68 GUIDE TO THE COKAL GALLERY. 



The tissues seldom remain soft ; they become impregnated with 

 horny matter or with carbonate of lime, or both. The horny skeleton 

 is continuous ; the calcareous consists of separate spicules more or 

 less closely packed. 



The differences at different ages in the amount of lime deposited 

 are well shown by the fine series of specimens of Isis (Case 14). 

 Colonies formed by budding and provided with a skeleton may 

 become of great length, as Juncella, or of great intricacy of inter- 

 lacement, as Gorgonella ; the fine GorgonelJa verriculata from Zanzi- 

 bar should be noticed ; often tliey are of exquisite beauty, as the 

 Calligorgia from Mauritius (Fig. 5) or the Hooherella from South 

 Japan suffice to show. Sometimes there is a continuous skeleton, as 

 in the noble red coral of the Mediterranean, good specimens of 

 which, showing the coral polyps expanded, and explanatory diagrams 

 of which are exhibited in Case 13. 



A particularly dense skeleton is developed in Heliopora (Fig. 6), 

 the only living member of a group (Coenothecalia) which formed a 

 large part of the coral fauna of Palaeozoic times. Long considered 

 to be a Zoantharian, the affinities of Heliopora to the Alcyonaria 

 were demonstrated by the late H. N". Moseley during the voyage of 

 H.M.S. Challenger. This skeleton is remarkable for being always of a 

 blue colour when collected on a reef ; but, as Mr. Stanley Gardiner's 

 collections show, the blue colour gets progressively paler as specimens 

 are obtained from deeper and deeper water. 



In other Alrgonaria a reduction of the skeleton seems to have 

 occurred, so that the axis is friable and breaks up into scattered 

 spicules ; this is the case with Paragorgia arhorea, a fine example of 

 which from the coast of Norway is shown with an illustrative 

 drawing as it appears during life, and a well preserved piece with 

 the polyps partly extended. 



In the Pennatididae reduction goes still farther, and little is left 

 ill the way of a skeleton save a horny axis, which extends along 

 the whole of the colony ; a striking example is to be seen in the 

 specimen of Osteocella on the south wall of the Gallery. 



Fine examples of Pennahila, Fimintlina, and others are shown, 

 as well as two beautiful plates of UmlelluJa encrimis, taken from the 

 Report of the Norwegian North Sea Expedition. The curiously 

 modified and kidney-shaped Renilla should be noticed. 



The general plan of the structure of the Octocoralla is shown by 

 Mr. Berjeau's water-colour drawing in the Gallery, which is shown, 

 reduced to a third, in the accompanying figure (Fig. 7), where we 



