4 SIDERASTREA RADIANS. 



and some forms of Isophyllia fragilis are usually found with it in such places. It is often 

 partly buried in the white calcareous mud of the flats, and yet seems healthy there. It is also 

 abundant in the small, shallow pools left on the flats by the tide. But it is equally common 

 on the reefs, where it often grows larger. It is also found well grown in Harrington Sound. 

 Exposure to the dry air, or even to the hot sun, for an hour or so, does not kill it, if it be wet 

 beneath. Probably its porosity enables it to absorb sufficient water to prevent drying up. 



The natural occurrence of the living coral under such varying condi- 

 tions as to purity of water and exposure indicates that this species, at any 

 rate, is not so extremely sensitive to its environment as corals generally are 

 found to be. 



The polyps are so small that their external characters can be fully made 

 out only with the aid of a lens or a low-power microscope. Further, they 

 differ so much in appearance, according to the amount of expansion and 

 retraction, that to obtain their complete characteristics it is desirable to keep 

 the colonies under observation for some time and subjected to various condi- 

 tions. As the stocks often lie free on the sea-floor, they can be collected 

 without any injury to the polyps, and are sufficiently small to be kept in the 

 laboratory in ordinary glass vessels. With a little attention no difficulty is 

 experienced in keeping the polyps alive. Indeed, the colonies continued to 

 increase in size during confinement, and while kept under observation new 

 polyps began to arise from the calices of previously dead areas. 



The boring bivalve Lithodomus appendiculatus Phi.* nearly always 

 occurs within the corallum, often several in each colony. The siphon has 

 a dark purple, funnel-shaped incurrent aperture and a small, tube-like 

 excurrent aperture, and protrudes a short distance beyond the surface of the 

 colony, the activity of the mollusc keeping up a strong circulation of water; 

 larvae and other small floating objects may pass in rapidly at one aperture 

 of the siphon, and after a brief interval may be shot out from the other. 

 Numbers of cirripedes (Pyrgoma) also frequently infest the corallum, and 

 assist in maintaining a current of water over the living colony. It seems 

 not unlikely that these infesting organisms may be of much importance in 

 clearing away the mud and sand which accumulate on the surface of the 

 polyps, and the currents produced by them may also bring food within reach. 



During the greater part of the day the colonies in the laboratory were 

 kept in the shade, or even in darkness, by placing the glass receptacles 

 under some cover. Under these conditions the polyps remained partly 

 expanded, but retracted when exposed for any length of time to the direct 

 rays of the sun. In the early morning and towards evening the vessels were 



* For the determination of this species I am indebted to Prof. L. P. Gratacap, of the American Museum 

 of Natural History. 



