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.
(1140). The nervous system of Notommata clavulata, as described by this indefatigable observer, is represented in fig. 230. It would seem to consist of several minute nodules (fig. 230, i i), exhibiting a somewhat symmetrical arrangement, and disposed apparently in pairs; some of these nodules, which are about ten in number, communicate with each other by delicate filaments, whilst others seem to be quite insulated from the rest.
(1141). Every one who is acquainted with the difficulty of conducting microscopical observations, especially with the high powers needful in detecting structures so minute as the nerves of the Rotifera, will be exceedingly cautious in admitting the complete establishment of facts involving important physiological principles; and we cannot help thinking that Ehrenberg has been misled by some appearances which it is impossible for the most correct observer always to guard against, in assigning to the Rotifera an arrangement of the nervous system so totally different from what is met with in any other class of animals as that represented in his figure, from which our engraving has been accurately copied.
(1142). All our ideas of the physiology of the nerves would lead us to suspect some error. The uses of ganglia, as far as we know at present, are either to associate nerves derived from different sources, or to serve as centres for perception, or else they are for the concentration of nervous energy. The position of the ganglia depicted in the figure as being in relation with the nervous threads would scarcely seem to be consistent with either of the above offices; and therefore we cannot but regard the observations that have been hitherto recorded concerning the nervous system of the Rotifera as far from being complete.
(1143). Professor Williamson observes that these small organs, which are so common amongst the Rotifera, and which Ehrenberg regards as nervous ganglia, are abundant in the Melieerta, but they afford no countenance to the hypothesis of the great Prussian Professor. They appear to be nothing more than small cells or vesicles, formed of granular viscid protoplasm, very similar to those into which the yelk of the egg becomes divided. Sometimes they float freely in the fluid which distends the integument and bathes the viscera; at others, thin ductile threads pass from one vesicle to another, as represented in fig. 229, h, where these objects are delineated as they appeared in one individual, in the clear space immediately below the viscera. They differ as widely as possible in their size, number, and proportion. So far from being nervous vesicles, they appear rather to be cells modified into a rudimentary form of areolar tissue. That they are hollow vesicles or cells, very viscous, readily cohering, and, owing to this coherence, easily drawn out by the movements of the various organs to which they are attached, are facts capable of easy demonstration.
(1144). Leydig conceives the nervous system of Lacinularia to consist of, first, a ganglion situated behind the pharynx, composed of four bipolar cells with their processes; secondly, of a ganglion at the beginning of the caudal prolongation, composed of four larger ganglionic cells and their processes. The last-mentioned cells are described by Professor Huxley as vacuolar thickenings - finding no difference whatever between them and the thickenings in the disk, which Leydig himself allows to be mere thickenings.
(1145). Professor Huxley's own view upon the subject is as follows*: - On the oral side of the neck of the animal, or, rather, upon the under surface of the trochal disk, just where it joins the neck, and therefore behind and below the mouth, there is a small hemispherical cavity, which seems to have a thickened wall and is richly ciliated within. Below this sac, but in contact with its upper edge, is a bilobed homogeneous mass, which Professor Huxley believes to be the true nervous centre.
(1146). On the nuchal region of many species of Rotifers are two remarkable organs (fig. 227,1, d d), which, from their structure, appear to perform the office of tentacula, although various uses have been assigned to them by different observers. Ehrenberg supposed them to be connected with the respiratory functions, while Dujardin compares them, with much greater probability, to the antennae and palpi of the Entomostracous Crustaceans. In Melicerta ringens1, these organs, when fully protruded, are seen to be terminated by a brush of fine divergent setae (fig. 227, 2, a), implanted on the convex side of a small deltoid body (b); from the flat side of this latter appendage there proceeds along the interior of the tube, towards the body of the animal, a delicate muscular band (c), which by its contractions draws the deltoid body backwards, thus inverting the extremity of the tube, and forming a double sheath protecting the setae. The whole apparatus, observes Professor Williamson, is very similar to that seen in the tentacles of the Snail, and appears to constitute rather a tactile than a respiratory organ. This is rendered the more probable by the fact that, when the animal first emerges from its tessellated case, the extremities of these two tentacles are the first parts to make their appearance, the two curved hooks, named by Schaffer the lips (fig. 227,1, b), being the next. The setae are usually half drawn into the inverted tentacle, but they project sufficiently forward to constitute delicate organs of touch, supposing the deltoid body, into which they are implanted, to be endowed with sensibility. The animal cautiously protrudes these tentacles before it ventures to unfold its rotatory organs; but it does not direct them from side to side, as an insect does its antennae.
Fig. 229. Lower half of the body of Melicerta ringens, highly magnified: a, lower stomach; b, c, lower portion of ovary and oviduct; d, intestine; e,filamentary spermatic tube (?); f, g g, retractor muscles; h, corpuscles swimming in the peri-intestinal fluid, regarded by Ehrenberg as nervous ganglia.