(1117). The class of animals that next presents itself for our consideration was, until very recently, confounded with the chaotic assemblage of minute creatures to which the name of Infusorial Animalcules was indiscriminately applied; but the information at present in our possession concerning their internal structure and general economy, while it exhibits in a striking manner the assiduity of modern observers and the perfection of our means of exploring microscopic subjects, enables us satisfactorily to define the limits of this interesting group of beings, and assign to them the elevated rank in the scale of zoological classification to which, from their superior organization, they are entitled.

(1118). The character whence the class obtains its name is derived from the peculiar organs placed upon the anterior part of the body, which are subservient to locomotion, and assist in the prehension of food. These consist of circlets of cilia variously disposed in the neighbourhood of the mouth, and having,when in action, the appearance of wheels spinning round with great rapidity, so as to produce strong currents in the surrounding water.

(1119). The annexed engraving of the StepTiano-ceros Eichhornii1 (fig. 224) exhibits an animal that would seem to be one of the connecting links by which the transition from the Articulata to the Mollusca is accomplished: the transparent cell, and ciliated tentacula around the mouth, would indicate this creature to be a Bryozoon, to be described hereafter; but the tentacula are visibly stunted and thickened at their base, thus approximating in character to the cilia-bearing lobes of a Rotifer, while the internal organs (the pharynx, gizzard, and stomach) conform exactly to the type of structure common to the Rotifera.

Stephanocero8 Eichhornii (after Ehrenberg): a, pharynx; b, Btomachal cavity.

Fig. 224. Stephanocero8 Eichhornii (after Ehrenberg): a, pharynx; b, Btomachal cavity; c, ova contained in the ovary; d, retractile muscle; e, gizzard, containing the masticatory apparatus; f, rotatory organs, resembling those of a Bryozoon.

* Rota, a wheel; fero, I bear. 1 Ehrenberg.

(1120). The body of the Wheel-Animalcules is enclosed in a delicate transparent envelope of considerable consistency, often terminating at the upper extremity in wavy indentations and tooth-like processes, as in Brachionus urceolaris (fig. 225, c c.) This harder integument is so constructed as to allow the animals to move at large in the element they inhabit. Continuous with the free margin of the shell is a delicate membrane connecting it with the bases of the cilia-bearing lobes around the mouth, so as to allow those organs, when not in use, to be retracted within the cell.

(1121). To the posterior extremity of the body is generally appended a pair of forceps, composed of two moveable pieces (figs. 225 & 230), used as anchors, or instruments of prehension; and by means of these the little creatures fix themselves to the Confervae or aquatic plants amongst which they are usually found.

In Brachionus urceolaris the prehensile forceps (fig. 225, p.) is attached to the extremity of a long flexible tail (o), wherein the muscular fibres destined for its motions are distinctly visible.

(1122). The cilia, whose action produces the appearance of wheels turning upon the anterior part of the body; are variously disposed; and from their arrangement Ehrenberg * has derived the characters whereon he bases the division of the class into orders. The peculiar movements excited by the vibration of these organs was long a puzzle to the earlier microscopic observers, who, imagining them to be really wheels turning round with great velocity, were utterly unable to conceive what could be the nature of the connexion between such appendages and the body of the animal. The apparent rotation has, however, been long proved to be an optical delusion, and to be produced by the progressive undulations of the cilia placed in the neighbourhood of the mouth.

Brachionus urceolaris (after Ehrenberg.)

Fig. 225. Brachionus urceolaris (after Ehrenberg): a, b, c, rotatory apparatus and marginal teeth of the shell; d, "calcar," or tubular prolongation of the shell, communicating with the visceral cavity; e, oculi-form spot; f, gizzard with its enclosed masticatory apparatus; g, stomachal cavity; h h, caecal appendages to the stomach; k, common outlet; l, lateral canals, to which the vibratory organs are attached m, contractile vesicle; n, ovaries; o, flexible tail, in which the muscular bands are distinguishable; p, terminal forceps.

* Abhandlungen der Koniglichen Akademie der Wissenschaften zu Berlin for 1833.

(1123). With respect to the agents employed in producing the ciliary movement in the Rotifera, we are as much in ignorance as we are concerning the cause of the same phenomenon in the Polygastrica. Ehren-berg describes the cilia as arising from a series of lobes, as represented in Notommata clavulata (fig. 230, a); these he regards as being muscular, and capable of producing by their contractions the rapid vibrations of the fibrillae attached to them. "We confess, however, that such lobes, even were their existence constant, seem very clumsy instruments for effecting the purpose assigned to them; and it is not easy to conceive how the rapid and consecutive undulations, to which the appearance of rotation is due, can be produced by organs of this description.

(1124). The observations of Dr. Arthur Farre* concerning the ciliary movements appear best calculated to throw light upon the nature of the action of these wonderful appendages, and to explain the cause of the apparent rotatory motion of the so-called wheels of the Rotifera. The very accurate observer alluded to remarks that, under high powers, the cilia have the appearance of moving in waves, in the production of each of which from a dozen to twenty cilia are concerned, the highest point of each wave being formed by a cilium extended to its full length, and the lowest point between every two waves by one folded down completely upon itself, the intervening space being completed by others in every degree of extension, so as to present something of the outline of a cone. As the persistence of each cilium in any one of these positions is of the shortest possible duration, and each takes up in regular succession the action of the adjoining one, that cilium which, by being completely folded up, formed the lowest point between any two waves, in its turn by its complete extension forms the highest point of a wave; and thus, while the cilia are alternately bending and unbending themselves, each in regular succession after the other, the waves only travel onward, whilst the cilia never change their position in this direction, having, in fact, no lateral motion.