﻿340 CONTRIBUTIONS TO THE NATURAL HISTORY 



and parallel to each other, at its lower margin diverging somewhat to reach the summit 

 of the disk, but keeping nearly parallel between the marginal spring and its medial arch. 

 From the summit the tubes also bend outwards and downwards, and terminate in blind 

 sacs below the middle of the flat disk, so that, seen in profile, as in Plate III. Fig. 2, and 

 Plate IV. Fig. 2, they appear like a tube terminating in a blind sac in the shape of a 

 hook ; but seen from the side of the animal, or by the broad surface of the disk, the 

 curved termination of the tubes covers the upper portion of their ascending branches, 

 as is seen in Fig. 5 of Plate III. 



The action of the filling of this tube is, therefore, to project the whole apparatus into 

 the tentacular cavity, and to stretch its upper hook outwards ; the impulse to a retro- 

 grade movement of the fluid is probably derived from the elasticity of the spring en- 

 circling the flat disk, and especially from the pressure of its curved middle branch, aided 

 perhaps by the action of a minute muscle, which descends obliquely from its lower ex- 

 tremity, and is attached to the inner wall. The figures of Plate IV. represent this 

 apparatus seen in various positions, in order to make all its details as clear as possible ; 

 and in order fully to appreciate them, its position in the main cavity of the body should 

 be contrasted with other figures illustrating the general arrangement of the circulatory 

 tubes, as seen in Plate III., and especially the vertical tubes (Fig. 2 and 5) rising into 

 the tentacular cavity. The base of this apparatus, being attached by its flat side to the 

 inner wall of the cavity, appears in profile, in a front view of the animal, so that the flat 

 disk is represented by a narrow margin, as in Fig. 2 of Plate IV., when its whole height 

 is apparent. Seen from the sides of the animal, its width becomes distinct, and the 

 elastic springs encircling its margin, and rising from the upper summit along the middle 

 to form the external hook, are seen in face, as in Fig. 5 of Plate IV. Seen in half 

 profile, or in a three-quarter view, both margins and hook become distinct, as in Fig. 1 

 of Plate IV. ; and the tentacle which arises from the hook can be traced from its origin 

 through the upper part of its course. In these three figures the whole height of the 

 apparatus is equally apparent ; but in Fig. 3 and 4 the cavity and apparatus are fore- 

 shortened, being seen from above in Fig. 3, and from below in Fig. 4. In Fig. 3 the 

 curves of the springs in the upper margin are plainly visible, and the two tubes ascend- 

 ing along the inner wall appear like two holes. In Fig. 4 the origin of the tentacle is 

 more particularly shown. In order to form a correct idea of the relations of the tentacle 

 proper with the flat disk from which it arises, it is necessary to keep distinctly in view 

 the arrangement of the springs encircling the disk. Whether these springs are a mere 

 swollen margin of a membranous coat, or a fold of the inner wall of the cavity, or an organ 

 ot a peculiar tissue, I have not been able to ascertain. However, so much is certain, 



