Hudson and Gosse's Monograph (1886-89) contains a history of the class to which, as to the whole book, we are deeply indebted; and a full systematic account of all published species.[250] C. Rousselet has introduced a method[251] of preparation of Rotifers in microscopic slides which enables workers to preserve the types they figure and describe for future identification and comparison. Gunson Thorpe has collected and studied Rotifera in China and {199}Australia. It would be unfair not to record here the invaluable services of the late Thomas Bolton, and his son of the same name, both of Birmingham, and of J. Hood of Dundee, who have found and widely distributed living specimens of new, rare, and interesting species. [Illustration: 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 {200}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.