ANATOMY OP MUSSA AND ETJPHYLLIA. 37 



deeper layers of the ectoderm. This view, which for a long 

 time held ground, was disputed by de Lacaze Duthiers, who 

 states in his paper on the development of Astroides caly- 

 cularis, that the calcareous tissue first makes its appearance 

 in the endoderm 1 ('Arch, de Zool. Exper. et Gen./ ii, 1873, 

 p. 269). But since he did not recognise the existence of a 

 third structureless layer — the mesoglcea — between the ecto- 

 derm and endoderm, and since his observations were not made 

 by means of sections, de Lacaze Duthier's statements on this 

 head are somewhat perplexing. It seems clear, however, that 

 he considered the calcareous tissue to be deposited in the 

 situation which further researches showed to be occupied by 

 the mesogloea, and it was from this layer that the corallum 

 was thought to be formed by succeeding writers on the subject 

 for some years. 



The more exact relations of the corallum to the polyp have 

 been chiefly worked out by G. von Koch, to whose works 

 complete references will be found in my own and Fowler's 

 papers on corals in this Journal. 



From his observations on the development of Astroides 

 calycularis, von Koch established the fact that the corallum 

 is a product of the ectoderm, secreted by it, and makes its first 

 appearance between the basal ectoderm and the surface to 

 which the young Astroides is attached. His method of obser- 

 vation is at once ingenious and convincing. Pieces of cork 

 were floated in the aquarium, to which free swimming larvse 

 soon attached themselves. As soon as it was apparent that 

 a deposit of calcareous matter was being formed the embryos 

 were killed in situ, and sections were cut through them and 



1 Fowler is in error in attributing to de Lacaze Duthiers the statement 

 that the corallum is formed externally to the polyp. It is true that the figure 

 to which he refers (fig. 27 of the memoir above quoted) fully bears the inter- 

 pretation which he has given to it, but de Lacaze Duthiers' own words show 

 that he took a very different view of the matter : " O'est au milieu, et dans 

 l'epaisseur de cette couche interne, toujours plus epaisse dans le milieu et au 

 bas des loges que se montrent les premiers nodules calcaires," and elsewhere 

 he expressly defines what he means by the " couche interne ;" it is the en- 

 doderm. 



