OF THE ACALEPHjE OF NORTH AMERICA. 265 



It is, however, at times more or less distended, owing to the greater or smaller quantity 

 of fluid circulated in them. The lower part of the vertical tube is movable to some extent, 

 inasmuch as it is situated along that portion of the gelatinous disk which alone is subject 

 to considerable movement by the contraction of its muscular bundles. This lower portion 

 of the tubes, however, is generally curved outward, (Plate I. Fig. 1,) so that it does 

 not meet the base of the sensitive bulb from above, but from its inner surface, as is well 

 shown in Plate I, Fig. 1, and in Plate II. Fig. 11, e, 13, b, and 17, d, d. It miijht 

 seem at first as if the bulb at the base of these vertical tubes was a mere sac placed 

 between the vertical tubes and the circular one of the lower margin, and it has really 

 been described as such a pouch or sinus ; but nobody, so far as I know, has as yet 

 perceived that there is a connecting tube between the four bulbs. Neither Mertens, nor 

 Brandt, nor Lesson, though all claim originality for their investigations, has seen any 

 such connection, and Professor Edward Forbes himself does not seem to have noticed it, 

 though he has known such a circular tube to exist in other allied genera. Now this 

 circular tube in Hippocrene is situated in the same relative position in which it occurs uni- 

 versally in other naked-eyed Medusae, extending in a circular disposition from bulb to 

 bulb, (Plate I. Fig. 2, 3, 4 ; Plate II. Fig. 11, Z>, 6, 13, o, a, 14, e, e, and 17,rt, «,) and form- 

 ing along the lower margin a continuous canal. Along the inner margin of this tube, the 

 nervous cord is distinctly seen, under no very high power of the microscope, as shown in 

 Plate I. Fig. 2, 3, 4 ; Plate II. Fig. 11, a, a, 14,/,/, and 17,6,6. But as here the eye- 

 specks and tentacles are grouped in four bunches, the circular tube is stretched more di- 

 rectly from one bunch to another, and though arched outside, it assumes an almost quad- 

 rangular form. In it, as well as in the vertical tubes, the circulation of most heterogeneous 

 granules, moving to and fro in all directions, and up and down through the vertical tubes into 

 the horizontal circular one, can easily be traced, even under low magnifying powers. 



So little is known of the structure of these animals, that I do not suppose that any 

 particulars respecting it will be thought superfluous, and I may, therefore, be permitted 

 to dwell at greater length upon the connection of the vertical and horizontal tubes with 

 the sensitive bulb. This very prominent part in the little Hippocrene, which to the naked 

 eye appears like a tuft of threads with dark specks at the base, consists of very hetero- 

 geneous organic elements. In the first place, I may say that it is by no means hollow, as it 

 has been supposed; that the tentacles themselves are also not hollow; and that there is 

 no direct communication whatsoever between the bulb and the chymiferous tubes. The 

 fact is simply, that, as a normal organic adaptation in all these animals, the sensitive ap- 

 paratus follows closely the disposition of the digestive and circulating apparatus. And, as 

 I have already mentioned above, all these systems of organs are everywhere in direct re- 



