Dr. F. Leydig on Hydatina senta. 280 



they formed a nearly continuous greyish-white stratum close to 

 the surface of the water. The Enter oplea^ which at first occurred 

 rather sparingly, had then also become so numerous, that per- 

 haps one Enteroplea might be counted to every twenty or thirty 

 Hydatimje, I studied the animalcule closely, and as my results 

 do not agree in all points with Cohn^s, it may not be superfluous 

 to give some details of the structure of Hydatina and Enteroplea, 



It is incorrect when the above-mentioned author says that 

 Hydatina senta " is one of the largest Rotatoria '.'^ it is rather 

 only one of middle size, for many other species, such as Notom- 

 mata myrmeleOy and especially Notommata Sieboldii, exceed it 

 three, four, and five times in size. That naturalist, indeed, well 

 remarks, that the form of the animal in its true outlines can 

 only be recognized when it is swimming about freely in a suffi- 

 cient quantity of water; but neither the figure nor the descrip- 

 tion given by him of the form of Hydatina can be indicated as 

 true ; his figure of the rotatory organ especially departs far more 

 from nature than that given by Ehrenberg, for which reason I 

 consider it necessary to publish a new representation of it. 



As regards the form of the animal, we perceive in individuals 

 which are swimming about quietly, revolving upon their axis, 

 that the body is divided into three principal parts, — a cephalo- 

 thorax, an abdomen, and a tail. If we disregard the abdominal 

 notch, the cephalothorax appears to be distinguished from the 

 abdomen, especially on the dorsal surface, by a tubercular infla- 

 tion, upon which the single setigerous pit {Borstengrube), beneath 

 which nerves terminate, is situated. This tubercle of the cephalo- 

 thorax, however, only makes its appearance distinctly when the 

 stomach is not exceedingly full, or the ovary is not too much 

 developed : when either of these circumstances occurs, the tubercle 

 must of course disappear more or less in consequence of the 

 inflation of the abdomen. The first or anterior third of the 

 cephalothorax is separated as the head by a sharp furrow. The 

 abdomen, which is faintly annulated, ceases with the segment 

 which bears the cloacal orifice ; then follow the segments of the 

 tail, of which even the first is narrower than the last segment of 

 the abdomen. The tail diminishes rapidly, and runs out into a 

 didactyle foot. 



The margin of the head or rotatory organ (PL VIII. fig. 1) is 

 not simple, but distinctly formed of two lips, with a tolerably 

 deep furrow between them. On the dorsal surface this furrow 

 rises into a papilliform process, lying in the median line, corre- 

 sponding to which the inner lip of the rotatory organ also gives 

 rise to a prominence ; the surface of the rotatory organ bounded 

 by the inner lip sinks into a funnel-shaped depression or buccal 

 cavity. As regards the ciliary coat, we may easily distinguish a 



Ann. 5f Maff. N, Hist. Ser. 2. Vol. xx. 19 



