Morphology of the Madreporaria. 143 



leading up to that represented, it was found that new bilateral 

 pairs of mesenteries are ad'Ied within the two directive ento- 

 coeles of an ordinary polyp, provided with only the six pairs 

 of ])rimary mesenteries. Tiie new mesenteries are established 

 in complete or incomplete pairs, in such a manner that when 

 six additional pairs are fully formed the twelve mesenteries 

 on each side of the enlarged polyp are arranged as in an 

 ordinary polyp of Madi-epora^ including the presence of two 

 pairs of directives. The stomodajal tube being also divided 

 all the way, it is clear that when the two moieties of the 

 enlarged polyp eventually separate, along the median plane, 

 each will exactly resemble the original parent polyp, as it 

 aj)peared before any additional mesenteries were formed. 

 Very rarely, one of the tission-polyps may contain more than 

 the usual six pairs of mesenteries, while the other will be 

 quite normal. 



Tlie final product has been found to be the same in the 

 enlarged polyps of Porifes, but, in any individual polyp of 

 this genus, the additional six pairs of mesenteries appear 

 succepsively within only one directive entocoele, which may 

 be either the dorsal or the ventral extremity. Polyps 

 bearing two oral apertures on a single disk seem very rare oa 

 colonies of the West-Indian species of Porites, but are more 

 frequent in Madrepora. 



The fundamental results in the present connexion are as 

 follows: — On any colony of Acropora (Madrepora) and 

 Pontes, a few polyps become much larger than the others, by 

 the appearance of new tentacles, mesenteries, and septa 

 beyond the usual number twelve. In general, the new 

 members conform in size and character with the primary 

 tentacles, mesenteries, and septa, and the additions continue 

 until the polyps are practically double the size of the ordinary 

 polyps, and then fission supervenes. Tlie two resultant 

 iiasion-polyps are exactly like the original, as regards the 

 arrangement and character of the mesenteries and other 

 organs. Half the mesenteries in each polyp are derivatives 

 of the primary polyp, and half are later formations. 



Knlarged and fission-polyps of the gemmiferous, imper- 

 forate coral, Cludocora arbuscula (Lesueur), have also been 

 studied in the same connexion. The polyps of this species 

 usually contain from t^velve to eighteen pairs of mesenteries, 

 and double this number of tentacles and septa, disposed 

 practically in two cycles. The mesenteries are arranged as 

 follows : — Six complete primary pairs, including two pairs of 

 directives ; -six incomplete alternating pairs, forming a second 

 cycle; two to six other pairs, whicli are still smaller and 



