36 CORALS 



worthy evidence concerning the mesenteries of the living 

 poh'p. It may be said, however, that wlien there are six or 

 twelve septa there are twelve and usually not more than 

 twelve mesenteries, and that when there are more than 

 twelve septa there are alwa^^s more than twelve mesenteries. 

 In the method of development of the septa there appears to 

 be a remarkable uniformity in corals belonging to widely 

 separated families ; but in view of the fact that our know- 

 ledge of coral embryology is still very limited, it would not 

 be right to assume that there are no exceptions to what 

 appears, at present, to be a general rule. 



When in the development of the polyp of a solitary 

 coral or the first polyp of a colonial coral the twelve primary 

 mesenteries (protocnemes) have been formed, six septa 

 appear simultaneously in the spaces between the mesenteries. 

 Two of these six septa are always found in the spaces between 

 the directive mesenteries and may be called the directive 

 septa, and the others in alternate intermesenteric spaces. 

 These are followed by another six septa situated in the 

 remaining six intermesenteric spaces. 



Twelve septa are thus formed, and, in conformity with 

 the terminology of the mesenteries, these twelve primary 

 septa may be called the " protosepta." 



In some cases (Astroides, Balanophyllia, etc.) the twelve 

 protosepta appear simultaneously, but there are many 

 reasons for believing that the former method of septal 

 sequence is the more primitive. 



The formation of the metasepta in the corals of Group 

 A follows very closely the formation of the metacnemes, a 

 single septum appearing in the space enclosed by a uni- 

 lateral pair of metacnemes. But in some cases septa are 

 also formed in the spaces between the unilateral pairs of 

 mesenteries, and thus a distinction has been drawn between 

 the septa that are formed inside a pair of mesenteries (Ento- 

 septa) and those that are formed between these pair (Ecto- 

 septa). Further discussion of the very complex problems 

 of septal sequence in recent and fossil corals would require 

 more space than can be assigned to the subject in this book. 



With these preliminary remarks on the points of structure 



