12 



SIDERASTREA RADIANS. 



in the second the penultimate cycle is assumed to be complete, whereas it is 

 made up of two different series of tentacles entotentacles and exotentacles 

 and simply represents an arrested stage in the hexameral plan. 



Fig. i is a diagrammatic representation of the arrangement of the tenta- 

 cles in a polyp of Siderastrea with the morphological enumeration added. 

 In the two inner cycles (i, n) six bilobed tentacles are complete, while only 

 five tentacles of the third cycle (in) are yet developed ; alternating with the 

 members of the three inner cycles of bifurcated tentacles is a simple tentacle 



Tic 



FIG. i. Diagram showing the relationships of the septa and tentacles in the same 

 polyp as that from which fig. 34, plate 6, was taken. All the entotentacles are 

 shown with a double apex and the exotentacles with a single apex. The exo- 

 tentacles are all arranged as if forming the fourth or outermost cycle, whereas 

 some will belong to the third and some to the fourth cycle, according as a third 

 cycle entotentacle is present or not. 



(x) belonging to the outermost series, but not forming a true cycle. The septa, 

 indicated by the thick radiating lines, show a corresponding arrangement. 

 For purposes of comparison the same polyp is also shown in transverse section 

 on plate 6, fig. 34, while the septa are diagrammatically represented in fig. 2, 

 p. 13. By comparing figs, i, 2, and plate 6, fig. 34, it is seen that the three 

 inner cycles of tentacles communicate with the entocoeles and correspond with 

 the entosepta, while the members of the outermost cycle communicate with 

 the exocceles and correspond with the exosepta. 



The tentacles of S. siderea are of the same character as those of S. radians, 



