ADULT COLONY. 



EXTERNAL CHARACTERS. 



The polyps are small and subpolygonal in outline, rarely exceeding 

 4 mm. in diameter when retracted, and 5 or 6 mm. on expansion. Six unequal 

 sides constitute the basal periphery of large polyps, while there are only four 

 or five in smaller examples. In general, the individuals in any colony are 

 irregularly disposed. In some regions an arrangement in parallel rows can 

 be made out, but usually this is interfered with by the intercalation of new 

 polyps among those fully grown. In most branching corals, alcyonarians, 

 and compound actinians, it is usual for the principal oral axis of the polyp 

 to be symmetrically placed with regard to the axis of the colony, but no such 

 regularity of orientation is manifest in the polyps of Siderastrea. 



The superficial polypal tissues are so thin that the white corallum 

 below shows through, and occasionally the internal arrangement of the 

 mesenteries around the stomodseum can be followed. This is especially 

 the case with polyps which have been preserved in formalin. Retracted 

 polyps usually exhibit radiating lighter areas, which correspond with the 

 cycles of septa below, and narrower alternating darker areas, which corre- 

 spond with the interseptal spaces and internal attachment of the mesenteries 

 (plate 6, fig. 32). 



The living polyps making up a colony are separable from one another 

 along a narrow polygonal groove which is always lighter in color than the 

 rest of the polypal wall. Along this groove the column wall is connected 

 with the corallum, not directly, but by the intermediation of the mesenteries, 

 and to whatever degree the polyps become expanded the wall is here never 

 much elevated. The polyps, however, are at no time raised far above the 

 corallum. On full expansion they rarely assume the true cylindrical form 

 with a flattened disc, like most coral polyps, but exhibit merely a dome-like 

 elevation of the walls over the calice, 2 or 3 mm. high (plate 6, fig. 31). 



At no place, except in simple larval polyps and at the margin of a 

 colony, is the boundary of the column wall of the individual polyp directly 

 connected with the skeleton; that is, it is nowhere continuous with the 

 skeletogenic basal wall. The margin of the column of any polyp passes 

 uninterruptedly into that of the surrounding polyps, and, as will be shown 

 later, the mesenteries pass from the column wall to the skeletogenic tissues 

 below, leaving narrow interspaces, which place the digestive cavities of all 

 the polyps of a colony in communication. 



7 



