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.”
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| Fig. 1.—Dolops ranarum (Stuhlmann). |
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
