OF THE ACALEPH^ OF NORTH AMERICA. 255 



In Sarsia, for instance, they are upon the outer surface of the base of the tentacles; in 

 Hippocrene, upon the inner surface. 



Another character common to all naked-eyed Medusec consists in tlicir ceulral nioulli. 

 Though many of them have been described as deprived of such a central aperture, there 

 can no longer be any doubt respecting its existence in all the well-characterized genera 

 of the family. Extensive differences may be noticed in tiie form of the mouth, and 

 in its connection with the inner cavity, but these differences are nowhere such as to 

 warrant more than generic distinctions. In Staurophora, it is a mere fissure, in the 

 form of a narrow extensive cross in the centre of the lower surface of the disk, with 

 low fringes all round its edges. In Tiaropsis, the opening is reduced to a small space 

 in the centre, but the margin of the mouth forms more extensive lobes, with thin, 

 light fringes. In various genera, this central cavity forms a sac more or less developed, 

 hanging from the centre, either with a simple margin, or with a lobed and fringed open- 

 ing. This central sac may assume a more permanent shape, as in Hippocrene, or be of 

 a more movable nature, as in Thaumantias, or even be developed into a very protractile 

 and retractile proboscis, as in Sarsia. In Hippocrene, we have perhaps the maximum of 

 division in the margins or lips encircling the mouth, as they form here a kind of dichoto- 

 mous tentacles; while in Sarsia the margin of the mouth is entire. Again, the organs 

 of reproduction are found in all, upon the outer surface of the digestive cavity, whether 

 they encircle the main, central cavity, or follow the radiating tubes. 



The characters of the naked-eyed Medusae may therefore be easily summed up in 

 this way. A gelatinous disk with a reentering margin; — a central digestive cavity with 

 radiating tubes meeting a circular tube along the margin ; — tentacles and eye-specks 

 along this margin ; — mouth and central cavity varying in size and form, but opening in 

 all in the centre ; — reproductive organs following the chymiferous system ; — and a dis- 

 tinct nervous ring along the circular marginal tube observed in all; — generation alter- 

 nate, one form Polypoid, and the other Medusoid. 



After having characterized the genus Hippocrene as above, and traced its intimate 

 relation to the diversified forms referred to the tyj)e of naked-eyed Medusae, I shall now 

 proceed to illustrate the minute structure of this animal, which I have been able to in- 

 vestigate satisfactorily in almost all its details. Form and structure are so intimately 

 connected in these low animals, I may say in Radiata at large, that the one is almost the 

 expression of the other, in a far more strict sense of the word than in any other group of 

 the animal kingdom. And this is so true, that giving a complete description of their 

 form is already to intimate the general arrangement of their organs, and that the de- 

 scription of their organs can never be full, unless it is mentioned in relation to their form. 



