154 PROCEEDINGS OF THE AMERICAN ACADEMY. 



in producing a reversal of the cilia on the lip zone of Metridium. Of 

 these substances 'potassium chloride, creatine, and albumoses are pres- 

 ent in the "extract (Street '• 08). The failure to obtain uniform rever- 

 sals may have been due to the varying differences in the degree of dilu- 

 tion of the stimulating materials, or to unknown factors associated with 

 the physiological condition of the polyps. As will be seen later, ciliary 

 action plays an insignificant part in the feeding process of the rose 

 coral, so that a prompt and definite reversal of the cilia of the oral disk 

 is not a necessary factor in the operation of securing food. 



Normal Feeding Behavior. This brief analysis of the responses of 

 the polyp to chemical and tactile stimuli leads to an understanding of 

 the way in which these various reactions are combined in normal feed- 

 ing behavior. 



It is generally believed that coral polyps take as food the small or- 

 ganisms found in the marine plankton, although there is little direct 

 evidence in support of this view. Examination of the coelentera of pre- 

 served specimens rarely reveals food material, the scarcity of which in 

 the digestive cavities of the Madreporaria has been commented upon 

 by Duerden, Hickson, Bourne, and Fowler (Pratt : os). It has even 

 been suggested that the zoochlorellae living in symbiotic relation with 

 many corals may elaborate a part of the latter's food, and thus com- 

 pensate for deficiencies in the supply from outside. 



On the other hand it is to be noted that the diurnal expansion and 

 contraction of coral polyps — their alternating periods of activity and 

 quiescence — coincide respectively with the appearance and disappear- 

 ance of the plankton in the surrounding waters. This can readily be 

 observed in the case of the rose corals. During the daytime, when 

 the sea about them is comparatively iree from small organisms, the 

 polyps appear partially contracted. No tentacles are to be seen. But 

 after dark, when the myriad components of the plankton begin to 

 swarm in the water, one may wade along the shore at low tide, and 

 with the aid of a water glass and light from a bicycle lamp, he may 

 readily see that the polyps are fully expanded, with tentacles well out. 

 They have every appearance of being on the alert for food. 



In order that I might observe under favorable conditions a polyp in 

 the act of feeding upon what we may suppose to be its natural food, I 

 placed a small colony in a dish of sea-water under a low-power binocular 

 microscope. This was done by lamplight at about half-past nine in the 

 evening when the polyps were well expanded. I then poured into the 

 dish a small ({uantity of living plankton which a half hour before had 

 been obtained with a tow-net in the locality from which the corals had 

 been collected. The plankton consisted of small Crustacea, including 



