the Morphology of the Bugosa. 469 



If the bilateral symmetry is the result of the bending, we must 

 imagine a young form, having a hereditary tendency towards radial 

 symmetry, varying from its ancestral type by growing in a curved 

 manner instead of straight. This curved growth became fixed in 

 its descendants, the curved form having been selected owing to some 

 advantage it gained over its straight fellows, possibly by presenting 

 a mouth more or less perpendicular to a horizontal current bearing 

 the food supply. The curvature of the corallum, caused perhaps 

 by unequal growth, perhaps by the falling over of the coral as its 

 bulk increased, or bj' some unknown agent, induced bilateral 

 symmetry in the soft parts by causing them to gravitate towards 

 the lower side of the mouth opening. The skeleton thenceforward 

 secreted was, therefore, bilaterally symmetrical, because it could only 

 take the shape of the soft parts by which it was formed. This is 

 in the main H. M. Bernard's view, who says that the formation of 

 the fossula would be a natural outcome of this gravitation of the 

 soft parts. 



On the other hand, Duerden has suggested that the bilateral 

 symmetry arose from the abortion of two of the six original primary 

 septa. The ancestors of the Eugosa (yet to be discovered !) were 

 radially symmetrical Hexacorals. Variations in these, whereby two 

 primary septa failed to grow above a certain size, were seized upon 

 by natural selection, for the possession of some unknown advantage, 

 and became fixed. The two defective septa were those primaries 

 adjacent to the counter septum. Moreover, in the loculi between these 

 and the counter septum no secondaries appeared. By subsequent 

 growth the secondary septa grew as large as the two defective 

 primary septa, and so the appearance of four primary septa arose. 



Of course the abortion of two septa may have been caused by 

 the bending of the corallum, in which case the two hypotheses 

 would only differ in that in the former a radially symmetrical 

 Tetracoral was the original ancestor, while in the latter a radially 

 symmetrical Hexacoral. And the settlement of this point might 

 have some bearing on the question whether the Hexacorals are the 

 descendants of the Eugosa, or whether both had a common ancestor, 

 and, if the latter, whether this ancestor was a Hexacoral or a 

 Tetracoral. 



Whatever view of the origin of the bilateral symmetry is taken, 

 the fact remains that the position of the main septum with regard 

 to the bending of the coral has no connection with the origin of 

 that bending. It is only suggested in explanation that the com- 

 parative rarity of corals having the main septum on the concave 

 side may be due to the fact that in these the lower part of the 

 mouth opening might be brought too near the sea-bottom to be clear 

 of the injurious sediment rapidly accumulating thereon. And, 

 further, uncertainty is expressed as to the value of the position 

 of the main septum as a systematic character ; whether to regard 

 it as of specific or of varietal importance. 



A new explanation of the origin of the fossula is suggested. 

 There would be a natural tendency for the soft parts to sink in the 



