236 THE VOYAGE OF H.M.S. CHALLENGER. 
EN. Endoderm. 
C. Canal cut transversely. 
Fig. 4. Portion of the superficial structures of Heliopora caerulea 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 he 
amongst these external ectodermal cells, and some of them (N) are 
shown in the deeper 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. 
KE. 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. 
EN. Endodermal cells. 
D. Layer of connective tissue cells. 
P. Residue of tissue after decalcification of a portion of the wall of a 
coenenchym tube. 
Fig. 5. Section vertical to the surface of the corallum of Heliopora caerulea, 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 tabule developed in them. 
In the cases of some of the tubes the tube walls are shown as cut parallel to 
their surfaces, in other places the cut edges of the walls only are seen. 
The tabulz being applied against the already formed insides of the calicles 
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 calicles are inclined 
upwards and outwards at a uniform angle from the vertical axes of these 
structures. 
