GONIOPORA. 25 



the walls of the central calicles have grown in height, while those (the youngest) round 

 the edges remain nearer to the low-walled primitive type, not having had time to grow high. 

 Interstitial buds naturally appear on the raised central portion owing to the enlarged surface. 

 And this portion tends to rise at a still greater rate than do the edges, which expand laterally 

 for a certain distance and then stop. Whatever be the reason for this, the lateral polyps 

 round the edges of the stock seem to fail to develop in these massive forms. In the explanale 

 stocks it is they which grow on, but in the massive forms it is those of the central regions 

 which develop. 



The polyps of this region rise in the lengthening calicles and secrete a tabular floor, and 

 from this new level they usually throw out a new edge which covers up and kills the 

 backward polyps round the original edge. The epitheca under the new edge is continuous 

 with the tabular floor. This process results in massive forms. The central growth is 

 continuous upwards through the stock, but there are a whole succession of edges one covering 

 over the other and each with its supporting and often overlapping epitheca. Whatever be the 

 specialisation of the deep central calicles, those on these free edges remain more like the 

 primitive low-walled type, though showing that type somewhat modified according to the 

 special modification of the central calicles. Not only do the walls remain thick and the fossa 

 shallow, but the central rosette and the typical septal formula persists, though all these features 

 are apparently completely lost in the peculiar specialisation of the central calicles. 



This gives us a simple and satisfactory explanation of the free edges which are character- 

 istic of this genus, and also of the fact that the " lateral " calicles of Goniopora always show so 

 many of the chief characteristics of the primitive type.* 



Secondary Modifications of the Explanate Growth.— Two modifications are shown by the 

 specimens in the National Collection. PI. XIII. fig. 10 shows an explanate stock on the 

 surface of which small areas have been stimulated to rapid budding and to the throwing up of 

 irregular columns, any parts of which can immediately readopt the explanate method of 

 growth if they come in contact with any foreign body which they can encrust. 



PI. XI. fig. 9 shows an explanate growth the edges of which run out into lobes ; these 

 bend up, curl round into knobs or cylinders, and are left behind on the surface by the further 

 growth of the edge. In one case certainly {G. Great Barrier Beef 12) and probably also in 

 G. China Sea 1, branching tufts are formed in this way. 



Variations on the Massive Growth-Form.— These may be numerous : so far they may be 

 brought under five headings, and they appear to depend solely upon differences in the speed of 

 growth of the central as compared with that of the lateral calicles. 



1. Fig. 2, diagram B, shows the lengthening of the calicles as being fairly uniform, and there 

 results a hemispherical or even globular mass, according as the edges hang do\vn or curl under. 



* These lateral calicles of mas.sive stocks also show a great thickness in the skeletal elements, 

 the columellar tangle frequently becoming quite solid. This is due to the fact that the whole 

 of a perforate colony is practically one organism ; the lateral calicles of massive forms thus become to 

 all intents and purposes basal, and their skeletons are accordingly thickened. 



