POSTLARVAL DEVELOPMENT. 85 



only after examination of a number of polyps. In a colony in which the 

 polyps are so closely arranged as in S. radians it is found that the individuals 

 rarely undergo their later development with perfect regularity all round- 

 some regions will be in advance of the normal sequence and others behind. 

 The polygonal form assumed by the adults is evidence that pressure is 

 exerted upon a form which otherwise would be circular, as in simple polyps 

 reared from larvae. Spatial difficulties may therefore be held sufficient to 

 account in a large degree for the many irregularities obtained in the estab- 

 lishment of the third mesenterial cycle. 



The mesenterial plan of two other polyps is given in figs. 3 (p. 26) and 

 1 1 (p. 100), and illustrates the variability encountered. In fig. 3 the sequence 

 is normal except that a pair of mesenteries (me) has appeared within the 

 ventral exoccelic chamber of the right middle system in advance of the pairs 

 in the dorsal exocceles of the ventral systems. The polyp represented in 

 fig. ii presents many departures from the normal regularity ; two third-cycle 

 pairs occur within three systems, one pair within another, and two systems 

 are without any third-cycle pairs. 



In Astrangia solitaria and Phyllangia americana, where the polyps 

 are practically separated from one another and retain their cylindrical form 

 throughout, the regularity of development all round is more pronounced, 

 and the general order of appearance of the mesenteries established in S. 

 radians is found to be maintained from beginning to end (1902, p. 459). 



Hitherto, the development of the third-cycle mesenteries has not been 

 actually followed either in actinians or corals. Faurot's studies in 1895 were 

 confined mostly to the tentacles. Carlgren* has described a condition of the 

 mesenteries in the actinian Condylactis cruentata, in which the twelve pairs 

 of third-cycle mesenteries as a whole, as well as the exocoelic chambers in 

 which they are situated, show a gradual decrease in size in passing from the 

 dorsal to the ventral border of the polyp. Pairs ma and mrf in the enumera- 

 tion of fig. 8, k, p. 82, were larger than pairs nib and ine, and these than 

 pairs me and in/; hence, if the condition obtained by Carlgren really repre- 

 sents the sequence followed in the growth of the third-cycle mesenteries in 

 Condyladis, it is altogether different from that of corals, being simple instead 

 of twofold. 



I believe it will be found in corals generally that the sequence of the 

 later mesenteries is by no means so regular as that of the earlier cycles. 

 The order followed by the organs in the first and second cycles is fairly con- 

 stant, but this can not be asserted of the third cycle, and probably the regularity 



*"Zur Mesenterienentwicklung der Aktinien," Ofvers afR. vet.-Akad. Forh., Stockholm, 1897. 



