OF THE ACALEPH^E OF NORTH AMERICA. 257 



below, and the width of the partition across the lower opening, prevent both an ex 

 tensive spreading and very powerful contractions, in consequence of which the move- 

 ments are reduced to repeated Jerks, and the whole animal seems rather to jump from 

 place to place by its sudden contractions, than to dart in one direction and to bend in 

 various ways, as the Sarsia does, or to progress slowly by repeated uniform contractions, 

 as we observe in Tiaropsis and Staurophora. In its natural position when at rest (Plate 

 I. Fig. 1 and 5), Hippocrene stands upright in the water, its tentacles more or less drawn 

 out and stretched outside at various angles from the lower margin. The tentacles, how- 

 ever, in that position, may be more or less contracted. When fully drawn out (as in 

 Fig. 1, 2, 5, 6), they are longer than the diameter of the body itself; but when fully 

 contracted (as in Fig. 4 and 11), they are very short and hardly visible isolatcly, but 

 form simple bunches of curved threads. If immovable, with retracted tentacles, the ani- 

 mal seems to sink slowly lower and lower in the water ; but with the tentacles stretched 

 out, it remains more steady in the midst of the water, and does not seem to change per- 

 ceptibly its place. As soon as it contracts to move, the tentacles are reduced to a certain 

 middle state of extension (as in Fig. 3), and brought together in a backward direction 

 (as in Fig. 9 and 10) ; but when dilating, they are drawn in through the lower aperture 

 in an inverted position (as in Fig. 7 and 8). From its peculiar form, this animal turns 

 easily every way, and moves with equal ease and elegance sideways or downwards, or 

 obliquely in all possible directions, as Fig. 7, 8, 9, 10, 11, show. The arrangement 

 of parts, however, is such, that, during the most powerful contractions, the lower opening 

 is brought into a square form, as noticed in Fig. 2, 3, 4, 9, 10, but when gradually relaxing 

 passes again into a more rounded shape, as in Fig. 1,5, 8, 11. When at rest, as in Fig. 



I, it is perfectly circular, and only the more straight course of the inferior chymiferous 

 canal, extending from one sensitive bulb to the other, preserves some indication of the 

 quadrangular outline. As far as I know, the species of Hippocrene which lives on the 

 shores of Massachusetts is the smallest perfect Medusa known. At least, I have not 

 observed any species with perfectly developed eggs of so small a size as this. Fig. 5 to 



II, indeed, though intended to give a correct idea of this animal in its natural appearance, 

 have been enlarged to twice their natural diameter, or at least exaggerated one half, as 

 I have never seen a single specimen fully equal in size to Fig. 11. But, as it would have 

 been almost impossible to give any thing like correct outlines of this little creature with- 

 out enlarging it, I have preferred to exaggerate somewhat its size, even in drawing its 

 natural attitudes, rather than to omit such details as will alone give a correct idea of its 

 particular appearance. Fig. 1, 2, 3, and 4, represent some of its attitudes considerably 

 enlarged. Fig. 1 is a profile view, slightly turned, to show, through the transparent 



