10 G. HERBERT FOWLER. 



be digestive in function, as is probably the case, A might 

 certainly be termed a " gastrozooid/^ 



But at present any explanation of the function of the struc- 

 ture above described, cannot be other than a mere speculation. 

 It cannot be regarded as a necessary result of the colonial 

 habit, since nothing similar occurs in the next species to be 

 described — M. as per a. It can hardly be connected with re- 

 production, as ova are of rarer occurrence in the modified than 

 in the unmodified polyps ; and an excretory apparatus is not 

 required by an organism whose cells are capable of amoeboid 

 activity, egestion as well as ingestion. 



The only evidence on the point is derived from the distribu- 

 tion of the zooxanthellse. These are most plentiful, firstly, in 

 the external canals just under the body wall; and secondly, 

 among the elongated cells of the mesentery. Assuming, as we 

 may fairly do, that nutriment and aeration were the determin- 

 ing factors of such distribution, it would seem that, in the first 

 case, there must be a strong current of nutritive " chyle- 

 aqueous fluid" (to use a word of the older zoologists) in these 

 external canals, and that aeration was effected by diffusion of 

 oxygen through the body wall from the surrounding medium ; 

 and in the second place, that the elongated vacuolated cells of 

 the mesentery were in some way assimilative, while oxygenation 

 of the tissues for these special digestive processes (and there- 

 fore secondarily and accidentally to the benefit of these 

 symbiotic algse), resulted from a constant stream of water 

 flowing through the central ectodermal canal of the 

 mesentery. 



That such a stream does pass through this canal is extremely 

 probable, for the longer ectodermic cells are all morphologi- 

 cally on the same side of the canal ; a wave of ciliary action 

 must therefore result in a current through the canal from one 

 of the apertures into the stomatodseum towards the other. 

 A comparison of fig. 8.7 with fig. 6 m will explain this arrange- 

 ment of the cells. 



It is interesting to note that in M. Durvillei, as in 

 Alcyonaria and Antipatharia, two mesenteries are distinguished 



