Structure of Heteropora neozelanica, Busk. 337 



sections, and have no doubt that they are in this species 

 really present throughout the whole of the peripheral part of 

 the skeleton. Their special interest arises from their being 

 in appearance precisely similar to the " septal spines " of so 

 many species of Favosites (using the term " septal " in its 

 proper signification). 



(b) Transverse sections. — The appearances presented by 

 transverse sections vary according to the part of the section 

 which may be looked at. The central portion of such a 

 section exhibits the tubes in the axial portion of the branch 

 divided at right angles. In the circumference of the section, 

 on the other hand, the tubes are divided more or less nearly 

 longitudinally, owing to their curvature on nearing the sur- 

 face, while this part also shows them in the thickened condi- 

 tion which they possess in the cortical portion of the branch. 

 The appearances presented by the periphery of transverse sec- 

 tions are therefore the same as those shown in the correspond- 

 ing region in longitudinal sections, and need not be considered 

 till we come to speak of the latter. In the central region of a 

 transverse section (fig. 2, B) we can study the condition of the 

 tubes in the axis of the branches before they bend outwards 

 to the surface ; and we find that their structure is very different 

 from that which they possess in the cortical region (as seen in 

 tangential sections). Instead of being rounded and thick- 

 walled, and provided with a largely developed canal-system, 

 they are now thin-walled, and angular or polygonal in shape, 

 and the canaliculi of the wall seem to have totally (or almost 

 totally) disappeared. There is also an apparent total absence 

 of the radiating spines which are developed in the cortical 

 part of the tubes. Lastly, the tubes in this region appear to 

 be almost entirely, or entirely, referable to the proper zocecia, 

 the interstitial tubes or cancelli existing only, or mainly, in 

 the cortical region. 



(c) Longitudinal sections. — These show precisely the same 

 differences, as regards their central and peripheral portions, as 

 have been already noted in transverse sections ; but it is now 

 necessary to briefly direct attention to both parts of the sec- 

 tion (fig. 2, C and D). In the central portion of the section 

 (supposing the slice to be taken in the median plane) the 

 tubes are seen in the axial portion of their course, where they 

 are nearly vertical, and where they exhibit the features which 

 I have pointed out as characterizing them in the central 

 region of transverse sections. That is to say, they are here 

 provided with thin and delicate walls, in which the canal- 

 system of the cortical region seems to be very slightly deve- 

 loped or wanting. The chief point to notice about the tubes 



