94 J- E. DUERDEN. 



A further stage secured in the growth of the twelve pairs of 

 third cycle mesenteries is given in Fig. Sr, where a pair has ap- 

 peared on each side within the ventral exocoele of the dorsal sex- 

 tant. Clearly, if the succession thus indicated were followed with 

 perfect regularity, other pairs would appear within the ventral 

 exocceles of both the middle and the ventral sextants, and the 

 cycle would then be completed according to Fig. Sd. No stage 

 exactly corresponding with Fig. Sd, however, has been obtained, 

 as the polyps of >$. radians very rarely, if ever, complete the third 

 cycle of mesenteries. Still the sequence so far as it can be traced 

 is such as to warrant the conclusion. 



The regularity in the dorso-ventral sequence of the mesen- 

 teries shown in Fig. 8 was secured only after an examination of 

 a number of polyps. In a colony in which the polyps are so 

 closely arranged as in 6". radians the individuals are rarely found 

 to 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 a certain pressure is exerted upon a form which 

 would otherwise be circular, as in the simple polyps reared from 

 larvae. Spatial difficulties may therefore be held sufficient to 

 account for the irregularities appearing in the growth of the third 

 mesenterial cycle. In Astrangia solitaria and PJiyllangia Ameri- 

 cana, where the polyps are practically free from one another, and 

 retain their cylindrical form throughout, the regularity of devel- 

 opment from one border to the other is more pronounced, and I 

 have found (1902, p. 459) the order of appearance of the mes- 

 enteries to be the same as that established for 5. radians. 



The normal sequence of the third cycle mesenteries in Sidcr- 

 astrea being now established we are justified in assuming that a 

 like succession will be maintained by the third cycle septa, as 

 individually the septa arise shortly after the mesenteries with 

 which they are associated. Hence the normal sequence for the 

 members of the first three cycles of entosepta will be that repre- 

 sented in Fig. 9. The six septa of the first order of entosepta 

 (I.) appear together as a cycle ; the six members of the second 

 (Il.aIl.c) follow a simple dorso-ventral succession ; the twelve 

 members of the third order (Ill.rt-III/) also appear in a dorso- 



