444 ROTIFERA. 



sembling those of fishes ; and, mistaking the movements of the gizzard 

 for the contractions of a heart, conceived these animalcules to be even 

 superior to insects in the organization of their vascular system. Ehren- 

 berg, however, thinks that he has discovered an internal respiratory 

 apparatus of a most extraordinary description. In Notommata centrura 

 he remarked seven vibrating points on one side, and six on the other, 

 attached to two long and undulating viscera, which he elsewhere describes 

 as being the testes of the animal (fig. 225, I) ; the above-mentioned 

 points were never at rest, and appeared to be placed in determinate 

 positions opposite to each other. Accurate observations, he says, have 

 shown each to be a peculiar little organ, provided with a tail resembling 

 that of a note in music, and to be thrown into vibration by three little 

 vesicles, or folds of their inflated extremity ; these organs floated freely 

 in the abdominal cavity by their enlarged portion, while by their tail 

 they were attached to the long tubular organ above referred to. 



(1149.) Ehrenberg's first idea on seeing these organs was that they 

 formed a vascular system, executing movements of pulsation ; but he 

 now considers them as internal branchiae, or organs of respiration, to 

 which the external water is freely admitted in the following manner: 



(1150.) In many species of the Rotifera, we find, projecting from the 

 neck of the animal, a horny tubular organ, called by Ehrenberg the 

 calcar or spur (fig. 225, d) : this he at first considered to be the male 

 organ of sexual excitement ; but he now regards it as a siphon, or a 

 tube of respiration, through which the circumambient water passes freely 

 into the cavity of the body. He thinks, moreover, that the periodical 

 transparency, and the alternate distention and collapse of the animal, 

 seen to occur regularly in almost all the Rotifera, are produced by the 

 introduction of water into the visceral cavity and its subsequent expul- 

 sion therefrom, upon which action the fluctuations observed in the in- 

 terior of the body would therefore depend. The supposition that water 

 is injected in this manner into the body seems to be favoured by other 

 appearances : for when the internal cavity is thus filled, all the viscera 

 appear isolated, so that the boundaries of each can be distinctly seen ; 

 but when the water is discharged, they approximate each other, their 

 limits become confounded, and the external membrane of the body 

 assumes a crumpled appearance. 



(1151.) Upon reviewing the above account of the mode of respiration 

 in the Rotifera, we must say that we consider that the office assigned to 

 the little organs called branchiae is extremely problematical, especially 

 as we have but the most vague intimations concerning the existence of 

 a circulating system at all, much less of such a double circulation carried 

 on in arteries and veins as the presence of such organs would infer. 

 "I presume," says Ehrenberg, "that the branchiae possess a vascular 

 system ; for when the local contractions occur in the bodv of the animal, 

 we see distinctly a certain number of filaments (vessels ?) loose and 



