260 CONTRIBUTIONS TO THE NATURAL HISTORY 



hauser's microscope, and that I could apply any higher powers after cutting the body 

 into halves ; for thus, without being sensibly altered, it would spread enough to admit 

 of its being placed under glasses of very short focus. Some details, however, were 

 examined from fragments cut entirely free. 



As there are few systems of organs so conspicuous in the body of Hippocrene as 

 the digestive system, I shall begin the minute descriptions of the parts of this animal 

 with an illustration of this apparatus, as a knowledge of its arrangement is the best 

 preparation to trace further the relations of other systems, and their combinations. 

 Mertens was undoubtedly correct, when he ascribed a mouth to the genus Hippocrene ; 

 and Brandt, in his more extensive description, when publishing the notes of this able 

 observer, was no doubt mistaken when he supposed that the fringes arising from the 

 main central cavity were a kind of suckers, by which the animal pumped its food in a 

 liquid state. I have, over and over again, seen the central mouth opening and shutting, 

 and assuming the most different outlines, as it contracted or expanded ; and, from a 

 microscopic investigation of the surrounding tentacles, I can positively say, that they 

 are deprived of any kind of opening at their termination, and are simply organs of 

 prehension. 



When shut, the mouth forms a prominent tubercle (Plate II. Fig. 18, a), between the 

 four bunches of tentacles, presenting a fissure in the form of a cross upon its apex. 

 This proboscis-like protuberance is a sort of protractile and retractile lip of whitish 

 cellular tissue, which, from its great transparency, is easily overlooked upon the dark 

 ground of the central cavity. This transparent tissue, however, forms the whole surface 

 of the central cavity (Fig. 18, b,b,b,b)\ it extends, also, sideways into the peduncle of 

 the tentacles, and rises upwards along the corners of the quadrangular mass, but it is 

 really capable of considerable dilatation and contraction ; for, though the mouth may be 

 protruded so as to bulge out between the tentacles for nearly half the diameter of the 

 central cavity, as represented in Plate II. Fig. 18, it spreads, at times, into a flat, nearly 

 star-shaped opening, as seen in Fig. 23, b, with a more or less undulating margin. In 

 this form the lips are considerably attenuated, and a narrow central cross-shaped fissure 

 (Fig. 23, c) appears in the centre as the entrance (Fig. 23, a) to the central cavity. When 

 examined from above in such a state of dilatation, as the upper wall of the central cavity 

 is very thin, it is easy to see through it, and the opening appears then as a small four- 

 rayed star, as in Fig. 19, 20, 22. The same appearance is also observed from below, as 

 drawn in Plate I. Fig. 2. But the mouth is capable of a still further dilatation, when 

 the narrow central opening is spread to the same extent as the lips, and the whole inner 

 surface of the central cavity is seen through the opening, as in Plate II. Fig. 21. In 



