OF THE ACALEPH^ OF NORTH AMERICA. 305 



than half the diameter of the disk, even in their most relaxed state. Usually they form 

 only a graceful fringe all round the disk, about one sixth of the whole diameter in 

 breadth. 



These tentacles are hollow, and their cavity, which is wider near their base than 

 towards the tip, opens into a circular canal, which extends all round the disk along the 

 bases of the tentacles. Underneath and within the tentacles, there is a further pro- 

 longation of the disk, in the shape of a seam stretching inwards, and forming, as it were, 

 a partition of the cavity below the disk. But as this seam is rather narrow, the cavity 

 itself is widely open, and the seam, in general, can hardly be considered as any thing 

 more than a mere membranous inner prolongation of the disk, or of the margin of the 

 disk, from which the tentacles issue. The existence of such a seam should not be 

 overlooked, as it is a point in the structure of the Medusa under consideration which is 

 probably common to all the genera, and only more or less developed in the different 

 types. For instance, the inferior opening of Hippocrene is very small in comparison 

 with the whole width of the animal, which, consequently, has the lower surface of its 

 disk almost shut out from immediate contact with the surrounding water. Such is 

 the case, also, in the genus Sarsia, whilst in Thaumantias, as in Staurophora, the flat- 

 ness of the disk and the narrowness of its inner scam make this feature less prominent. 

 As in all the Medusae in which it has been observed, this seam is contractile ; concentric 

 fibres prevailing in it, though there are also a few radiating ones, by the powerful con- 

 traction of which this seam is at times reversed outwards, so as to cover the base of 

 the tentacles. Specimens which have died in such a position might also easily be 

 mistaken for peculiar species. 



At the base of each tentacle there is, on its inner surface, a black eye-speck, varying 

 in size in proportion to the size of the tentacle itself. The form of these eye-specks 

 is less regular than in most other naked-eyed Medusae ; the pigment-matter forming 

 rather irregular heaps, dark in the centre, with the margin vanishing away into the sur- 

 rounding substance. But, however indefinite these specks may be, (and they appear 

 as perfectly distinct dots to the naked eye, and only under higher magnifying powers 

 seem less defined at their margin,) they must, physiologically speaking, be considered 

 to be the same organs as similar specks upon the margin of the disk of most other Medusae. 

 The tentacles themselves, in their contraction, are not only shortened and lengthened ; 

 but they coil spirally, and the coil is twisted from right to left downwards, or from left to 

 right upwards, all around the disk, without antitropic direction of those of different parts 

 of the periphery. 



The diversity of the forms which Staurophora assumes is truly wonderful. It may 



