12 G. HERBERT FOWLER. 



future polyp cavity before it is perforated for communication between 

 the coelenteron and the exterior, and also apparently before any 

 mesenteries are formed. The cavity into which it is invaginated is 

 already of considerable diameter, and larger than the ordinary canals 

 of the colony ; though smaller than that of a fully formed polyp, at 

 that point it is probably never enlarged by re-absorption of coral, but 

 its continuation upwards by future growth of the polyp possesses 

 a gradually increasing diameter. 



In a young polyp in which the stomatodseuin was invaginated, 

 but not yet perforated below, the latter appeared to be supported by 

 tissue surrounding the future septa, just as the external body wall 

 is supported by tissue enclosing the costse. In sections below the 

 stomatodseum, and unconnected with it, were seen two small mesen- 

 teries with filaments, which appeared to be growing upwards towards 

 the stomatodseum, and to have not yet joined it. It is therefore 

 possible that these grow upwards from the canal system, and are 

 formed quite independently of the rest of the polyp. This view is 

 further supported by the observation that, in sections quite at the 

 top of a branch, above the plane of any lateral polyps, occur in the 

 canals one, sometimes two, little mesenteries with filaments, which 

 I believe to be growing upwards towards the sites of future polyps. 

 They appear to take rise, near the cavity of the apical polyp, from 

 the wall of the canals. 



In the only other stage of development from which any observa- 

 tions could be made, six mesenteries had appeared ; of these the two 

 furthest from the axis carried muscles on the outer faces, though it 

 does not necessarily follow that they were the abaxial " directives " 

 of the adult. The muscles of the other two pairs were not sufficiently 

 developed to allow of their arrangement being recognised. 



Conclusion. — From M. DurvUlei, the present species is widely 

 separated by a strong morphological distinction, the absence of 

 dimorphism ; since the difference between the apical and lateral 

 polyps in M. aspera is hardly strong enough to be reckoned as such. 

 That such a distinction should exist between two species of a genus is 

 very remarkable • but, considering the great antiquity of these forms, 

 the similar structure of the colony in both, and the fact that they 

 exhibit a similar differentiation of certain mesenteries, it is not to be 

 inferred that their systematic relations are unsound. 



