Nervous system and organs of the senses. — The undoubted existence of a muscular sys tem, and in some cases exhibiting undoubted indications of the strim seen in the voluntary muscles of the higher animals, would prepare us for the existence of a nervous system in the Rotifera. Such a system has been pointed out by Ehrenberg, and indicated in several species by ganglia and nervous cords. In Hydatina senta (fig. 293.) there is a large mass occupying the place of the supra-ceso phageal ganglia in other families, and to which Ehrenberg has given the name brain. From this mass there proceed several cords, which are presumed to be nervous. There are also other ganglia in various parts of the same animal (fig. 293. c). In the expanded rotatory organ of' the Megalotrocha fiavicans, we also see nervous ganglia, from which cords diverge in several directions (fig. 291. a, a). Masses of the same kind are visible in the Ste phanoceros Eichornii ( fig. 292. b,b).
A more complicated arrangement of gan glia and nerves has been described by Ehren berg in Notommata elavulata. Whilst some naturalists have admitted the correctness of Ehrenberg's conclusions with regard to these organs, others have thrown doubts on his observations alto7ether. It is not improbable that, with regard to some of the parts he has figured as nervous cords, that they may be more correctly referred to other systems. But, a priori, it was not improbable that a ner vous system should exist in these animals ; and it is more probable that the organs in some parts, called nerves by Ehrenberg, should be so, than that they should be anything else. Under these circumstances, we are disposed to regard the conclusion, that these knots and cords perform the functions of nerves, as the most philosophical. An objection has been urged, against regarding the white masses as nervous ganglia, that ganglia are only seen under certain circumstances, such as the as sociation of nerves, the concentration of nervous energy, or acting as centres of per ception. Ehrenberg has pointed out the frequent co-existence of what he calls ganglia with the red spots, which he regards as eyes. These spots, which are seen also in the Poly gastria, are very common in the Rotifers. Sometimes there is but one, at other times two, and not unfrequently three or more. It Is upon the characters afforded by the pre sence or absence, and situation of these eyes, that Ehrenberg has constructed his genera of Rotifers. These eyes have been attacked from many quarters. Morren has seen their red colour distributed over the whole body in various Polygastria. Dujardin objects that
they are present in some species and absent in others ; that they enlarge and decrease, or disappear altogether with age. Rymer Jones says, that they possess no organisation that would lead us unhesitatingly to designate them organs of vision. At the same tinie, the resemblance between these eye-specks and those of the Medusm and the Molluscs, which are admitted to be organs of vision, would still, we think, give the balance of probabili ties in favour of regarding these red spots as true rudimentary eyes. If then we may re gard these red spots as eyes, the masses near them, on the same grounds, may be looked upon as associated nervous ganglia.
Reproductive systenz. — Although all ob servers are agreed that the Rotifers are truly hermaphrodite, they have not agreed upon the functions of many parts supposed to belong to the reproductive system. The female or gans consist of an ovigerous sac or ovary, which exists very generally in the family (fig. 294. c ; fg.289. d, d ; fig. 296. c ; fig. 303. i ; fig. 29:?. fp. It is sometimes a simple sac, at others provided with two projecting processes, terminating by a narrow opening in the common cloaca. In some cases the ovary produces but one ovum ; in others there are several ova. Sometimes the young burst the shell before they leave their parent, as in the Rotifer vulgaris. In the Hy datina senta and others, the eggs are deposited some hours before the young are hatched.
The development of the ova can be seen with great distinctness through the transpa rent shells of many of the Rotifers. Ehren berg has detailed this process as it occurs in the Hydatina senta. At first the ova are seen as little vesicles filled with a glairy fluid, pro bably albumen. In the course of a few hours a dark speck is seen in the vesicle, which may be called the yolk (fig. 303. n). In this state the ovum becomes fecundated, and is extruded from the cloaca. Three hours after extru sion the germinal vesicle, which had before been pushed to one side by the yolk,disappears, and the yolk occupies the whole of the egg. Six hours after extrusion, a dark spot appears upon the yolk, which can be discerned to be the head of the young animal with its masti catory apparatus. In eleven hours the rota tory organs are developed, and their cilia beginning to work, the young creature moves about in its shell. At the end of twelve hours The movements become so powerful US to burst the shell.