THE STRUCTUHB AND RELATIONS OP TUBIPOPtA. 663 



Towards the free end of the corallite the walls become much 

 thinner, and above this, in spirit specimens, perfectly free 

 spicules can be found scattered in the mesoderm. I have 

 nothing further to add to the excellent description of these free 

 spicules given by Professor Percival Wright (26). 



The tabulae consist of a simple network of spicules, the extent 

 to which the spicules are joined together being much less than 

 it is in the wall of the corallite (fig. 7). 



II. Anatomy of the soft parts of Tubipora purpurea. 

 — When a transverse section is made through the wall of a 

 polype of Tubipora from Avhich the skeleton has been removed 

 by decalcification, it is seen to be composed of three principal 

 layers (figs. 9, 10, and 12). Externally there is an ectoderm 

 (^^2) composed of a single row of oval cells situated with their 

 longest diameter parallel with the layer of mesoderm upon 

 which they rest ; internally there is an endoderm {^Pi com- 

 posed of two or three rows of spherical cells, and between the 

 endoderm and ectoderm a mesoderm composed of a homo- 

 geneous jelly-like matrix containing a few scattered mesoderm 

 cells (c) and fibres (f) ; and in this mesoderm are seen a 

 number of large spaces (s s) occupied by the spicules before 

 decalcification. The ectoderm is composed all over the surface 

 of the polypes of a single row of oval cells in which, even after 

 prolonged staining in borax carmine, I have been unable to 

 discover any nucleus. In places, especially in the older parts 

 of the polyps, the ectoderm is entirely destroyed by parasites 

 of which endless different kinds both animal and vegetable 

 may be found (see figs. 9 and 10 d a.^). 



Where the ectoderm is invaginated into the corallite (fig. 8.^) 

 its character changes. In the first place the cells are consider- 

 ably smaller in size. Whilst outside their longest and shortest 

 diameters are respectively "03 and '02 mm., in the invaginated 

 portion they are never larger than '01 by '003 mm. In the 

 second place, there are two or three rows of cells instead of only 

 one, and cells of the most superficial row by being elongated 



' For a description of the Foraniinifera infesting Tubipora see Carter (3). 



