14 SIDERASTREA RADIANS. 



confusion may arise ; indeed, the polyps were for a long time under observa- 

 tion before any regular arrangement could be established with certainty. 

 It was only after observing the Jamaica polyps under various phases of 

 expansion that the cyclic plan of the first tentacles could be determined, 

 while, as regards the two outer cycles, such an arrangement is more 

 theoretical than actual. The tentacles on plate 6, fig. 32, from a camera 

 drawing of a preserved polyp, do not readily suggest a hexarneral cyclic 

 plan. 



Siderastrea is apparent!}' the only madreporarian genus with true 

 dimorphic tentacles, the peculiarity having been first recognized by Agassiz. 

 The independent origin of the two moieties of the bifurcation, as described 

 later, is also a remarkable feature in the tentacular development of the 

 Zoantharia. Dimorphic tentacles are occasionally met with among the 

 tropical Actiniaria, e. g., Rhodactis, Phymanthus ; but here they are the disc 

 tentacles as compared with the marginal tentacles which show the distinc- 

 tion, and the morphological value of the former is uncertain. At any rate, 

 the disc and marginal tentacles of actiniaus are not comparable with the 

 entotentacles and exotentacles of Siderastrea. 



DISC AND MOUTH. 



The naked portion of the disc is very limited in extent. It is smooth, 

 circular, and usually partly depressed within the calice. So transparent is 

 the wall that the cyclic arrangement of the internal mesenteries and septa 

 can be easily followed. The central part or peristome is not separable from 

 the rest of the disc, except that it is usually elevated in a conical manner, and, 

 owing to a greater concentration of the endodermal Zooxanthellae, it appears 

 darker than the rest of the polyp. 



The mouth is small and slit-like when closed, oval or circular when 

 open. The lips are not specially marked off from the peristome by any 

 tumidity, such as occurs in many anemones. In living polyps the mouth 

 sometimes closes along the middle by the approximation of the lips, but 

 leaves a small opening at each end. The apertures, however, can not be 

 regarded as representing a siphonoglyph or gonidial groove, for microscopic 

 examination reveals no difference in the character of the stomodseal wall 

 between the sides and extremities. On narcotization the circular muscles of 

 the disc relax, and the oral aperture remains widely open and circular; the 

 stomodseal wall is then seen to be smooth, without ridges and grooves. 



