236 THE VOYAGE OF H t M.S. CHALLENGER. 



EN. Endoderm. 



C. Canal cut transversely. 

 Fig. 4. Portion of the superficial structures of Heliopora ccerulea as viewed in a thin 

 vertical section. 



E. Ectoderm, consisting of elongate club-shaped cells running out below 

 into fine processes, which traverse the next succeeding layer (M) of 

 homogeneous connective tissue. At the bases of these cells are others 

 of similar structure but irregular form. Small nematocysts lie 

 amongst these external ectodermal cells, and some of them (N) are 

 shown in the deej>er regions. At EN the wall of one of the tubular 

 cavities of the ccenenchym is seen in section at its edge, showing its 

 three layers and the residue of animal matter (P) left by parts of its 

 calcareous wall after decalcification. 



Between EN and the section of the wall of the canal (V) a narrow 

 strip of the inner surface of the tubular cavity is viewed from its 

 surface aspect. 



E. External ectodermal layer. 



M. Layer of homogeneous connective tissue. 



N. Nematocysts. 



C. Connective tissue cells. 



V. Canal of superficial system. 

 V. Canal of deep system. 

 E N. Endodermal cells. 



D. Layer of connective tissue cells. 



P. Eesidue of tissue after decalcification of a portion of the wall of a 

 ccenenchym tube. 

 Fig. 5. Section vertical to the surface of the corallum of Heliopora ccerulea, showing 

 the structure of the hard tissue. 



A. Former calicular cavity shut off from the recent calicle A' by the tabula, C. 



The tube cavities on either side have similar tabulae developed in them. 

 In the cases of some of the tubes the tube walls are shown as cut jmrallel to 

 their surfaces, in other places the cut edges of the walls only are seen. 



The tabulae being applied against the already formed insides of the ealicles 

 and tubes as a later formation, the line of the old wall of the calicle in each 

 case can always be traced up for some distance past the level of the tubula, 

 which appears as the bottom of a second tube fitted within the first. The 

 calcareous fibres forming the walls of the tubes and ealicles are inclined 

 upwards and outwards at a uniform angle from the vertical axes of these 

 structures. 



