CALCAREOUS SKELETON OP THE ANTHOZOA. 541 



culty in admitting the fact that an Alcyonarian spicule can 

 grow by deposition of successive crystalline layers within its 

 sheath, there can be no difficulty in admitting the possibility 

 that the Madreporarian corallura grows in the same way, by 

 addition of new material elaborated by the calicoblasts and 

 passed through the membrane which lies between them and 

 the corallum. I do not wish to push the analogy further, but 

 only to emphasise the fact that an explanation which is satis- 

 factory in the case of such complex bodies as Alcyonarian 

 spicules, with their bewildering variety of form and their 

 complex arrangements of warts and spines, cannot be held 

 to be unsatisfactory in the case of Madreporarian coralla, 

 whose fundamental plan is not strikingly variable, and 

 whose detail is scarcely more complicated than that of the 

 spicules. 



Of course there is no real explanation in either case. We 

 are as ignorant of the laws which govern the formation of these 

 organic crystalline growths as we are of the molecular laws 

 which determine why a given mineral solution shall crystallise 

 out according to a given system. But I enter my protest 

 against the discovery of aDeusex machina in the form of 

 calcified cells. 



I have been mainly occupied in the course of this paper in 

 criticising a particular statement made by Mrs. Gordon, and I 

 may appear to have been unappreciative of her excellent 

 memoir on the structure of Madreporarian corals. I must 

 conclude by saying that I do not conceive that her most 

 important conclusions are affected by the error which she has 

 made in the matter of the calicoblasts. The acceptance of my 

 views on the formation of the corallum would not detract in 

 the least from the importance and truthfulness of her observa- 

 tions on the different types of microscopical and macroscopical 

 structure in recent and fossil Madreporaria, nor would it make 

 them less useful as a basis for an amended classification of the 

 group. Saving this question of calcified ectoderm, and some 

 remarks on the anatomy of Fungia with which I shall have to 

 deal on another occasion, I have found her descriptions of fact 



