534 Transactions of the Society. 



which form the swollen portion, being nearly equal, and the three 

 terminal ones diminishing successively in length. The last seg- 

 ment terminates in a foot or forked tail, formed of two small 

 triangular points, which can be separated or brought together at 

 pleasure. This segmentation becomes less and less distinct with 

 age, in consequence of the distension of the body caused by the 

 eggs, and finally it is transformed, as was said above, into a rounded 

 sac, with two prolongations formed by the head and tail (Fig. 9). 



Seen in profile, the body seems in its anterior part obliquely 

 bevelled, at the expense of the ventral face. The dorsal face is 

 prolonged into a projecting and protractile superior lip (Fig. 3), 

 from the base of which descends on each side a fold of the external 

 tegument, furnished with vibratile cilia at its edge, and uniting 

 beneath with that of the opposite side, so as to circumscribe the 

 opening of what is called the mouth, but which is more correctly 

 the vesfihular orifice, as it gives access to a cavity, at the bottom of 

 which is the true mouth and the rotatory organ. To this cavity 

 I give the name of buccal vestibule (Figs. 2, 6, vb). 



The superior and lateral lips are soft and wonderfully contrac- 

 tile. The former especially is endowed with surprising agility, the 

 movements being evidently tactile. (See Figs. 2-7, 1.) 



Inside the buccal vestibule is the rotatory organ (Figs. 2-7, or), 

 which is the apparatus of locomotion and the organ of touch. It 

 shares the excessive mobility of the surrounding parts. (See Figs. 

 3, 4, and 5.) 



Ehrenberg and Leydig * differ in describing the rotatory 

 apparatus, but it is certain that in N. Wernechii the cilia form, on 

 the surface of the rotatory disk, an isolated bundle completely 

 independent of the other cilia of this region. 



I have never discovered the long threads which Ehrenberg, 

 from Werneck's description, said existed at the sides of the mouth 

 in the adult. 



It is only at a very early stage that the animal uses its rotatory 

 apparatus for progression, as it soon commences its parasitic exist- 

 ence in the tubes of Vaucheria, where it is of course incapable of 

 swimming in the dense plasma. 



In the free stage the young Notommata advance rapidly in the 

 water, either gliding with a uniform movement or '" striding/ " along 

 after the manner of the rotifers ; that is, by resting alternatively on 

 the two opposite extremities of the body. 



The energetic contractions imply a pronounced development of 

 the muscular system which I have not been able to observe with 

 sufficient accuracy to describe here. 



The digestive organs are composed of the buccal cavity, of the 



* See Leydig's " Ueber don Bau und die systematische Stcllung der Eader- 

 thieie," ' Zeitsehr. wisss. Zoo].,' vi. 1855, p. 68. 



