38 GILBERT C. BOURNE. 



the cork to which they were attached. A diagrammatic 

 drawing of a section made in this way, tangential to the 

 surface of the polyp, is given in fig. 13 ; it is slightly modified 

 from von Koch's original drawing for the sake of greater 

 clearness. 



Previous to von Koch's researches on Astroides, von Heider 

 ('Sitz. der Kais. Akad. in Wien./ lxxxiv, 1881) had shown that 

 in Cladocora there exists everywhere between the structureless 

 mesogloea and the corallum a layer of rounded cells, which 

 apparently have the function of secreting the coral substance ; 

 to these cells von Heider gave the appropriate name of calico- 

 blasts, and considered them to be a product of the " meso- 

 derm " (mesogloea), the layer from which he supposed the 

 corallum to be derived. It is clear, however, from von Koch's 

 researches, that the layer of calicoblasts is nothing more than 

 the persistent ectoderm which secretes the corallum, and lies 

 everywhere between it and the mesogloea. Unfortunately we 

 are not acquainted with the development of any Madreporarian 

 other than Astroides, but since calicoblasts have been found in 

 all corals recently examined, we may assert with certainty that 

 the corallum is a product of the ectoderm and is 

 always external to the polyp. 



In describing the formation of the theca, the septa, and the 

 costae, the statements of de Lacaze Duthiers and von Koch 

 differ widely from one another. The former states distinctly 

 that the theca appears independently of the septa, the latter 

 arising as twelve radiating rods, bifurcated at their peripheral 

 extremities. As far as the number and shape of the rudiments 

 of the septa are concerned, von Koch's statements are in ac- 

 cordance with those of de Lacaze Duthiers, but he gives a very 

 different account of the formation of the theca. The skeleton, 

 he says, first appears as a ring-shaped basal plate, incomplete 

 in its central portion, this plate always making its appearance 

 between the basal ectoderm and the surface to which the larva 

 is attached. It is composed of an aggregation of small spheri- 

 cal nodules, each of which is made up of rhomboid calcareous 

 crystals grouped in concentric layers. As growth proceeds the 



