16 Ediuund B. Wilson 



direct. That is, the buds do uot as a rule arise as direct oiitgrowths 

 frolli older polyps , bat by means of an interposed network of entoder- 

 mic canals. This is shown for instance in fig. 3, where the young polyp 

 is Seen to arise in the middle of a canal-system which communicates 

 with the cavities of older polyps. 



Now we find , as a rule, that the canal-system communicates with 

 the basal part of the zooid or young bud (seefig. 13), andhence the 

 food-supply must enter at this part of the bud. The most active part of 

 the bud or zooid is however at the opposite end where the powerful 

 ciliated cells of the Oesophagus, which draw in the water, are in a state 

 of intense though interraittent activity, and where the organs of the 

 young polyp are developing. It seems therefore clear that it must be 

 advantageous to have strong and steady currents flowing from the basal 

 to the upper part of the stomach-cavity — and this is effected by the 

 ectodermic bands of the dorsal filaments. In some cases these organs 

 actually extend out into the canal system. In fig. 5 [KopJiohelemnoti) 

 the enormously developed dorsal filaments, d.f.^ extend far out into 

 the canal-system. The Communications of this system with the main longi- 

 tudinal canals of the colony are shown at a, a, a, and an iuspection of 

 the figure will show, I think, that the upward currents aloug the ectodermic 

 bands must tend to transport the nutritive fluids from the canal system to 

 the upper portion of the bud where development is actively going forward. 

 In other cases the dorsal filaments may be followed down to the very 

 mouths of the canals, but there stop, as I have observed in Gorgonia. 



As a rule, however, the dorsal filaments extend only to the base of 

 the polyp , as shown in fig. 13, but even bere it seems clear that their 

 action must tend to bring about the same result. In the adult polyps 

 this is always the case , so far as known , and we can now understand 

 the object of this arrangement. It must often happen that certain polyps 

 of a colony òbtain a larger supply of food than others , though it is 

 clearly advantageous that ali should be equally well-fed. The nutritive 

 fluids derived from these polyps must pass iuto the canal-sj^stem, though 

 we are entirely in ignorance as to how this is effected — whether by 

 the contraction of the polyps, or by a reversed action of the cilia, or by 

 a specialisation of certain canals for this purpose. But, however this 

 is effected , it seems pretty clear that the less well-fed polyps profit by 

 the fact that the ciliated bauds extend down to the region where they 

 communicate with the canal-system. 



We may now inquire after the cause of the exceptions to the gene- 

 ral rule that the dorsal filaments are the first to appear in the buds, and 



