This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
Fig. 227. Melicerta ringens (after Prof. Williamson): a a, ciliated lobes, constituting the rotatory apparatus; b, hooks, called by Schaffer "the lips;" c, rotatory flap, or "pellet-cup" (Gosse); d d, tentacular organs; e, gizzard, containing the gastric teeth; f, upper stomachal cavity; g, inferior stomachal cavity; h, anal outlet; i, protuberance occasioned by the act of defecation; k, ovary; I, dilated oviduct; m, filamentary spermatic tube (?); n n, muscles of caudal appendage; o, prehensile organ; p p, corpuscles floating in the perivisceral fluid; q q, ova nearly ready for expulsion.
* Quarterly Journal of Microscopical Science, vol. i. p. 1.
(1135). Prom the upper extremities of the two crushers there project upwards and backwards two slender prolongations (fig. 228, d), which are united by a sort of double hinge near this apex, where they not only play upon each other, but also on a small central fixed point (fig. 228, e) lodged in a little conglobate cellular mass. Ehrenberg only describes three transverse bars on each side, which he regards as teeth; it is obvious that he has only noticed the three upper and larger pairs. It is equally evident that these transverse teeth, as he terms them, do not move upon the strong longitudinal plates, as he imagines, but are firmly united with them. Muscles are either attached to the divergent peripheral processes, or to the cellular mass in which these processes are imbedded, causing the entire apparatus to separate along the mesial line by means of the hinge-joint (fig. 228, e), the so-called teeth merely transmitting the motor force to the two longitudinal plates. These latter appendages are thus made to play upon each other with great power, and to act as efficient crushers, bruising the food before it passes into the stomach, as is the case with the gastric teeth of the Crustacea.
(1136). From the above remarks it will be seen that, though in its construction the dental apparatus is more complex than is represented by Ehrenberg, in its mode of working it is less so. The conglobate organ in which this apparatus is imbedded is transparent, and composed of numerous large cells, each of which contains a beautiful nucleus with its nucleolus. The cells are only seen when the organ is ruptured between two plates of glass, when they readily separate from one another; but the nuclei, with their contained nucleoli, are distinctly visible in the living animal. Delicate muscular threads most probably penetrate this organ to reach the dental apparatus; but Professor Williamson has hitherto been unable to detect their presence satisfactorily.
Fig. 228. Gastric dental apparatus of Melicerta ringens: a a, crushing plates; b b, c c, lateral framework; d, handle-like processes; e, central fixed point. (After Prof. Williamson).
(1137). After passing the dental organs, the food enters an elongated stomach (fig. 227, 1, f) with very thick pulpy parietes. In young examples these walls are colourless and transparent, but in more matured specimens they exhibit a bright-olive hue. The whole cavity, as well as the oesophagus leading to it, is lined with cilia that are constantly playing. On rupturing this organ, it is seen to be composed of a thin pellucid external membrane, exhibiting no distinct structure, but within which is a thick layer of large, tinged epithelial cells. These are easily detached from the membrane, when each one is seen to be spherical, containing numerous yellow granules, and very often a nucleus with its nucleolus. The cilia are attached to one side of these cells, the great length of the former constituting the most marked feature of this arrangement; it often, indeed, equals the entire diameter of the cell. Some of the cells exhibit no cilia, others are only furnished with them on one side, while a few appear to be fringed with them throughout their entire circumference. Professor Williamson supposes that in the latter case the cells have projected considerably into the cavity of the stomach.
The yellow granules are absent from those of young animals, showing clearly that it is these contained granules that give the colour to the parietes of the stomach.
(1138). This stomach appears to be chiefly a receptacle for the food. From time to time, especially when the viscus is distended, portions of its contents pass down into a lower stomach (fig. 227, 1, g), which is separated from the upper one by a marked but varying constriction. The second stomach is also lined with cilia even larger than those of the upper viscus; but the parietes are very much thinner and more transparent, the cells being less easily traced. The diameter of the organ is nearly the same in each direction, so that it is almost spherical. The mass of food with which it is usually distended is constantly revolving, the motion being due to ciliary action. This process goes on for some minutes, after which the creature contracts its body and forces the entire contents out of this viscus into a long narrow cloaca that terminates externally by an anal outlet (fig. 227,1, h.) As it does this, it everts a considerable portion of the cloaca, thus almost bringing the cloacal outlet of the stomach to the exterior, and causing at the same time a large transparent protuberance (fig. 227, 1, i) to be developed on the corresponding side of its body. At other times the creature can draw in these appendages, so that scarcely any trace of a cloacal canal is visible.
(1139). Notwithstanding the microscopic size of the Rotifera, and the consequent difficulty of detecting the more minute details of their structure, Ehrenberg thinks he has succeeded in discovering filamentary nerves, and nervous masses, distributed in different parts of their body, - an arrangement which not only would account for the complete association of their voluntary movements, but, from the presence of ganglia, would render these animals capable of possessing some of the local senses; indeed, Ehrenberg imagines he has discovered such to exist in the shape of red specks, to which he gives the name of eyes. The organ alluded to is a minute red spot, indicated in the figures (figs. 225 and 230); nevertheless no organization has been described of such a nature as to entitle us unhesitatingly to designate it an organ of vision, even if it should, as he intimates, invariably be in connexion with a nervous mass, which, from examining his drawing of the arrangement of the nerves, we should have little expected to be the case.