THE STRUCTURE AND RELATIONS OF TUBIPORA. 507 



down the tube (fig. 10, M. F.). Each mesenterial filament 

 consists (fig. 10, M. F.) of an enormously thickened, columnar, 

 ciliated epithelium supported by a portion of the mesoderm of 

 the mesentery, but my specimens were not sufficiently well 

 preserved to allow me to enter into any further details of theii 

 histology. 



Formation of the Platforms. — Professor Wright, in his 

 description of Tubipora, says : " I think it is pretty evident that 

 the external tabulae (i. e. platforms) are formed in the first 

 instance as flattened offshoots from the upper edges of the 

 tubes." I am able entirely to confirm this view, as many of 

 the polypes in my spirit specimen exhibit the earliest indications 

 of a platform in the form of a thin rim spreading out from the 

 lip of a polype. This thin rim, as it increases in size, either 

 meets and fuses with other similar rims proceeding from neigh- 

 bouring polypes, or else it simply surrounds the lips of the 

 adjacent polypes and fuses with them (fig. 11). At first the 

 young platforms are quite pale, but soon delicate pink spots 

 may be seen scattered over their surface, and as the platform 

 increases in size and thickness these spots unite together into 

 a delicate network, and eventually the whole surface assumes, 

 to the naked eye, a deep red homogeneous colour. An exami- 

 nation of sections through these young platforms shows that at 

 first the rim consists of a fold of ectoderm containing a thin 

 lamina of mesoderm ; subsequently, however, as the lamina of 

 mesoderm becomes thicker, canals lined by endoderm are pushed 

 into it, and soon ramify in its substance, forming the canal 

 systems of the platforms (vide von Koch 10, fig. 10). 



The ectoderm of the young platform is of the same nature as 

 the ectoderm of the invaginated portion of the retracted polype 

 (vide supra, p. 563, fig. 8.^), consisting of a number of small 

 cells arranged in more than one row, and giving the appearance 

 of being in a condition of rapid multiplication and growth ; the 

 mesoderm, too, does not contain the characteristic pyramidal 

 cells and fibres of the other parts of the polypes, but contains 

 numerous groups of small round cells, which have sunk down 

 into the matrix from the ectoderm. 



