Fig. 108.—Diagrammatic views of disc of Rotifers. Cingulum represented by a black line, groove shaded; trochus dotted; the black spot represents the mouth. 1, Simple disc of Microcodon; 2, Bdelloid disc of Rotifer or Callidina, the star represents the ciliated proboscis; 3, disc of Hydatina, groove represented by lobes bearing ciliated styles; 4, disc of Melicerta, the star represents the ciliated ventral cup with openings into it from the groove; 5, disc of Conochilus; 6, disc of Stephanoceros, cingulum (?) of setose lobes, trochus horseshoe-shaped, mouth central.
The front of the body constitutes the HEAD, which is scarcely distinct, though usually separated by a slight neck-like constriction. The DISC, which terminates the head, varies greatly in shape and in the arrangement of its parts. Imagine a circular funnel, finely ciliated within, and with the mouth at the bottom, the prominent rim bearing two zones of cilia, the inner or anterior being the coarser, and termed the "trochus" or hoop; the outer finer, and termed the "cingulum" or girdle, while a very finely ciliated groove lies between the two zones. Either or both of these zones may be interrupted on the dorsal or ventral median line, or both; and the funnel-shaped mouth may be shifted—usually ventrally, so that it forms only a dilatation of the ciliated groove. Again, the wreath as a whole may be festooned or lobed; or the lobing may be confined to the area between the cingulum and trochus, as in most Ploima (Figs. 106 and 108, 3). Very frequently on these lobes adjacent cilia are fused together during life, producing "vibratile styles," whose true nature is only revealed after death. In Microcodonidae the structure of the disc (Fig. 108, 1) nearly conforms to the primitive type; but the ciliated groove is absent, and the "trochus" is in two separate half-elliptical bands. In the Flosculariaceae (Fig. 108, 6) the mouth is also central, the disc is funnel-shaped, and the trochus is a horseshoe-shaped ridge, with its ends dorsal and raised into prominent knobs. The margin of the funnel is in Flosculariidae (Fig. 115) usually lobed, and furnished either with exceptionally strong cilia, or else with very long bristles which are usually passive. However, by the retraction of the lobes that bear them they are clasped together like casting-nets to enclose prey brought into the funnel by the action of the trochal cilia. An external ring of cilia in Floscularia mutabilis and F. pelagica serves for swimming. In Apsilidae the margin of the disc bears neither cilia nor bristles, but is either simple and ring-like, or is produced into tentacles (Fig. 112, C). The oral funnel is probably represented in Flosculariaceae by the continuation of the small central mouth into a ciliated tube (Fig. 115, C, tf), open below, and hanging freely down into the crop.
In all other cases the mouth is displaced, and lies in the groove and on its ventral side (except in Conochilus, where it is dorsal, Fig. 108, 5). In the Bdelloida the disc is prolonged into two great lobes like kettle-drums, round the posterior, external, and ventral edges of which run the trochus, cingulum, and ciliated groove (Fig. 108, 2). All three are interrupted behind in the median line; ventrally the groove widens into the oral funnel, the cingulum is continued into a sort of spout-like lower lip (Fig. 109, C, D, l), and the trochus is absent. The body is prolonged dorsally above the lobes into a two-jointed proboscis, ending in a ciliated cup overhung by two dorsal flaps: this we regard as a detached portion of the wreath.
This "Bdelloid" type of wreath occurs also in Scirtopoda (Fig. 117), and in the Ploimal genera Triarthra, Pterodina, and Pompholyx. A simpler wreath of essentially the same type occurs in Asplanchnaceae and Melicertaceae; the disc is not prolonged into drum-shaped lobes, but is thin at the rim, where it bears the triple ciliated zone, interrupted on the dorsal median line and depressed ventrally into the oral funnel. In the Melicertidae, moreover, the disc is widened into a great plate-like extension, often beautifully lobed; and in many of the species a ciliated cup lies ventral to the lips, and is connected with the groove by a short ciliated channel on either side (Figs. 108, 4, and 116). Even the simpler wreath of Asplanchnidae is complicated by stronger lobes on either side bearing vibratile styles.
The most complex discs are found in Ploima, especially in Brachionus, Hydatina, and Synchaeta, since the groove is replaced by a zone of lappets, as above mentioned. In Proales the whole face of the disc is strongly ciliated. The wreath is reduced in the parasitic genera Drilophagus, Albertia, Balatro, and the Seisonaceae; in Adineta and Taphrocampa it is only represented by a general but scanty ciliation of the disc.
Fig. 109.—Callidina symbiotica. (After Zelinka.) A, Ventral view, with the disc half expanded, proboscis extended; B, lateral view, proboscis extended; C, ventral view of anterior segments with expanded disc; D, lateral view of same (proboscis retracted). a, Antenna; bl, bladder (enlargement of rectum); c, ciliated cup of the proboscis; ci, cingulum; cl, cloaca; cp, group of pores, the openings of cement glands; di, disc; g, gizzard; gm, germarium (that of the opposite side seen at a higher level); gr, ciliated groove; k, kidney; l, lip; m, mouth; pr, proboscis; sp, spurs of foot; tr, large cilia of trochus, showing vertical movements; vm, yolk-gland. The body muscles are represented by shaded bands.
The head is very frequently retractile, as a whole, by strong muscles. In Bdelloida the disc proper is retracted when the animal crawls, while the proboscis is exserted (Fig. 109). Ciliated patches occurring outside the region of the disc point to a primitive condition when the whole surface of the body was ciliated, as does the partial ciliation of the foot in certain groups. Synchaeta and many Notommatidae possess a pair of lateral, hollow, ciliated pits on the body, which can be everted to serve as additional swimming organs; these are termed "auricles."
The cuticle varies much in texture. It may be smooth and flexible, dotted or shagreened, or in the Loricata firm and of definite shape, constituting a lorica, which may be more or less distinctly divided up into areas or separated into distinct pieces. In this case it resists decomposition, and several species are only known by this "skeleton." In Ploesoma it is much thickened and looks like a honeycomb. A regular alternation of harder and softer zones effects the annulation of the body in certain genera.