Chap. II. STRUCTURE OF THE ADULT. 63 



continues, in a measure, to move to and fro from the main cavity, through the 

 radiating tubes and back again, the contractions of the injured margin obhterating 

 the canals through which it woukl otherwise ooze out at the periphery. The same 

 is the case with the frhiges along the margin of tiie oral appendages ; they grad- 

 ually drop off, and with them j^arts of the arms themselves, especially' toward their 

 extremities, which become blunt. Evidently they are now in a dying condition, 

 and can scarcely regulate their course. They are frequently capsized, and air 

 accumulates in the cavities of the body, especially in the genital pouches, the lower 

 floor of Avhich is also destroyed. As soon as air has been lodged in these cavities, 

 the Aurelia is forced to the surface of the water, where it floats at the mercy 

 of the elements. No sooner has it ceased to reo-ulate and control its motions, 

 than swarms of little shrimps fix themselves upon its surface, and enter its interior 

 cavities, where they are occasionally found crowded in hundreds. A small species 

 of Hyperia seems particularly to delight in resorting to our species. The gelatinous 

 disk is the last part of our Medusa which may be found floating in this way 

 upon the water, deprived of all its appendages. But such is the continuity of 

 the tissues of the umbrella, in Aurelia, that it does not break up in regular organic 

 segments, as does that of our Cyanea. 



The manner in which stranded Medusae are sometimes covered in hot, dry, and 

 windy days, by floating sand, and moulded in it, explains the jiossibility of the 

 preservation of Acalephs in a fossil state. The few specimens found in the fine- 

 grained limestone of Solenhofen were probably 2>reserved in that way. 



With a view to a closer comjjarison of these animals with other Eadiates, it 

 may not be out of place to notice here, that the whole upper floor of the body of 

 the Meduste bears the same relations to the main cavity and its radiating tubes, 

 as the roof of a Starfish does to its furrowed imder surface. We are, therefore, 

 justified in .considering this disk as an abactinal structure; and it may well be said 

 that a Medusa, with its convex bell-shaped umbrella, resembles closely some of the 

 bell-shaped Crinoids, the abactinal parts of which form the calyx, so called, while 

 the ambulacra! area may be compared to the lower surface of a Medusa, since 

 the absence of a stem in Comatula has already taught us, that this support is not 

 an essential element of the structure of a Crinoid. Moreover, while attached to 

 their Hydroids, the naked-eyed Meduste do not differ from the Crinoids, even in 

 that respect. 



