SCYPHOMEDUSAE COLLECTED BY STEAMER " ALBATROSS. ' ' 181 



work upon this subject, but the more recent study of S. Hatai is far 

 more elaborate and convincing, and should be consulted by all stu- 

 dents of the subject. 



Indeed the rate of growth, and the ultimate size that an individual 

 jellyfish attains before becoming mature, is a measure of its success 

 in obtaining food, and it is interesting to see that the largest jelly- 

 fishes are those of the cold seas where the floating animal life is 

 more abundant than in the Tropics. In common with the Corals, 

 Sea-anemones, Alcyonaria, Siphonophores, and Hydromedusae the 

 Scyphomedusae are, so far as is known, exclusively carnivorous and 

 do not feed upon plant life. Thus it is possible that the more rapid 

 growth rate of Pacific corals in comparison with those of the At- 

 lantic may be due to the better food supply derived from the deep 

 lagoons of the Pacific, whereas in the Atlantic the lagoons are shal- 

 low and the water is charged over these vast flats with a precipitate 

 of calcium carbonate which collects upon all floating animals and 

 tends to smother them ; whereas in the Pacific this does not appear to 

 be the case. 



Jellyfishes are, then, all carnivorous, and, while few devour others 

 of their own species, they often greedily feed upon other sorts of 

 medusae. These animals are, indeed, an important factor in destroy- 

 ing the eggs of cod and other fishes whose spawn floats in the sea. 



The prey is seized by the mouth, and after being held and par- 

 tially digested in the stomach, the remnant is ejected through the 

 mouth. 



The central stomach is a space in the middle of the umbrella, but 

 this always gives rise to an outwardly radiating system of pouches or 

 tubes which may form a complex network of vessels under the mus- 

 cular layer of the concave side of the umbrella. As this system of 

 pouches is connected with the stomach, and nutrient fluids derived 

 from the food circulate through it, it is often called the gastrovas- 

 cular system, for it is both a sort of " chymiferous system " as well as 

 a digestive and circulatory space. 



In all the larger jellyfishes, or Scyphomedusae, we find within 

 the stomach four clusters of tentacle-shaped organs of unknown 

 function, placed at the broad sides of the cruciform mouth. The 

 smaller jellyfishes, or Hydromedusae, lack these stomach-tentacles or 

 gastric cirri, as they are often called ; and in still another structural 

 detail do they differ from the Scyphomedusae, for the Hydromedusae 

 have a diaphragm-like membrane (velum) extending inward from 

 the bell-margin and partially closing the opening of the umbrella, 

 but the larger jellyfishes (Scyphomedusae) do not have a diaphragm 

 of this sort, although it is true that the Carybeidae, or sea-wasps, 

 appear to have such a diaphragm, but it is not strictly comparable 

 with that of the Hydromedusae. 



