62 SIDERASTREA RADIANS. 



column wall completely over-folding the tentacles and covering nearly all 

 the disc (plate 3, fig. 18). 



After the first few days the polyps were able to feed upon small pieces 

 of molluscs or chaetopods placed upon the disc by means of forceps or a 

 needle. The tentacles would close upon the fragment and hold it as the 

 forceps were withdrawn. If the pieces offered were small enough they were 

 taken bodily within the gastric cavity, but when too large to be engulfed a 

 portion only would be drawn within the stomodseum, remain there for two or 

 three hours, and then be pushed away over the side of the disc. 



For some time Zooxanthellas were present within the ectoderm of the 

 disc and upper part of the column, and rendered these regions nearly opaque ; 

 but later they wholly disappeared from the outer layer, and, as in the adult, 

 were restricted to the eudoderm. As the polyps grew larger their walls 

 became thinner and more transparent. The internal yellow cells were 

 thickly distributed along the two sides of the mesenteries and within the 

 endoderm of the stalk of the tentacles, but were sparse towards the lower 

 part of the column. They formed an important aid in determining the course 

 of the mesenteries within the living polyps. 



The young polyps displayed great power of recovery from injury. In 

 two or three instances the glass to which they were attached was broken in 

 such a way that a polyp was completely divided into two, the halves adherent 

 to different fragments ; yet the two parts continued to live, and each became 

 a distinct, though smaller, polyp, nearly circular on healing, and restoring 

 in every way the normal form. 



On the polyps attaining partial transparency, the internal cavity could 

 be seen to contain granular matter and Zooxanthellae in a regular rapid 

 movement, and, by focussing at different heights, it was possible to make out 

 the complete course of the circulation. The particles passed directly upward 

 along the column, then transversely across the disc, downward along the 

 endodermal surface of the stomodseum, and thus right into the central cavity 

 of the polyp ; they then continued radially across the base outwards towards 

 its periphery, and once more up the column. The regularity of the course 

 was a little disturbed at the opening of the tentacles into the gastric cavity. 

 Some of the particles would circulate around this region in an eddy-like 

 manner, or pass a little within the tentacular cavity, and afterwards be 

 wafted along in the general current. The course is indicated in fig. 6. 



Occasionally the lips of the stomodasum would meet in the middle and 

 remain open at each end, but no incurrent or excurrent streams were ever 

 detected through the apertures thus formed. A bolus of organic debris, 



