64 Amei'ican Fisheries Society 



HOW MEDUSAE KILL FISH. 



It is remarkable how few cases have been observed of 

 the actual seizure and digestion of small fishes by medu- 

 sae. This lack of observations, in view of the vast 

 abundance of jellyfishes in most seas where small fishes 

 abound, is astonishing, and adds, I think, to the interest 

 of the study which I shall briefly set forth. At Canso a 

 few years ago I captured a young Obelia less than a half- 

 inch in diameter which had captured a small fish more 

 than half an inch long. Obelia has no long tentacles for 

 seizing prey and its hanging manubrium, with mobile 

 mouth opening, is not specially well-adapted to grasping 

 active living creatures such as fishes. Yet I found that 

 this specimen had not only caught a small fish, but that 

 it was nearly masticated and digested, and only the head 

 and the bright metallic eyes were recognizable, barely 

 protruding from the mouth. Apart from the squeezing 

 action of the manubrium the pulsations of the medusa 

 enlarged and diminished the capacity of the radial 

 gastro-vascular canals, the pumping, or rather, suction 

 action aiding in breaking down and tearing off frag- 

 ments of the body of the larval fish which had been 

 ingested tail first. Now it is well known that the oral 

 and gastric endoderm cells copiously pour out a secretion 

 which has the action of a ferment or solvent upon food 

 materials. A process of digestion commences as soon as 

 an object like a small fish passes into the manubrial 

 aperture. The food is broken down by mechanical press- 

 ure and squeezing, and is mingled with the ferment and 

 sucked inwards. Particles of the fish could be seen pass- 

 ing into the fundus of the gastric cavity whence four 

 radial canals pass off to join the circular marginal canal. 

 The endoderm of these canals is ciliated, though sparsely, 

 causing a circulation of the watery gastro-vascular fluid 

 in which float the macerated food particles and the dis- 

 solved liquid elements. Intra-cellular digestion, as 

 Metchnikoff and Ray Lankester found, takes place over 

 limited areas of the internal walls, but the nutrient 

 matters are probably not transferred to the deeper tis- 



