414 



ROTIFERA. 



constitute the rudiments of a respiratory 

 system. Whatever may be the true function 

 of these parts, it cannot be supposed that the 

 oxygenation of the nutritive fluid, takes place 

 entirely in these organs. Not only is the 

 alimentary canal supplied with vibratile cilia, 

 but its peritoneal surface, and the cavity in 

 which it is placed, so that on these surfaces 

 the respiratory changes might take place quite 

 independent of these organs. Dujardin refers 

 the office of certain contractile vesicles (fig' 

 297. b ; fig. 288. a.), regarded by Ehrenberg 

 as connected with reproduction, to the respira- 

 tory function. 



Nervous system and organs of the senses. 

 The undoubted existence of a muscular sys- 

 tem, and in some cases exhibiting undoubted 

 indications of the striae 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 (Jig. 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, r). 

 Masses of the same kind are visible in the Ste- 

 phanoceros Eichornii (fig. 292. b t 6). 



A more complicated arrangement of gan- 

 glia and nerves has been described by Ehren- 

 berg in Notommata clavulata. Whilst some 

 naturalists have admitted the correctness of 

 Ehrenberg's conclusions with regard to these 

 organs, others have thrown doubts on his 

 observations altogether. 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 aUo in the Poly- 

 gastria, are very common in the Rotifera. 

 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 eves, 



that Ehrenberg has constructed his genera of 

 Rotifera. 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 time, the 

 resemblance between these eye-specks and 

 those of the Medusae and the Mollusca, 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 system. Although all ob- 

 servers are agreed that the Rotifera 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 ; 

 fig.289. d, d ; fig. 296. c ; fig. 303. i ; fig. 292. ff). 

 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 vulgnris. 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 Rotifera. 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 as 

 to burst the shell. 



According to Ehrenberg, the male organs 

 consist, first, of a series of tubular prolonga- 

 tions, some of which we have seen are covered 

 with the tremulous gills, and, second, of ve- 

 sicles which are capable of contracting. In 

 some cases the tubes are present without the 

 vesicle ; in other cases, as in Ptygnra meli- 

 certa (jig. 288. a), the vesicles a reseen but not 

 the tubes. Those who regard these as a male 



