STRUCTURE MD RELATIONS OE CERTAIN CORALS. 
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ever, freely anastomose. The transverse canals lead directly from one 
polyp-cavity to another, with a course nearly parallel to the surface plane of 
the Sarcophyton, or from the polyp-cavities to the zooid-cavities. The vertical 
system of canals has a tortuous, branching, freely anastomosing course. The 
zooid-cavities contract at their lower extremities, and pass directly into this 
system of canals. Prolongations of the vertical system of canals pass up to 
the surface between the zooid-cavities, and between these and the polyp- 
cavities. 
Z. Zooids. 
PM. Protractor muscle. 
EM. Eetractor muscle. 
C. One of the canals of the transverse system. 
Fig. 3. Transverse section across a polyp of Heliopora ccerulea , taken just below the 
mouth (decalcified). The tips of four of the tentacles cut across are retained 
in situ ; the other four have fallen out. 
D. External layer of connective-tissue cells. 
C. Layer of homogeneous connective-tissue giving off a series of stout 
offsets (A), which pierce the layer (D) and project externally. This 
layer is seen to form the central plate of the mesenteries and a 
wall around the stomach. 
EN. Entoderm lining the whole of the intermesenteric chambers. 
M. Mesenteries. 
EM. Eetractor muscles. The “ Dorsalfach ” of Kolliker lies below in 
the drawing, the “Yentralfach” above. The muscles are covered 
by the entoderm. 
TT. Tentacles cut across. 
S. Stomach. B. Its muscular layer. 
Fig. 4. Section parallel to the surface of Heliopora ccerulea , to show the relation of the 
hard parts to the soft. 
TC, TC and the similar oval spaces represent tubular cavities of the 
ccenenchym cut across. The interspaces between these cavities 
are occupied by the hard tissue. 
The hard tissue is fibrous in structure, the fibres radiating 
from a series of axes here seen cut across (C). Suture-like lines 
(S) occur occasionally where the peripheries of the various 
radiating systems join. 
D. Layer of connective-tissue cells. 
C. Layer of homogeneous connective-tissue. 
EN. Entodermic layer. 
Fig. 5. Polyp of Heliopora ccerulea and the immediately surrounding structures as 
viewed in a thick horizontal section from the outer surface. 
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