Medusae of the Philippines and of Torres Straits. 165 



Indeed 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 jellyfishes 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, exclu- 

 sively carnivorous and do not feed upon plant life. 



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

 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 muscular 

 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 gastrovascular system, for 

 it is both a sort of "chymiferous system" and a digestive space. 



In all of the larger jellyfishes, or scyphomedusae, we find within 

 the stomach 4 clusters of tentacle-shaped organs of unknown function, 

 placed at the broad sides of the cruciform mouth. The smaller jelly- 

 fishes, 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 which grows inward from the bell-margin 

 and partially closes the opening of the umbrella, but the larger jelly- 

 fishes (scyphomedusae) do not have a diaphragm of this sort, although 

 it is true that the Cubomedusse, or sea-wasps, appear to have such a 

 diaphragm, but it is not strictly comparable with that of the hydro- 

 medusae. 



The sexual organs of the larger jellyfishes (scyphomedusse) are found 

 in the stomach peripheral to, and closely associated with, the four clus- 

 ters of gastric cirri. The sexes are usually separate, the animals being 

 either male or female, although in rare instances, as in Chrysaora, they 

 are hermaphroditic, or male when young and female when old. When 

 mature at the breeding season the males and females usually come to 

 the surface in great numbers and may congregate in vast swarms many 

 square miles in area. The larvae or eggs may then be cast out into the 

 water by the breaking down of the stomach-wall or the larvae may un- 

 dergo a part of their development within the stomach, or mouth-parts, 

 of the mother, finally to be cast out through the mouth. 



In any event the larvae soon develop into minute pear-shaped crea- 

 tures about as large as a pin's head, their bodies being covered with 

 vibrating cilia which enable them to spin through the water. For 

 a few days or even weeks they may remain thus swimming near the sur- 

 face and be drifted far by tide and ocean current. Soon, however, the 



