viii THE VOYAGE OF H.M.S. CHALLENGER. 



The advantages of a diverse distribution of the polyps is gained in the higher forms 

 by a different method. The colony, instead of growing over foreign bodies, becomes 

 raised from an attached base, and expands freely. One surface of the colony gives origin 

 to the individual polyps, while the other represents the attached surface of an encrusting 

 form. From mechanical causes, however, the colony does not remain of a flattened 

 shape, but becomes coiled up in a tube-like fashion, so as to bring the polyps to the 

 outside, placing the former basal surface to the interior of the coil. At the same time 

 spicules are differentiated in the coenenchyma and become closely aggregated, so as to 

 form a supporting axis. These conditions are seen in some of the lower forms belonging 

 to the Rriareida3, such as Solenocaidon, while in the higher types the axis is better 

 developed, occupies the interior of the colonial mass, and forms a cylinder, which is 

 surrounded by the polyp-bearing ccBnenchyma. In this way we may suppose that the 

 division of Scleraxonia, of which Corallium represents the highest form, has developed. 



In another series of forms, the favourable distribution of the individuals is otherwise 

 attained. Bundles of polyps, the walls of which have thickened into a common ccenen- 

 chymatous mass, grow out into long cylindrical masses, and develop from their coenenchyma 

 new polyps at diverse heights. These may again form similar polyp bundles. In this 

 way there arise those lobed masses as seen in Alcyonium and Lohulmia, or if but a few 

 tubes are united in each bundle, more bushy or tuft-like colonies as in Nepthyidas. 



Lastly, as another stage in the evolution, we find that a single polyp with a canal- 

 icular coenenchyma grows out into a long cylindrical axial polyp, from the walls of which 

 small polyps with short cavities and long tubular buds arise which may again bear small 

 lateral polyps. Such a condition is exhibited by Telesto among the Cornulariidse. For 

 the development of a very expanded colony the hollow axial polyp forms by itself too 

 weak a support, and there develops a more or less solid horny or calcareous central axis. 

 The forms which thus arise fall into two divisions. The one (Pennatulacea) includes free, 

 the other (Holaxonia or Axifera of v. Koch) more or less fixed colonies. 



The advantages gained by the formation of an upward branching colony may, 

 however, under special conditions become lost. This is seen in some of the axis-bearing 

 Gorgonacea, when they occur at great depths. For here the food is not swept about 

 by movements in the water, but falls from above to the bottom. It would be under 

 these circumstances an advantage if the colony could be formed on the incrusting type, 

 but since organic development progresses and cannot at once revert to an original type, 

 the characters already established have to become adapted to the altered conditions, so 

 we find that the Goi'gonid stem in such instances becomes a creeping polyp-bearing axis 

 and the polyps develop only on the one side, as in some species of Stroplwgorgia, 

 Bathygorgia, and others. In this way all the polyps come to lie in the one plane, and 

 each has an equal chance of obtaining the food as it falls from above. 



And yet in another way may a support for the ascending colony be developed. In 



