1898), with far over a thousand species (Hansen, 1900), embraces forms of extreme diversity, although, when species are known in all their phases and both sexes, they constantly tend to prove that there are no sharply dividing lines between the free-living, the semi-parasitic, and those which in adult life are wholly parasitic and then sometimes grotesquely unlike the normal standard. Giesbrecht and Hansen have shown that the mouth-organs consist of mandibles, first and second maxillae and maxillipeds; and Claus himself relinquished his long-maintained hypothesis that the last two pairs were the separated exopods and endopods of a single pair of appendages. Thorell’s classification (1859) of Gnathostoma, Poecilostoma, Siphonostoma, based on the mouth-organs, was long followed, though almost at the outset shown by Claus to depend on the erroneous supposition that the Poecilostoma were devoid of mandibles. Brady added a new section, Choniostomata, in 1894, and another, Leptostomata, in 1900, each for a single species. Canu in 1892 proposed two groups, Monoporodelphya and Diporodelphya, the copulatory openings of the female being paired in the latter, unpaired in the former. It may be questioned whether this distinction, however important in itself, would lead to a satisfactory grouping of families. In the same year Giesbrecht proposed his division of the order into Gymnoplea and Podoplea.

In appearance an ordinary Copepod is divided into fore- and hind-body, of its eleven segments the composite first being the head, the next five constituting the thorax, and the last five the abdomen. The coalescence of segments, though frequent, does not after a little experience materially confuse the counting. But there is this peculiarity, that the middle segment is sometimes continuous with the broader fore-body, sometimes with the narrower hind-body. In the former case the hind-body, consisting only of the abdomen, forms a pleon or tail-part devoid of feet, and the species so constructed are Gymnoplea, those of the naked or footless pleon. In the latter case the middle segment almost always carries with it to the hind-body a pair of rudimentary limbs, whence the term Podoplea, meaning species that have a pleon with feet. It may be objected that hereby the term pleon is used in two different senses, first applying to the abdomen alone and then to the abdomen plus the last thoracic segment. Even this verbal flaw would be obviated if Giesbrecht could prove his tentative hypothesis, that the Gymnoplea may have lost a pre-genital segment of the abdomen, and the Podoplea may have lost the last segment of the thorax. The classification is worked out as follows:—

1. Gymnoplea.—First segment of hind-body footless, bearing the orifices of the genital organs (in the male unsymmetrically placed); last foot of the fore-body in the male a copulatory organ; neither, or only one, of the first pair of antennae in the male geniculating; cephalic limbs abundantly articulated and provided with many plumose setae; heart generally present. Animals usually free-living, pelagic (Giesbrecht and Schmeil).

This group, with 65 genera and four or five hundred species, is divided by Giesbrecht into tribes: (a) Amphaskandria. In this tribe the males have both antennae of the first pair as sensory organs. There is but one family, the Calanidae, but this is a very large one, with 26 genera and more than 100 species. Among them is the cosmopolitan Calanus finmarchicus, the earliest described (by Bishop Gunner in 1770) of all the marine free-swimming Copepoda. Among them also is the peacock Calanid, Calocalanus pavo (Dana), with its highly ornamented antennae and gorgeous tail, the most beautiful species of the whole order (fig. 4). (b) Heterarthrandria. Here the males have one or the other of the first pair of antennae modified into a grasping organ for holding the female. There are four families, the Diaptomidae with 27 genera, the Pontellidae with 10, the Pseudocyclopidae and Candaciidae each with one genus. The first of these families is often called Centropagidae, but, as Sars has pointed out, Diaptomus (Westwood, 1836) is the oldest genus in it. Of 177 species valid in the family Giesbrecht and Schmeil assign 67 to Diaptomus. In regard to one of its species Dr Brady says: “In one instance, at least (Talkin Tarn, Cumberland) I have seen the net come up from a depth of 6 or 8 ft. below the surface with a dense mass consisting almost entirely of D. gracilis.” The length of this net-filling species is about a twentieth of an inch.

Fig. 4.—Calocalanus pavo (Dana).

2. Podoplea.—The first segment of the hind-body almost always with rudimentary pair of feet; orifices of the genital organs (symmetrically placed in both sexes) in the following segment; neither the last foot of the fore-body nor the rudimentary feet just mentioned acting as a copulatory organ in the male; both or neither of the first pair of antennae in the male geniculating; cephalic limbs less abundantly articulated and with fewer plumose setae or none, but with hooks and clasping setae. Heart almost always wanting. Free-living (rarely pelagic) or parasitic (Giesbrecht and Schmeil). This group is also divided by Giesbrecht into two tribes, Ampharthrandria and Isokerandria. In 1892 he distinguished the former as those in which the first antennae of the male have both members modified for holding the female, and the genital openings of the female have a ventral position, sometimes in close proximity, sometimes strongly lateral; the latter as those in which the first antennae of the male are similar to those of the female, the function of holding her being transferred to the male maxillipeds, while the genital openings of the female are dorsal, though at times strongly lateral. In 1899, with a view to the many modifications exhibited by parasitic and semi-parasitic species, the definitions, stripped of a too hampering precision, took a different form: (a) Ampharthrandria. “Swimming Podoplea with geniculating first antennae in the male sex, and descendants of such; first antennae in female and male almost always differently articulated.” The families occupy fresh water as well as the sea. Naturally “descendants” which have lost the characteristic feature of the definition cannot be recognized without some further assistance than the definition supplies. Of the families comprised, the Mormonillidae consist only of Mormonilla (Giesbrecht), and are not mentioned by Giesbrecht in 1899 in the grouping of this section. The Thaumatoessidae include Thaumatoessa (Kröyer), established earlier than its synonym Thaumaleus (Kröyer), or than Monstrilla (Dana, 1849). The species are imperfectly known. The defect of mouth-organs probably does not apply to the period of youth, which some of them spend parasitically in the body-cavity of worms (Giard, 1896). To the Cyclopidae six genera are allotted by Giesbrecht in 1900. Cyclops (O. F. Müller, 1776), though greatly restricted since Müller’s time, still has several scores of species abundantly peopling inland waters of every kind and situation, without one that can be relied on as exclusively marine like the species of Oithona (Baird). The Misophriidae are now limited to Misophria (Boeck). The presence of a heart in this genus helps to make it a link between the Podoplea and Gymnoplea, though in various other respects it approaches the next family. The Harpacticidae owe their name to the genus Arpacticus (Milne-Edwards, 1840). Brady in 1880 assigns to this family 33 genera and 81 species. Canu (1892) distinguishes eight sub-families, Longipediinae, Peltidiinae, Tachidiinae, Amymoninae, Harpacticinae, Idyinae, Canthocamptinae (for which Canthocampinae should be read), and Nannopinae, adding Stenheliinae (Brady) without distinctive characters for it. The Ascidicolidae have variable characters, showing a gradual adaptation to parasitic life in Tunicates. Giesbrecht (1900) considers Canu quite right in grouping together in this single family those parasites of ascidians, simple and compound, which had been previously distributed among families with the more or less significant names Notodelphyidae, Doropygidae, Buproridae, Schizoproctidae, Kossmechtridae, Enterocolidae, Enteropsidae. Further, he includes in it his own Enterognathus comatulae, not from an ascidian, but from the intestine of the beautiful starfish Antedon rosaceus. The Asterocheridae, which have a good swimming capacity, except in the case of Cancerilla tubulata (Dalyell), lead a semi-parasitic life on echinoderms, sponges, &c., imbibing their food. Giesbrecht, displacing the older name Ascomyzontidae, assigns to this family 21 genera in five subfamilies, and suggests that the long-known but still puzzling Nicothoë from the gills of the lobster might be placed in an additional subfamily, or be made the representative of a closely related family. The Dichelestiidae, on account of their sometimes many-jointed first antennae, are referred also to this tribe by Giesbrecht. (b) Isokerandria. “Swimming Podoplea without genicullating first antennae in the male sex, and descendants of such. First antennae of male and female almost always articulated alike.” To this tribe Giesbrecht assigns the families Clausidiidae, Corycaeidae, Oncaeidae, Lichomolgidae, Ergasilidae, Bomolochidae, Clausiidae, Nereicolidae. Here also must for the time be placed the Caligidae, Philichthyidae (Philichthydae of Vogt, Carus, Claus), Lernaeidae, Chondracanthidae, Sphaeronellidae (better known as Choniostomatidae, from H. J. Hansen’s remarkable study of the group), Lernaeopodidae, Herpyllobiidae, Entomolepidae. For the distinguishing marks of all these, the number of their genera and species, their habits and transformations and dwellings, the reader must be referred to the writings of specialists. Sars (1901) proposed seven suborders—Calanoida, Harpacticoida, Cyclopoida, Notodelphoida, Monstrilloida, Caligoida, Lernaeoida.

Authorities.—(The earlier memoirs of importance are cited in Giesbrecht’s Monograph of Naples, 1892); Canu, “Hersiliidae,” Bull. Sci. France belgique, ser. 3, vol. i. p. 402 (1888); and Les Copépodes du Boulonnais (1892); Cuenot, Rev. biol. Nord France, vol. v. (1892); Giesbrecht, “Pelag. Copepoden.” F. u. fl. des Golfes von Neapel (Mon. 19, 1892); Hansen, Entomol. Med. vol. iii. pt. 5 (1892); I. C. Thompson, “Copepoda of Liverpool Bay,” Trans. Liv. Biol. Soc. vol. vii. (1893); Schmeil, “Deutschlands Copepoden,” Bibliotheca zoologica (1892–1897); Brady, Journ. R. Micr. Soc. p. 168 (1894); T. Scott, “Entomostraca from the Gulf of Guinea,” Trans. Linn. Soc. London, vol. vi. pt. 1 (1894); Giesbrecht, Mitteil. Zool. Stat. Neapel, vol. xi. p. 631; vol. xii. p. 217 (1895); T. and A. Scott, Trans. Linn. Soc. London, ser. 2, vol. vi. p. 419 (1896); Hansen “Choniostomatidae” (1897); Sars, Proc. Mus. Zool. St Petersburg, “Caspian Entomostraca” (1897); Giesbrecht and Schmeil, “Copepoda gymnoplea,” Das Tierreich (1898); Giesbrecht, “Asterocheriden,” F. u. fl. Neapel (Mon. 25, 1899); Bassett-Smith, “Copepoda on Fishes,” Proc. Zool. Soc. London, p. 438 (1899); Brady, Trans. Zool. Soc. London, vol. xv. pt. 2, p. 31 (1899); Sars, Arch. Naturv. vol. xxi. No. 2 (1899); Giesbrecht, Mitteil. Zool. Stat. Neapel, vol. xiv. p. 39 (1900); Scott, “Fish Parasites,” Scottish Fishery Board, 18th Ann. Rep. p. 144 (1900); Stebbing, Willey’s Zool. Results, pt. 5, p. 664 (1900); Embleton, Journ. Linn. Soc. London, vol. xxviii. p. 211 (1901); Sars, Crustacea of Norway, vol. iv. (1901). (T. R. R. S.)