Tegumentary system.— The Rotifers are all covered with a resisting tegument, more or less flexible, and which is the last putt of the body to decompose. The composition of this tunic, although possessing various degrees of density, appears to be entirely organic ; and the absence of siliceous or calcareous matter will account for these animals being never seen in a fossilised state. The investing membrane is open in front, to allow of the contact of the fleshy interior with the water in which the creatures live. There is, also, an anal orifice. In those species in which this membrane is not hardened, so as to form a shield, it is capable of being folded by the action of the muscles, and possesses a number of False articulations. The anterior part, to which are attached the vibratile cilim consti tuting the rotatory organ, is capable of being retracted into, or thrust out from, the rest of the body. All the parts of the body retract within the skin into a kind of globule, when the animal is removed from the water. The tegument has attached to it various _organs, as the claws in Emydium, the cirrhi, or fins, of Polyarthra, and the elongated setm of Tri arthra (fig. 297.), the teeth in the dense tegu ment, or lorica, of Brachioncea (fig. 296. g, g). The tail, or foot, must be regarded as an elongation of the tegument. It varies much in size and length. Sometimes it consists of a single styliform seta, as in Triarthra seta (fig. 297.) ; in the genera Monura and Monosty/a it is styliform, but is also articu lated. In some of the species of the genus Anurma there is no tail at all. In most in stances the tail is forked, as in Hydatina, Eueldanis, Philodina, 1?otifer, Brachionus, (figs. 293-296.). Sometimes the tail is vided from the point of its origin with the tegu ment of the body, as in Notommata longiscta (fig.298.) and in Hydatina senta (fig.293.). ..More frequently a portion intervenes between the body and the tehninal processes. This is soft and movable in every part in Brachio nus pala (fig. 296.); forms a series of sheaths in others, as Dinocharis paupera (fig. 302.), and many of the Philodinvea. The tail is opening and closing the processes on each side, and apparently holding on to any object by their means. Many genera, as Conochilus, Floscularia, Stephanoccros, and others, have no fork, but remain fixed by their tails. Even in species which have forked tails, as in the Philodinma, the creatures seem to have the power of fixing themselves independently of their fork. It would thus seem not impro bable that the t.il in these cases acts as a kind of sucker.
often furnished with supplementary setm, or bristles, and in Pterodina it is terminated with a row of vibratile cilia. The tail is used as a rudder, an oar, and a hold-fast. When styli form, it seems used as a rudder, although in some cases apparently employed to propel the animal. When furcated it has the power of Projecting from the upper part of the ex tern tegument, in many species, is a little process, which Ehrenberg calls a spur, or siphon (fig. 299. d), and which he thinks is connected with the function of respiration, and therefore calls it a respiratory tube. It corresponds with an orifice in some species (fig.293.g), which Ehrenberg calls the respira tory orifice. He has also hinted that they may be connected with the reproductive func tion. Two of these organs are seen in some of the Arotonnnata and other genera, and they are sbmetimes covered with cilia. Dnjardin thinks that they resemble more closely the palpi and antennw of the Entomostraca.
The rotatory organs, or wheels, must be also regarded as a portion of the tegumentary systetn. They are fleshy retractile lobes, covered with vibratile cilia, capable of being contracted or expanded at the will of the animal. The movement of the cilia vvhen the
lobes are expanded gives the appearance of a wheel moving upon its axis, an appearance which was a source of much wonder to the earlier observers of these creatures. In addi tion to the vibratile cilia, there are frequently found, on the rotatory lobes, setw, or bristles, which have not the power of moving. This is the case in Floscularia, if, indeed, the organs called rotatory in that genus are truly homologous with the rotatory organs in other species. The true homologue of the rotatory apparatus in Floscularia appears to us to be seated within the external ciliated lobes, where an evidently active motion is constantly going on. The form of the lorica varies greatly ; in some species it is flat and depressed, as in Pterodina and Monostyla ; in others it is pris matic. as in Mastigocerca, or gaping, as in Euchlanis (fig. 294.). Some species, as Ste phanoceros (flg,. 292.), Floscularia, Melicerta, and others, have a soft skin, very contractile, which secretes externally a case, and which Ehrenberg calls a lorica; but this is essen tially a different organ from the lorica. Where this case occurs, it seems to stand in the same relation to the animal as the Polypidon of the zoophytes. The animals which form these cases are also fixed, and retract their bodies within their case in the same manner as the Polypiferw. The Floscularia may be compared to the Hydroid Polyps, while Ste phanoceros, with its ciliated tentacula-like pro cesses, vvould appear to have a relation with the Ascidoid polyps.
Motory system. — As the movements of the Rotifers are rapid and various, so we find their muscular system complicated. The principal organs of locomotion are the rota tory organs, by vvhich alone the great mass of the Rotifers appear to move. The move ments effected by these organs are performed principally by the agency of the vibratile cilia. Although no tissue has yet been dis covered in the cilia of the Rotifera and Poly gastria, Professor E. Forbes has observed fibrous tissue in the cilia of a species of Me dusm, and there can be little doubt that the movements of the cilia, like those of organs to which muscles are attached, are of two kinds, one of which is under the control of' the will, and the other not. In the Rotifera, the vibratile cilia of the rotatory organ appear to be under the control of the will of the animal. The extension and contraction of the rotatory apparatus is under the influence of longitudinal muscular bands, which are very evident in most of the species (fig. 293. e,e,e,e ; fig. 296.; fig. 291. b). Not only is it evident, from the action of' these muscles, that they are under the con trol of the will of the animal, but Ehrenberg has described soine of these muscles as pos sessing the striated character of the voluntary muscles of animals higher in the scale of or ganisation. Euchlanis triquetra (fig. 294. b) and the species of Eosphora are those in which striated muscles have been observed. This fact is interesting in connection with Mr. Busk's observation of the exi,,tence of muscular striae in Anguinaria spathzdata, a form of ciliobrachiate polyps. It affords a proof not only of the relation of these two families, but also of both, to the articulate tribes rather than to the Mollusca. Mr. Busk, after the most patient research, has not been able to discover the presence of strim in the muscular system of the Mollusca. Not only have the longitudinal bands been regarded as active agents in the movement of the rotatory organ, but also certain transverse bands. These bands ( fig. 293. f, f,f; f ; fig. 299. g, g, g) have been described by Ehrenberg as trans verse vessels. There seems to be little proof that such is their office.