Fig. 106.—Hydatina senta, ventral view. (After Plate.) al, Lateral antenna; bl, bladder; ci, cingulum; e, e, eggs in uterus; fg, foot gland; g, gizzard; gg, gastric gland; gm, germarium or ovary; gr, ciliated lobes of "groove"; i, intestine; k, k, kidneys; m, mouth; ns, nephrostome; oe, oesophagus; rc, renal commissure, transverse tube uniting kidneys above mouth; s, stomach overlaid by reproductive organs; tr, trochus; u, uterus; vm, vitellarium or yolk-gland.
Definition of the Class.—We may define Rotifera as a class of minute bilaterally symmetrical animals, with a chitinous integument, a soft terminal "disc" fringed by a complex ciliary "wreath," an anterior or subventral mouth, and a dorsal cloacal aperture, beyond which the body is usually prolonged into the "foot" or process bearing cement glands, and serving for attachment, temporary or permanent. The body-cavity has no epithelial lining, and is traversed by nerves and muscles. The alimentary canal possesses a chitinous gizzard or mastax of peculiar arrangement, and it usually opens into a cloaca. The nervous centre consists of a ganglion on the dorsal side of the pharynx, to which a second one on the ventral side is sometimes connected to form a complete ring; eyes and bristle-bearing feelers are usually present as sense-organs. A paired system of renal tubes serves for excretion, opening through a median contractile bladder into the ventral side of the cloaca. The sexes are distinct; but the males (Fig. 107), which mostly lack digestive organs, occur rarely, and the females are usually viviparous, or carry about the eggs till they are hatched; while, owing to the rarity of the males, parthenogenesis is habitual. Fission and budding are alike unknown. The fertilised eggs are of the kind termed "winter" or "resting" eggs, and resist conditions adverse to life.
The Rotifera are of cosmopolitan distribution; most of the species inhabit fresh water, whilst some are brackish, and a few are marine; 84 genera and about 700 species have been described.
Fig. 107.—Male Rotifers. (After Hudson.[[252]]) 1, Floscularia campanulata; 2, Lacinularia socialis; 3, Notops brachionus; 4, Synchaeta tremula; 5, Asplanchna ebbesbornii; 6, Brachionus urceolaris; 7, Salpina mucronata; 8, Pedalion mirum.
External Features.[[253]]—The body is divided into three regions: (1) the head, ending in the disc, which bears the ciliary wreath; (2) the trunk, containing the viscera; (3) the foot, which only contains muscles, nerves, and cement-glands. The general form of the BODY varies greatly: it is spherical in Trochosphaera, ovoid in Asplanchnidae, conical in Scirtopoda, Triarthridae, and Synchaeta; moderately elongated in the majority of the Ploima, among which some forms are very flat, like Pterodina, Metopidia, and Brachionus; shortly elongated and cylindrical in Hydatina (Fig. 106), Notommatidae, and many others. In Taphrocampa it is cylindrical and segmented, while the segments are telescopic in the Bdelloida, both ends being retractile into the middle segment. In most attached, tube-dwelling forms the body is ovate, tapering behind into the elongated stalk-like foot.
The FOOT at the hinder end of the body is usually more or less jointed; in Pterodina and Brachionus it is long, transversely wrinkled, and retractile. Usually it terminates in a couple of acute, mobile toes, perforated at the tips by the ducts of the pedal glands (Fig. 106, fg), whose viscid secretion serves to anchor the animal. In Rotifer there are three of these toes, which are retractile, and in addition there are in this genus, as in most of the Bdelloida, toe-like pointed spurs in pairs on the more proximal joints of the foot. In Callidina the spurs are often perforated, and the toes are replaced by numerous openings on the last joint of the foot (Fig. 109, A); while in Discopus the end of the foot expands into a large disc, with numerous pores for the exudation of the pedal cement, and there are no spurs. In Pedalion mirum the foot is represented by two tubular processes ciliated at the apex and at the outer side near the base (Fig. 117, f). These are inconstant in size and form, that of one side being sometimes reduced or absent, while both are absent in the closely allied species P. fennicum.
In Melicertidae and Flosculariidae the long foot ends in an expanded disc, which is cupped and ciliated in the larva (Fig. 112, B) and in the larva-like male (Fig. 107); but in two species it is prolonged into a long flexible thread which is not contractile. The foot is also elongated in the Bdelloid genus Actinurus and the Ploimal genus Scaridium. It forms a mere ventral disc in Apsilus (and Atrochus?), and is absent in Asplanchnidae (except Asplanchnopus), Triarthridae, and Anuraeidae, and in the genera Trochosphaera (Melicertaceae) and Pompholyx (Pterodinidae).
The fringed spines of Triarthridae are jointed appendages moved by powerful muscles; in Triarthra one is median and ventral, the others being attached to the shoulders. In Polyarthra, there are twelve flattened and serrated spines, a bunch of three being attached to the dorsal and ventral faces of either shoulder. An easy transition leads to the hollow appendages of Scirtopoda, which end in a fringe of bristly hairs, themselves feathered with finer hairs (Fig. 117). These processes are in Pedalion six in number, two median (respectively dorsal and ventral), two antero-lateral, and two postero-lateral. As they contain proper muscles, and the postero-lateral pair contain part of the nephridia and bear the lateral antennae, they are true outgrowths of the body, and are not homologous with the spines of Triarthridae.