534 



MEDUSA; RHIZOSTOMA. 



But another set of eight subdivide and ramify, so as to form a 

 sort of net-work of vessels, which appear to serve for the nutri- 

 tion of the portion of the structure at a distance from the 

 stomach, and also to expose the nutritive fluid to the aerating 

 action of the surrounding water. The Medusa often attain con- 

 siderable size. It is said that their disks have been seen three 

 or four feet in diameter ; and that the animals have weighed as 

 much as 60 Ibs. It might be inferred from the extreme delicacy 

 of their structure, that they are supported only on food most 

 easily obtained ; but this is by no means the case ; for in their 

 stomachs are found small Crustacea, Mollusca, and even Fishes. 

 It would seem that their tentacula, like those of the Hydra, 

 possess considerable muscular power ; and that they are capable 

 of drawing towards the mouth almost anything which comes 

 within their reach. Even large Fishes are occasionally found 

 entangled amongst them. Very probably their stinging power 

 is of use in weakening the resistance of their prey. 



1187. There are many Discophora which resemble the Me- 

 dusce in general form, but 

 which yet differ from them in 

 many important and curious 

 points of organisation. Some- 

 times the mouth is prolonged 

 into a sort of proboscis, formed, 

 as it were, by the union of the 

 bases of the tentacula, so as 

 much to resemble the stalk 

 of a mushroom. Occasionally 

 the oral tentacula almost dis- 

 appear, and then the marginal 

 tentacula are largely a4& 

 abundantly developed, and 



probably replace them in function. The most curious modifica- 

 tion, however, is that which is displayed to us in the Rhizostoma 

 (Fig. 724), an animal bearing a close external resemblance to 

 the Medusa. Here the central mouth is entirely absent ; but the 

 tentacula are channelled through their whole length, and their 



FIG. 724. -RHIZOSTOMA. 



