THE ANATOMY OF THE MADREPORARIA. 3 



of the apical polyp. All other polyp cavities converge towards, and, 

 by means of canals, eventually open into, this central cavity, but no 

 more definite connection is traceable. Tissues not unlike mesenteries 

 are sometimes visible in it, but the alcohol in which the specimens 

 were killed did not penetrate sufficiently rapidly to preserve the 

 central parts in good histological condition. In some sections the 

 six septa are not recognisable, and the axis of the branch is occupied 

 by a wide-meshed network of coral ; this is probably due to re- 

 absorption of part of the skeleton. 



In transverse section are also seen concentric series of longitudinal 

 canals (c 2 ) permeating the corallum ; their arrangement appears to 

 indicate that the radial growth of the branch is effected in the 

 following manner. Directly beneath the external body wall of the 

 colony a series of longitudinal canals runs between the costse 

 (Fig. 4, c 1 ) ; and it is probable that, for increase in the diameter of 

 the branch, the costse grow outwards, and then, bulging laterally, 

 fuse over these canals, so as to enclose them entirely in corallum 

 (cf. Fig. 10, x). Thus there results a series of internal longitudinal 

 canals, concentrically arranged, with radii of coral between them 

 which represent former costse. Not only does the appearance of 

 such a transverse section as Fig. 3 suggest that this is the mode of 

 growth, but also " dark lines of growth " (Fig. 5) run radially from 

 each costa towards the centre, so continuously as to indicate that 

 what was a costa when the diameter of the branch was very small, 

 has continued to grow as such, and to be still such when the 

 diameter is very much larger. New costse, when required owing to 

 the increased circumference of the branch, appear to take their origin 

 from the point of fusion of previous costse. 



More minutely, growth is effected, presumably by the activity of 

 calycoblast cells, through the addition to and formation of crystalline 

 ellipsmds, similar to those described by von Koch in Stylophora (2). 

 These ellipsoids have a distinct sweep from one " line of growth" to 

 the next. 



The calyces are all of approximately the same size, and that so 

 minute as to render investigation of the anatomy difficult. 



The septa are very irregular of occurrence ; the complete number 

 appears to be six, but three are rarely to be seen in one section^ 

 often none at all. They are not constant through the whole depth 



