224 BEHAVIOR OF THE LOWER ORGANISMS 



food withdraws, by an extension and displacement of the mouth, till 

 there is nothing beneath the food body, and it is pressed by the oeso- 

 phageal lobes into the internal cavity. The lobes then withdraw and 

 the mouth closes. 



The determining factors in the food reaction are partly internal, 

 partly external, the variations of the former playing perhaps the most 

 important part. Many of the sea anemones are voracious, taking food 

 until the body forms a distended sac. But in most species, if not all, 

 the behavior changes decidedly as the animal becomes less hungry, and 

 after a time it refuses to take food, even removing it if the food is ap- 

 plied to the disk. The changes in reaction as hunger decreases seem 

 less marked in those species in which the food is taken mainly by ciliary 

 action. 



Specimens that have not been fed for a long period frequently swallow 

 indifferent bodies, such as pellets of paper, grains of sand, and the like. 

 This has been observed in Aiptasia (Jennings, 1905 a), Sagartia (Torrey, 

 1904), Metridium (Allabach, 1905), and in a number of Mediterranean 

 anemones (Nagel, 1892). In Stoichactis the taking of such indifferent 

 bodies is rare, but sometimes occurs. In Sagartia and Metridium such 

 indifferent bodies cause a reversal of the beat of the oesophageal cilia, 

 just as is occasioned by actual food. All together, it is clear that in 

 hungry specimens of various sea anemones mechanical stimuli acting 

 alone may cause the food reaction. 



In some cases chemical stimuli acting alone produce the food re- 

 action. If filtered crab juice is applied to the tentacles of Metridium, 

 they arch over toward the mouth. If the juice reaches the mouth, the 

 cilia of the oesophagus are reversed, striking inward, just as when a 

 piece of meat is present. The swallowing movements of the oesopha- 

 gus may likewise take place under chemical stimulation. Parker (1905) 

 has lately found that certain inorganic chemicals, containing potassium, 

 will cause the cilia to reverse and beat inward ; this is the case for ex- 

 ample, with KC1 and KN0 3 . But the reversal which takes place under 

 the action of meat juice is not due to the potassium salts which it con- 

 tains, for it requires a concentration of the potassium salt to produce 

 this result that is much greater than that existing in meat juice. In 

 Adamsia, according to Nagel (1892), the tentacles react to sugar in the 

 same way as to meat juice; this is not true for Metridium and Sagartia. 



As sea anemones become less hungry they usually cease to react to 

 such indifferent bodies as grains of sand, pellets of paper, etc., though 

 they still take crab meat readily. In Metridium and Sagartia bits of 

 paper no longer cause the reversal of the oesophageal cilia, by which 

 particles are carried to the mouth, while crab meat still produces this 



