492 



MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



It must be borne in mind that the protocnemes are practically established at the time the 

 larva settles, and that no skeletal formation takes place before the sedentary stage is assumed. 

 During the subsequent growth of the coral, the new septa (metasepta) are found to arise in a 

 dorso-ventral succession, approximately pari passu with the pairs of mesenteries, and it may be 

 conceived that if the larva? settled with only a few pairs of mesenteries developed then the primary 

 septa would show a corresponding succession in their appearance. 



In most adult polyps of Porites and Madrepora the protoseptal stage is retained, six ento- 

 ccelic members and six exocoelic forming a larger and a smaller cycle. Very often some of the 

 twelve septa are wanting, and usually one or both of the directive septa are much larger than 



Fig. 12. 

 SfcU rastrsea radians. — Series Illustrating the development of the protosepta in larval polyps. Tin- outer white ring represents the epitheca. 

 a, six equal entosepta are present, developed simultaneously. I. six equal smaller exosepta now occur, also developed simultaneously, 

 and alternating with the six primary entosepta. <■, Successive appearance of the exosepta from the <lors:il to the ventral aspect of the 

 poly]., observed in ;i few cases. 



the Literal, thus exhibiting a bilateral symmetry, corresponding with that of the mesenteries and 

 tentacles. 



The young bud of Madrepora, represented in section on PI. Ill, proves that in asexual 

 growth all the protocnemes may make their appearance before septal development commences, so 

 that it seems not improbable that the protosepta may arise simultaneously in bud as well as in 

 larva] polyps. 



It is manifest from the examination of buds of other genera that the septa occur in accordance 

 with the number of the mesenteries at any particular stage, or at any rate their downward 

 extension is in conformity with the mesenteries present. It is found that as the mesenteries 



