MEDUSA ; RHIZOSTOMA. 



473 



of orifices at the edge of the disk. But another set of eight sub- 

 divide and ramify, so as to form a sort of net- work of vessels, which 

 appear to serve for the nutrition 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 

 Medusce often attain considerable 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 sup- 

 ported 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 

 thei rprey. 



1032. There are several Acalephte, which resemble the Medusce 

 in general form, and especially 

 in the possession of this um- 

 brella-shaped disk, by the un- 

 dulations of which they move 

 through the water ; but which 

 yet differ from them in many 

 important and curious points 

 of organisation. Sometimes 

 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 mush- 

 room. Sometimes the tentacula 



are of much smaller proportional size, and then we usually find 

 a series of filaments hanging from the free margin of the mantle 

 (Fig. 610). Occasionally the tentacula almost disappear, and 

 then these filaments are largely and abundantly developed, and 



FIG. 612. RHIZOSTOMA. 



