26 SIDERASTREA RADIANS. 



in each sextant the third-cycle pair (iii) is situated on the dorsal aspect 

 of the second-cycle pair [ii). In the right middle sextant, where two pairs of 

 third-cycle mesenteries occur, a pair is present on both the dorsal and 

 ventral aspects of the longer pair. It is shown later that the mesenteries 

 of the first and second cycles are developed in a very definite sequence, 

 and such would probably be the case for the members of the third cycle were 

 the polyps isolated and free to develop normally all around. The individual 

 polyps in a colonial coral like Siderastrea^ however, are so closely arranged 



Illb 



I 



KiG. 3. Diagram of mesenteries of fig. 34, plate 6 i^cf. figs. I and a, pp. 12, 13). 



that their growth after the first two cycles becomes largely influenced by 

 spatial necessities. Hence, in the particular polyp from which plate 6, fig. 

 34, is taken, one region, the right middle sextant, has progressed farther 

 than the others. Throughout the studies little constancy has been found in 

 the order of development of the members of the third C3'cle. The amount of 

 variability in the number and disposition of the mesenterial pairs is repeated 

 in the septa (p. 48). In no two polyps, among a dozen or so studied in 

 transverse sections, was the mesenterial plan the same for the third cycle ; 

 also, in none was the full complement of twelve pairs present. The normal 

 sequence of the mesenteries is more fully described in connection with the 

 development of the polyp. 



