8 SIDERASTREA RADIANS. 



On partial retraction the column wall comes to rest upon the corallum,* 

 but the central area of the disc and the tentacles remain a little elevated, the 

 peristome especially so. On fullest retraction these also come to lie closely 

 against the skeleton and follow it all the way, nearly to the bottom of the 

 calice. Unlike the polyps of most corals, the column wall in Siderastrea is 

 incapable of folding over the tentacles and disc upon retraction ; both the 

 column wall and disc are simply drawn down upon the skeletal tissues, 

 leaving the tentacles and mouth exposed. Where, in other coral polyps, an 

 overfolding of the polypal wall occurs, it is brought about by the action of 

 the circular endodermal musculature of the column wall, sometimes in the 

 form of a sphincter ; but histological examination reveals that the columnar 

 musculature in S. radians is of the weakest character. 



The genus Siderastrea, in many respects, is allied to the mushroom 

 coral Fungia, both being included within the section Madreporaria Fungida. 

 The genus Fungia is not found in West Indian shallow waters, but the 

 various figures by Dana (1846), Bourne (1887, 1893), and Saville-Kent (1893), 

 of different species of Fungia found elsewhere, show that in this genus 

 also the column wall in retraction is not drawn over the disc. The West 

 Indian species of the allied genus Agaricia are also unable to overfold the 

 column wall. 



Apparently the outer surface of the polyps of S. radians is not ciliated, 

 for light particles dropped upon it remain there, and no ciliary motion is 

 recognizable when the living polyps are examined under a moderate magni- 

 fication. Similar particles dropped upon colonies of Manicina areolata are 

 gently swept away and over the sides in such a manner as to plainly indicate 

 ciliary activity, so that probably different coral species vary in this respect. 

 The outer surface is always uniformly ciliated in larvse (plate i, fig. i). 



COLUMN WALL. 



The columnar and discal areas of the polyps are not sharply separated 

 from one another under any condition of expansion or retraction. The outer 

 cycle of tentacles, which in ordinary cases marks the peripheral boundary of 

 the disc, here forms but an irregular circle, and its members are comparatively 

 widely apart (plate 6, figs. 31, 32). Including as column all that part of the 

 polypal wall between the polygonal line of attachment of the polyps to one 

 another and the outermost cycle of tentacles, its extent is very limited, being 



* The free superficial regions of the polyp (column and disc) never come into actual contact with the 

 skeleton, as the living parts of the latter are always covered with the basal skeletotrophic tissues. When 

 they are said to rest upon the corallum the tissues lining the corallum always intervene. 



