of the Organ-pipe Coral. 381 



a. The horny axis of Gorgoniche and Antipathidse, and 

 the homy internodes of Isis. 



b. The more or less calcified lamellose axis of Gorgonida; 

 (Prtmnoa, Plicaurella, Isis, &c.) and Pennatulidae. 



3. Crystalline structure, which seems to increase through a 

 deposit of chalk from a preexisting structure, as, after the 

 removal of all the salts, there is still left a small, almost 

 inappreciable organic residue. Here are placed : — 



a. The greater number of those polyps with merely super- 

 ficial skeletons (Tubipora) ; and 



b. Structures like the chalky skeletons of the Madrepores. 



The structure, however, of the skeleton of Tubipora, as will 

 be seen from the above, is certainly not crystalline ; and the 

 manner in which it is deposited differs in no essential parti- 

 cular from that described in section II. 1. Fusiform spicules 

 are secreted by the ectodermic layer ; these spicules around 

 the base of the tentacles are of a white colour, and in many 

 cases are simply fusiform, not warty ; but those at a little 

 distance from the base of the tentacles not only assume a light- 

 red colour, but become crowded over with warty excrescences, 

 and there is then to be found a gradual growing; together and 

 consolidation of those around the edge of the tube — that is, 

 where this is formed. In the case of a young bud, there is 

 at first no tube, the spicules having not yet become coalesced ; 

 they are here simply placed side by side. 



I regret very much that I had no opportunity of watching 

 the development of the egg of Tubipora, or even of seeing the 

 formation by budding of the attached zooid forms. From an 

 examination, however, of a large series of specimens, it is, I 

 think, pretty evident that the external tabula? are formed in 

 the first instance as flattened offshoots from the upper edges of 

 the tubes. Thus in many instances flat plates will be found 

 to project from the upper and still soft portion of the tube ; this 

 plate will consist of a fold of ectoderm, into which some of the 

 endodermic layer is tucked : spicules are freely secreted in the 

 outer layer of this fold, which is of a bright-red colour ; and in 

 one or two instances a small swelling was seen to arise from 

 the free end of this lateral fold-like prolongation of the tube. 

 I have little doubt that these swellings were the starting- 

 points of fresh polyps. It must not be forgotten that while in 

 some masses of Tubipora the skeleton-tubes were all close to- 

 gether, and the polyps all on the same level, in many others 

 the masses vere very much less compact and the polyps were 

 growing in an irregular manner. 



The polyp certainly can and does constantly add to the 



