Thomas Roscoe Rede Stebbing

“ 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. ”

“ 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. ”

“ 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. ”

“ 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.” ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ 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 ”

“ 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 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. ”

“ 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. ”

“ 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. ”