Page:EB1911 - Volume 23.djvu/803

Rh ganglion cells below invertebrate sense-organs. Moreover, the body cavity of the Rotifers is a primitive archicoele; the persistent or accrescent cleft between epiblast and hypoblast, traversed by mesenchymal muscular bands. Thus we regard Rotifers as an independent stem branching off at the outset of the rise from the Platode type to higher Invertebrata. The (q.v.) which in many ways recall Rotifers, appear to be equally independent.

The following classification of Rotifers is our modification of that of Hudson and Gosse, further altered through considerations put forward by C. Wesenberg-Lund, which, however, we do not consider wholly convincing. He notably regards an oblique disk with uniform ciliation as primitive, a view which we cannot adopt.

Classification:—

(A.) Disk usually with well-marked strong trochus, ciliated groove and more delicate cingulua interrupted by an antero-dorsal median gap, usually more or less bilobed.

(i.) Trophi incudate: (ii.) Trophi malleoramal: (iii.) Trophi ramate: (iv.) Trophi uncinate: (B . ) Ploimaeae; disk variable, often circular, sometimes with a lobed trochus bearing membranelles (vibratile styles); trophi complete, malleate, submalleate, virgate, or forcipate; anus subapical; foot usually short, and usually bearing two toes which may be much elongated. (C.) Seisonaceae. Body elongated with a narrow neck above the disk; foot ending in a terminal perforated disk. Trophi virgate exsertile; germary paired; genito-urinary cloaca opening above the neck in the male, subapically in the female. Gut blind (Paraseison), or opening into cloaca (Seison). Males resembling females, common. All known species are parasitic on the Crustacean Nebalia; Seison Claus; Paraseison Plate.

Habitat and Habits.—The Rotifera are all aquatic, the majority dwelling in fresh water with Protozoa and Protophyta, as well as Entomostracous Crustacea. This association with Protophyta accounts for their study by many distinguished botanists, such as W. C. Williamson and F. Cohn. Some are moss-dwellers, inhabiting the surface film of water that bathes these plants: such especially are the Bdelloids, with their exceptional capacity for resisting desiccation. Others—the majority—live among weeds, the tubicolous ones mostly upon them. A few are sapropelic, haunting the looser debris that forms the uppermost layer of the bottom ooze of quiet waters: we may cite the aberrant Floscularian Atrochus. Widely different are the habits of the plankton forms, which float or swim near the surface, and are often provided with long