OF THE ROTATORIA. 403 



Entomostraca and Cypns, and that no trace of the entrance or exit of water 

 is perceptible, even when particles of colouiing matter are diffused through 

 the liquid, calculated to indicate the slightest current." 



Since this was written, Perty, Gosse, Williamson, Huxley, and Leydig in 

 particular, have minutely studied the point in dispute, and coincide with the 

 French naturalist as to the non-perforated character of the organ, and its ho- 

 mology with antennae. Mr. Gosse writes — '^ The tubes or spurs on each side 

 of the head (of Melieei^ta) below the chin (XXXVII. 17 c? d) are evidently 

 consimilar with the antennae of Rotifer, &c. There is a slender piston in 

 each, capable of being retracted, and beaiing at its extremity a tuft of very 

 fine, divergent, motionless hairs." Mr. Williamson's account is more detailed ; 

 he calls them '' tentacles," and states that, when fully protruded, they are 

 seen " to be terminated by a brash of fijie divergent setae implanted on the 

 convex side of a small deltoid body {the jpiston, Gosse) (XXXVII. 12) ; 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 (XXXVII. 

 13, 14), which, by its contractions, di^aws the deltoid body backwards, thus 

 inverting the extremity of the tube, and forming a double sheath protecting 

 the setae (XXXVII. 14). This inversion of the tube was, we believe, first 

 noticed by Dutrochet. The whole apparatus is, as suggested by Schaffer, very 

 similar to that seen in the tentacles of the snail, and appears to constitute a 

 tactile rather than a respiratory organ. This is rendered more probable by 

 the fact that, when the animal first emerges from its tessellated case, the ex- 

 tremities of these two tentacles are the fii'st parts that make their appear- 

 ance (XXXVII. 11 del), — the two curved hooks being the next (XXXVII. 

 17 6). The setae are usually half drawn into the inverted tentacle ; but they 

 project sirfficiently foi-ward to constitute delicate organs of touch, supposing 

 the deltoid body, into which they are inserted, to be endowed with sensi- 

 bility. The animal cautiously protrudes these tentacles before it ventures 

 to uiofold its rotary organs, but it does not dii^ect them in an exploratoiy 

 manner from side to side, as an insect does its antennae." 



But there are many strictly homologous processes with a terminal tuft of 

 setae which are tubular and not retractile, or other\\dse neither tubular nor re- 

 tractile, but horn-like in figure, or merely conical. Examples occur in Notom- 

 mata Myrmeleo (XXXVIII. 26 h) and N. SieholdH (XXXVIII. 32 g), and 

 in the shorter conical elevations on the disk of Syncliceta and Polyarthra, and 

 in the horns of the last-named genus. 



A further departure from the highly- developed antennae of some Rotatoria 

 is exemplified in the fossae, pits, or apparent apertures (XXXVIII. 28-30), 

 oftentimes with elevated edges, containing a tuft of biistles, which are met 

 with usually on the necks of the animals. These fossae, as weU as the 

 retractile and non-retractile antennae of all forms, Leydig believes to be in 

 immediate and special relation with nerves which extend to the base of the 

 brush of rigid ciHa. The number of such fossae varies in different species. 

 In^ accordance with his hj^othesis of respiration, Ehrenberg called them 

 " ciliated respiratory openings." In Enteroplea (XL. 2), Hydatina (XL. 1), 

 Biglena, Otoglena, in Euchlanis trlquetra, and in several NotommatcB, an 

 apparent aperture exists on the neck. More than two are seen in Poly- 

 arthra, Notommata Myrmeleo, and in N. SieholdH (XXXAT:II. 29), arranged 

 along the back; and in Asplanchna. BrightwelJii (Gosse), Dalrymple met 

 with two on the back, which he supposed to be and described as lateral 

 apertures, but which, Leydig afiiiTas, have the imbroken cuticle lining them 



Interesting variations are found in No fens, in which Ehrenberg describes 



2d 2 



