420 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



distance above the coralluin, while the tentacles ai'e erect or overhanging. The colonies remain 

 in this state nearly all night, unati'ected by any artificial light employed in observing tliem; even 

 when the strong light from a condensing lens rests upon a polyp for some time there is no 

 response. In the morning the poh'ps are again found retracted. 



If injured too much in the process of collecting, as when a portion of a colony is with diffi- 

 culty broken off a large mass, the polyps of most i-orals arc unable to recover sufficiently to 

 expand at night. The liest specimens for laboratory study are the colonies found lying free on 

 the sea floor, for these can be removed without nnich disturl)ance to the living animals. 



On the reefs, Jhemnlrhici, VolpdplnjJl'iii^ and OrhinlJa arc found partly expanded during 

 the day, only the tips of the tentacles and part of the disk being visible; Maniciiui will some- 

 times protrude its tentacles, ))ut hophiilUd rarely so. On the other hand, Madrepora and Purites 

 are usually fully expanded; colonies of l)oth species are often met with in situ on which, by 

 means of a water glass, all the polj'ps are seen protruding to their full extent. 



The corals found in very shallow water in Kingston Harbor are mostly retracted during the 

 day, but if collected with care, and placed in shallow glass vessels exposed to the full rays of the 

 sun, such species as 21(()uci iia areolata , Porites fiin-uta^ Side)'a><tr:r(( mdia/is, VJudocm'a arhiwcida, 

 and Oeulina diffusa will expand fully. Further, when in th(> laboratoiy these species have 

 been kept shaded from the sun during the early i)ai"t of the nK)ruing and are then brought into 

 its direct rays, they soon begin to expand, and remain so for some time. Also, on bringing corals 

 which during the whole day have been kept in a cool, shaded place into the rays of the setting 

 sun they nearly always respond to the change and expand fully. It may be that in such experi- 

 ments it is not the strong light bat rather the slight increase of temperature of the water which 

 exercises some stinuilating influence on the polyjjs. 



The general experience is that if colonies arc placed in shaded spots during the day the 

 polyps respond to the change, and expand to a greater or less degree, but if exposed to full 

 light they remain retracted. 



Much difl'erence is experienced in the readiness with which various coral .species expand. 

 In the laboratory Agaricla seems to open less freely than others; L'ladocora and Oculiita are 

 among the readiest to open out. In some instances the polyps of the latter remained fully 

 distended for two or three da^s together without ever retracting. 



It ma}', therefore, be taken as a general rule that coral polyps expand to their full degree 

 daring the night, but that under artificial conditions they may respond to an increase of light and 

 temperature. The whole question of their response to external conditions is full of interest, l)ut 

 can be solved only by a long series of observations and experiments. 



That night expansion is not restricted to tropical corals may be gathered from the observa- 

 tions of Gosse (18(!0, p. 312) on L'aryiqdnjUia Sinithii. This British species was also found to 

 expand most freely at night. 



Many sea anemones exhibit the same phenomena as coral polyps, though not to the same 

 degree. Colonies of the Zoanthid Pahjthoa are found in plenty on the reefs; during the day the 

 polyps are mostly in the retracted state, and in the laboratory night is always found to be the most 

 suitable time for examining them in the fully distended condition. 



In the course of his examination of the coral reefs of Funafuti, Rotuma, and Fiji, ^Ir. Stan- 

 ley Gardiner (1898) found unich the same results as regards the time of expansion of coral 

 polyps; Euphyllia^ Sytn.phyllia^ and Jlufsm were the only corals observed by (-iardiner to be fully 

 expanded in the daytime. He further states, as is also the case in West Indian waters, that only 

 during the night is the tow net able to collect in any quantity the minute larvse, eggs, and 

 other small organisms which probably constitute the food of coral polyps. During the day such 

 pelagic forms evidently sink to the deeper waters, reappearing nearer the surface at night, and 

 becoming most abundant in the early morning. 



It may be that night exi)ansion and day retraction of the coral polyps are in some way con- 

 nected with this distribution of their food, and it is not unlikely that the phenomenon may be 

 associated with the strong local sea breezes which usually disturb tropical waters during the day, 

 and produce a cloudiness some distance around the shores. At night and early morning, the 



