OCTINEON LINDAHLI. 473 



special localisation of digestive cells on ridges (which may be 

 compared to a typhlosole) would lead to a concentration of 

 reproductive cells in their neighbourhood for a better nutri- 

 tion ; and that the general musculature of the body-wall, pro- 

 bably originally both circular and longitudinal, like that of 

 Hydra, might, when carried out along the ridge, become 

 specialised on one side into retractor (longitudinal), and on 

 the other side into protractor (circular) muscles. 



The alternative hypothesis — that the ridges or mesenteries 

 grew out for support like buttresses, and that the concentra- 

 tion of digestive and reproductive cells occurred on them 

 secondarily — does not seem so probable, and has neither 

 analogy nor observation in its favour ; but the identical con- 

 centration of the three functions in Coelenterate groups which 

 have evidently been long independent of one another (e. g. 

 Madreporaria and Alcyonaria), and the analogy of a typhlo- 

 solar increase of absorptive and secretive surface, occurring in 

 many divisions of the animal kingdom, are in favour of the 

 idea first suggested. The way in which one function may be 

 dropped and another retained is well seen in the Ceriantheae, 

 where the contractile function has been taken up (? per- 

 manently retained) by the body-wall, and digestive alternate 

 with reproductive mesenteries. Any mesentery may become 

 specially utilised for a particular function ; in Seriatopora and 

 Pocillopora (Fowler, 'Quart. Journ. Micr. Sci.,' xxviii, 1) the 

 six mesenteries specialised for digestion and reproduction are 

 precisely those six which in Madrepora pocillifera and 

 tubigera are arrested, are sterile and devoid of filaments^ 

 (Fowler, 'Quart. Journ. Micr. Sci.,' xxvii, 1). 



* These two species of Madrepora were originally identified for me as 

 belonging to the species M. aspera and Durvillei. In the recent 'Cata- 

 logue of the Madreporarian Corals in the British Museum,' vol. i, 'The 

 Genus Madrepora ' (Lond., 1893, 4to), the author, Mr. George Brook, whose 

 premature death has recently deprived zoology of a careful student of the 

 Anthozoa, has assigned them to the species quoted in the text. 



In this case (Pocilloporidse and Madreporidse) I think we may safely com- 

 pare the twelve mesenteries of the polyps concerned, because of the orienta- 

 tion afforded by their axes when compared with the axes of the colony and 



