290 Dr. F. Leydig on Hydatina senta. 



border consisting of fine long hairs, which runs uninterruptedly 

 round the rotatory organ and belongs to its outer edge. The 

 hairs strike outwards. Perceptibly different from these, are some 

 strong or bristle-like cilia, which stand next to them on the inner 

 lip of the rotatory organ, and also form a continuous series ; 

 similar bristles also fringe the true margin of the buccal cavity, 

 which lies further inwards ; and even between the two series of 

 bristles above indicated, cilia, of equal strength and irregularly 

 distributed, are seen working; lastly, the two papilliform pro- 

 cesses previously mentioned also bear a tuft of about six bristles. 

 All the thick, bristle-like cilia strike like hooks inwards. 



The ceaseless movements of the animal, and the constant in- 

 version and eversion of the cephalic extremity, add not a little 

 to the difficulty of observing it ; and in order to convince one- 

 self of the true form of the rotatory organ, it is advisable to kill 

 the animal slowly, and without its inverting itself. For this 

 purpose I made use of an extremely weak solution of bichromate 

 of potash, in which they continued to swim about for hours, and 

 remained extended after death. That a covering glass is not to 

 be used in examining them, is a matter of course. 



The external integument (PI. VIII. fig. 2), as has been shown 

 to be the case in other Rotatoria, consists of two different layers, 

 namely of the external structureless cuticula, and the "granular 

 layer" situated beneath this. I should not have considered this 

 worth mentioning, had not Cohn said that the granular layer 

 described by me is scarcely to be distinguished as such in ma- 

 ture individuals of Hydatina. Both in fresh objects and in 

 animals which have been acted upon by acetic acid, the separate 

 nuclei of the soft cutaneous layer in question may be seen as 

 clearly as can be desired, and in many individuals single fatty 

 points lie in this cutaneous stratum. 



With regard to the muscles, it may be stated, that besides the 

 longitudinal and annular muscles, of which the former are the 

 broadest, and also exhibit a differentiation into a clear cortical, 

 and granular axial substance (PI. VIII. figs. 1 & 2, b), there are 

 also branched muscles, and that not only in the head, where they 

 are especially striking, but also in every segment of the body. 

 In the broad, longitudinal muscles (figs. 1 & 2) the granules of 

 the axial substance are sometimes observed to be very regularly 

 arranged, so as to remind one of a delicately-marked transverse 

 striation. Cohn's statement, that the substance of the muscles 

 sometimes " appeared frothy from vacuoles," can only refer to 

 altered or dead muscles. 



Of the brain and the nerves radiating therefrom, I thought 

 I might also give a new figure (PI. VIII. fig. 2, c) : this 

 organ, when seen from above, is nearly quadrangular; when 



