264 CONTRIBUTIONS TO THE NATURAL HISTORY 



The vertical tubes do not form so simple an arch as in the genus Sarsia ; for in Hippocrene 

 the lower surface of the gelatinous disk is not simply arched ; but there is in its centre a 

 more prominent knob, below which the central digestive cavity hangs. This knob, with 

 the central cavity, forms, therefore, a very prominent mass in the centre of the main 

 cavity of the body ; and the chymiferous tubes, which arise from the upper corners of 

 the digestive cavity, have first to ascend along the gelatinous protuberance, and then 

 merely bend over in a narrow arch before they descend along the lateral walls of the main 

 cavity to reach the base of the sensitive bulb. The curve which all these four tubes de- 

 scribe is that of the crosier. (Plate I. Fig. 1.) As the upper portion of the disk 

 scarcely ever changes its form, but slightly contracts and dilates, we perceive no marked 

 change in the curve of these corners, and the changes in the direction of the current of 

 chyme are consequently less frequent than in Sarsia. 



To these immovable central parts of the animal, we must, no doubt, also ascribe the 

 special adaptation of the central digestive cavity, which is precisely the reverse of that of 

 Sarsia. In Sarsia, the extremely elongated form of this cavity allows it to be used as a 

 proboscis, stretching in all directions for the food, and snapping at it with the greatest 

 quickness and precision. Here the same organ is changed into a square, immovable, large 

 cavity, the opening of which is capable of a most extensive dilatation, and surrounded by 

 immovable branched tentacles, the surface of which is covered all over with nettling cells, 

 by the agency of which the prey is retained. The appendages of the border of the 

 mouth, by their peculiar structure and power of dilatation, perform in Hippocrene the 

 same office with reference to securing the prey as the exceedingly active proboscis of 

 Sarsia, which is, at the same time, both a catching apparatus and the digestive cavity. 

 The chymiferous tubes are nearly as apparent upon the disk of the animal as the central 

 digestive cavity, the nourishing fluid which is circulated in them being generally of a light- 

 brown color. The position of the tubes within the main cavity is also characteristic. 

 They are not attached loosely to the inner surface of the gelatinous disk, but follow 

 its more permanent outlines, being kept steady both by the muscular bundles which extend 

 in a circular arrangement round that surface, and by the epithelium which lines it. 

 Their position is properly between this circular or concentric layer of contractile muscular 

 cells, and the gelatinous disk itself, alternating with the main vertical masses of muscular 

 fibres. Yet these tubes are not a mere furrow or canal hollowed out between other tis- 

 sues : they have distinct walls of their own, which are exceedingly thin, structureless, homo- 

 geneous, and transparent, consisting of a hyaline membrane, in direct continuation, how- 

 ever, with the walls of the central digestive cavity, as may be seen from Plate I. Fig. 1 

 and 4, and Plate U. Fig. 7, 8, 9, 10. Their diameter is uniform for their whole length. 



