STRUCTURE AND RELATIONS OF CERTAIN CORALS. 
105 
often somewhat curled up. I have been unable to determine the connexion of these 
tags of tissue with the calicular wall, Plate 8. fig. 3 *. Beneath the uppermost tabulae 
scarcely any organic lining remains to the tubes, if any at all, and the deeper central 
parts of the corallum are, in the specimen of Heliopora which I have examined, 
almost entirely filled with the tubes of the boring annelids ( Leucodora , sp.). Thus 
when a mass of Heliopora, after being hardened, is decalcified, the whole of the 
deeper parts are removed, and a thin layer of soft tissue only remains behind, which 
above presents a similar appearance to that of the surface of the undecalcified coral, 
but beneath is seen to be composed of a series of villi with the bottoms of the calicular 
sacs appearing as tubercles amongst them. Since the tubes of the ccenenchym and 
calicles have no lateral connexions with one another except close to the surface of 
the corallum, in decalcified preparations they are, excepting at their very upper 
extremities, entirely separated from one another ; hence it is extremely difficult to 
prepare fine transverse sections in the deeper regions, since the structures afford no 
support to one another. 
Canal-systems . — The summits of the cavities of the sacs of soft tissue lining the 
ccenenchymal tubes communicate freely with one another and with the cavities of the 
polyps by means of a system of short transverse canals, which cross over the margins of 
the walls of the calcareous tubes at the lower parts of their mouths, as already described, 
p. 97, and shown, Plate 9. fig. 7. The tubes are mostly very short ; they are circular in 
section, and have the same three layers in their walls as have the sacs within the tubes. 
In older parts of the coral, where the calcareous tubercles on the surface are much deve- 
loped and the mouths of the ccenenchymal tubes contracted, a series of open channels 
appear in the corallum at the bases of the superficial papilliform eminences, when the 
coral is looked at with a hand-magnifier. It is in these channels that the system of 
transverse canals runs. This canal-system I have termed the “ deep canal-system,” to 
distinguish it from the system of smaller canals lying superficially to it. The tube- 
cavities communicate with the polyp-cavities by means of the transverse canal-system, 
through a system of large apertures shown in Plate 9. fig. 8. These apertures open 
in the intermesenterial spaces all around the summit of the calicle, a single one being 
situate in the space formed by each externally projecting fold of the calicular wall. 
The superficial canal-system consists of a series of small canals and sinus, which 
take mostly a more or less vertical direction, and communicate directly with the 
deep canal-system. These superficial canals anastomose with one another by hori- 
zontal offsets. A series of horizontally extended canals of this system surrounds 
each contracted polyp, the canals taking a radial arrangement. One such canal is 
shown in vertical section in Plate 9. fig. 7, and the appearance of the summits of the 
canals as seen from the surface of the coral is shown in Plate 8. fig. 5. The superficial 
canals are not only lined by, but also always more or less filled with entodermic cells. 
* Exactly similar structures occur iu Tubijpora , being specially developed around the lower part of the 
polyps. 
