ADULT COLONY. 9 



represented b}' a zone not exceeding 2 mm. across. When the polyps are 

 expanded the proximal region of the column remains somewhat polygonal in 

 outline, but distally it becomes circular in section, rarelj^ assuming the true 

 columnar form. 



The column does not present the two topographical subdivisions, calicinal 

 and pericalicinal, like most corals. Instead it extends directly from the edge 

 of the calice, so that it is wholly within it; in other words, no "Randplatte" 

 or " edge-zone " occurs in Siderastrea. 



Structurally, the column wall is very thin, partly transparent, and 

 smooth ; on expansion vertical lines are clearly visible, indicating the internal 

 attachment of the mesenteries. 



TENTACLES. 



The oral disc is divisible into an outer tentaculiferous region and an 

 inner naked area or peristome, with the mouth in the middle. The tentacles 

 are arranged widely apart, and thus occupy a comparatively broad zone of 

 the disc, leaving only a very restricted smooth central area. As can be seen 

 from plate 6, fig. 32, the tentacular region during retraction includes the 

 greater proportion of the exposed part of the polyp, while the columnar area 

 is very narrow, and little remains of the naked portion of the disc. This 

 exceptional relationship of the columnar, tentacular, and discal areas results 

 from the wide intervals which separate the different cycles of tentacles, a 

 similar condition being apparently characteristic of the Fungidse generally. 

 In most coral and actinian polyps the tentacular cycles are placed close 

 together, the bases of the tentacles of one cycle being partly embraced b}' the 

 bases of the next cycle, and thus the tentacular zone is very narrow compared 

 with the remainder of the disc. 



The tentacles of S. radians are remarkable among all West Indian 

 coral polyps on account of their form and arrangement. Under certain con- 

 ditions of partial or even full expansion of the polyps the organs can be 

 discerned with a lens as small sessile tubercles, arranged only approximately 

 in cycles, some as pairs and others singly, and corresponding in position 

 with the septa. In most cases they stand out as small, delicate, simple or 

 bifurcated finger-like outgrowths of the disc. As seen externally each 

 appears to originate over the central termination of the septum with which 

 it corresponds, that is, before the margin of the septum commences to dip 

 downward. 



When the tentacles are fully extended and much enlarged they appear 

 as shown on plate 7, fig. 37. The stem consists of a broad basal part which 



