164 Papers from the Department of Marine Biology. 



the rim. These tentacles are hollow, the stomach cavity being con- 

 tinued into them as it is also into the sense-clubs; indeed, there is every 

 reason to believe that the sense-organs are only highly modified tentacles. 



Powerfully developed circular, or a combination of circular and radial, 

 muscles are found in the under, concave part of the umbrella, and here 

 also there is a network of nerve-fibers connecting the muscles with 

 the sense-clubs. Curiously, there are no muscles, and probably no 

 nerves, over the outer convex part of the umbrella. 



Those who have handled jellyfishes know that they are capable of 

 inflicting a sharp sting, the tentacles being especially active in this 

 respect. Closely clustered over the surface of the tentacles, and other 

 parts of the jellyfish, there are minute cells, each containing a hollow 

 tube coiled rope-like within the cell. Upon excitation these little tubes 

 are turned inside out and shot forward, and their outer ends, which are 

 barbed, penetrate the skin, causing a sharp sting due in part, it appears, 

 to formic acid. Thus it is that these large jellyfishes are among the 

 most persistent enemies of the fishes, for many an incautious victim is 

 ensnared among their stinging tentacles only to be paralyzed and finally 

 drawn upward into the mouth of the jellyfish. It is, however, a poor 

 rule which does not work in both directions, and certain kinds of small 

 fishes often accompany the jellyfishes, swimming in and out among 

 the dangerous tentacles, even biting off small pieces of the jellyfish 

 itself and occasionally themselves failing a prey to the stings, but in 

 general enjoying a peculiar protection from the attacks of larger fishes 

 who dare not venture too near the jellyfish. 



The mouth, or mouths, of the jellyfish may be surrounded by veil- 

 like lips or, if the mouths are numerous, as in the so-called Rhizostomse 

 (root-mouthed) jellyfishes, by complex frills lined by minute tentacles 

 which at intervals bend to and fro and sweep, as it were, for food, 

 for the jellyfishes are all carnivorous. In the higher animals the intes- 

 tine is a tube which lies suspended within the body cavity, but as long 

 ago as 1849 Huxley showed that the jellyfishes have no body-cavity, 

 and consist simply of a stomach and an outside with a mere structure- 

 less lamella, or a solid mass of jelly, between the stomach-wall and the 

 outer skin layer of the animal. 



This gelatinous substance may serve as a store of food for the animal 

 in case of starvation, and Cassiopea can live at least 42 days without 

 food, the weight of the jellyfish declining to less than one-hundredth 

 its original magnitude. The loss of weight on each day is, however, 

 proportional to the weight of the animal at the beginning of that day 

 and thus the lighter it becomes the less the weight lost. 



Thus, if W be the original weight of the jellyfish, and y its weight 

 after x days of starving, then yW (1 a)""; where a is a constant, 

 less than unity. 



