Unisegmental Vermes with a retractile trochal apparatus at the anterior end of the body, and a posterior foot which is a ventral process of the body. There is a single ganglion dorsally placed, a pair of nephridial tubes, and a coelome. Circulatory organs are absent. The sexes are separate, and the male with rare exceptions arrested in development. Parthenogenetic (?).

The body is protected by a cuticle secreted by an underlying layer of ectodermic protoplasm with scattered nuclei, i.e. by a syncyticum. This cuticle is often delicate; gelatinous in Notommata centrura; sometimes thickened as a series of rings which gives to the body a segmented appearance, or as a variously shaped shield or lorica protecting the body more or less perfectly. A moult has been observed only in one instance (Apodoides stygius). One or two Rotifers are furnished with long spines, e.g. Triarthra; and Pedalion mira has six hollow processes, terminating in a number of feathered setae and containing muscles like the limbs of Arthropoda. Some few Rotifera develope round themselves a gelatinous case, which they inhabit permanently (Flosculariadae) or temporarily (Philodina), the origin of which is unknown. Melicerta fashions a case of pellets out of material collected in a ciliated groove below the trochal apparatus, and cemented together by a gland. The trochal apparatus is sometimes absent altogether, e.g. Balatro, or feebly developed, e.g. Albertia. It appears to consist typically of an internal prae-oral ring of long cilia, the trochus, and an external ring of finer cilia, the cingulum which leads into the mouth; the former is the homologue of the prae-oral ring of cilia in the Trochosphere, the latter of the adoral ciliated band, seen not in all but in many examples of the same larval form.

Both rings may be interrupted dorsally, and the trochus ventrally. The latter is sometimes lobed or broken up into isolated eminences. The cingulum forms five long processes fringed with stout cilia in Stephanoceros, or five knobs each with a bundle of long cilia, usually motionless, in Floscularia.

The foot is sometimes absent, e. g. Asplanchna. It may be short or long; sometimes transversely wrinkled, e. g. Pterodina, or segmented; it terminates with two styles,or in a disc, as in tubicolous forms and occasionally in free, e. g. Pterodina. It is in the latter case surrounded by cilia, as it is in tubicolous forms before they become attached. The coelome does not extend into it, but it contains two glands, the secretion of which serves for attachment.

There are muscles for the retraction and extension of both the trochal apparatus and the foot, disposed partly in a circular, partly in a longitudinal direction. They are colourless and sometimes striated. The jaw apparatus and the digestive tract have their special musculature. Connective tissue cells with fine processes unite the various organs which lie in a well-developed coelome. The latter is probably to be considered an archicoele. The blood (haemo-lymph) is clear, as a rule colourless, and often contains granules but no corpuscles.

There is a single dorsally placed nerve-ganglion composed of round cells. It lies above the pharynx and gives off nerves anteriorly as well as to the dorsal sensory organ or antenna (calcar). The latter is represented by a bundle of sensory hairs, sometimes borne upon a peduncle, and it may be double or probably divided. In many Rotifera there is a pair of similar but laterally placed organs, somewhat posterior in position, but not supplied directly from the ganglion. An unpaired or paired eye generally lies behind, upon, or in front of the ganglion. It consists of a reddish pigment, in which a clear lens is found imbedded in some species. Many Rotifers possess a remarkable globular or kidney-shaped mass of calcareous granules in connection with the ganglion, the significance of which is unknown.

The mouth is anterior, nearly terminal in tubicolous forms, but more or less ventral in others. The digestive tract is usually ciliated throughout. The mouth leads into an oesophagus, followed in Flosculwiadae by a crop, but usually directly by a muscular pharynx or mastax containing the chitinous jaw-apparatus or 'trophi.' These consist of two hammer-like bodies, the mallei, which work against an incus or anvil. Each- malleus consists of a manubrium jointed to an uncus; the incus of two pieces, rami, borne upon a single piece, the fulcrum. The shape of the 'trophi' is variable. The stomach is globular or cylindrical, and receives the secretion of two glands placed at its commencement and in some of a large simple or lobed glandular mass as well. The intestine is pyriform and opens into a rectum or cloaca, which also receives the duct of the nephridi and the oviduct. It is rarely ciliated, and always opens dorsally. Both intestine and cloaca are absent in the Asplanchnidae. The mouth, oesophagus, and mastax originate from the stomodaeum; the cloaca from the proctodaeum; the stomach, its glands, and the intestine from the archenteron. The nephridia or excretory organs consist of a right and left tube, much convoluted in places, especially anteriorly.

These tubes have nucleated walls and are probably intracellular. They are connected by a transverse canal close behind the ganglion in three or four Rotifers, Lacinularia soeialis, Floscularia (sp. ?), Apsilus lentiformis, Hydatina senta. They carry laterally a variable number of ciliated organs, each of which consists of a pyriform canalicule, lodging at its free broad end a flame-cell. The canalicule is closed (Plate), or has a lateral aperture (Eckstein). The two nephridia with a few exceptions end in a contractile vesicle which opens into the cloaca. Except in the Philodinadae the ovarium consists of a germarium and vitellarium (Plate), which are inclosed in a membrane. This membrane is continued into an oviduct which opens into the cloaca. Seison and the Philodinadae have a double, other Rotifera a single, ovarium.

The male resembles the female in structure in Seison. That of Apodoides stygins does so at first, but with the evolution of the generative organs the alimentary canal becomes reduced to a cellular cord. The male Euchlanis also resembles the female but with the same difference. In all other instances there is a dimorphism. The male is small, with the body pointed posteriorly, a simplified trochal disc, and a soft cuticle. The alimentary tract is represented by an irregular cellular cord. The ganglion is unchanged, but the tactile antenna is never stalked. The contractile vesicle of the nephridia may be absent even when present in the female, and then the two vessels have been observed opening on the penis in Hydatina senta: they may unite with the vas deferens. The testis is single, variable in shape; its duct ciliated, and either ending in an invaginable dorsally placed penis, e. g. Hydatina, Brachionus, etc, or in the posterior pointed termination of the body, e. g. Conochilus. In coitus the penis is inserted at any spot into the coelome, in which the spermatozoa move about freely for a time but after a time perish.

The female is therefore parthenogenetic (Plate). The male has not been seen as yet in the Philodinadae.

The ova are of three kinds, small male ova, thin-shelled summer ova, and thick-shelled winter or, better, resting ova; and a given female lays only one kind (Plate). The ova are sometimes laid and then either float loose or are attached to water-plants or to the mother. The Philodinadae are viviparous, as are some other forms. The ovum then developes in the oviduct or in the coelome, as in Rotifer vulgaris (Zacharias). How the young Rotifer escapes from the parent in this latter case is unknown. There appears to be an epibolic gastrula.

Rotifera inhabit both fresh and salt water and damp moss. Some are parasitic, either externally, as Seison on Nebalia, or internally, as species of Albertia in the coelome of Ltimbricus and the intestine of Slugs (Lima-cidae). They are said to resist the effect of drying. The Philodinadae certainly can do so, as they surround themselves with a gelatinous layer. The affinities of the class are doubtful, but certain resemblances to the Chaetopodan Trochosphere are to be traced.

Hudson and Gosse, 'The Rotifera,' London, 1886.

Die Rotatorien der Umgegend von Giessen, Eckstein, Z. W. Z. xxxix. 1883. Beitrdge zur Naturgeschichte der Rotatorien, Plate, J. Z. xix. 1886 (with literature quoted). An atte?npt to re-classify the Rotifers, Hudson, Q. J. M. xxiv. 18841.

Pedalion, Hudson, Q. J. M. xii. 1872. Trochosphaera, Semper, Z. W. Z. xxii. 1872. Rotifer vulgaris, Zacharias, Z. W. Z. xli. 1884; A. N. H. (5), xv. 1885. Asplanchna, Hudson, Journ. R. Micr. Soc. (2), iii. 1883. Melicertidae, Joliet, A. Z. Expt. (2), i. 1883.

Chitinous parts ofmastax, Gosse, Ph. Tr. 146, 1856.

Nephridia, Plate, op. cit. pp. 56-7, 76-7, 98-100.

Power of resisting drought, Gavarret, A. Sc. N. (4), xi. 1859; Davis, Slack, and Hudson, Monthly Microscopical Journal, xi. 1873; and winter habits, Cubitt, op. id. v. 1871.

Egg formation and development, Tessin, Z. W. Z. xliv. (1 and 2), 1886. . .