ee? THE VOYAGE OF H.M.S. CHALLENGER. 
manner. The arrangement will be seen in Plate I. fig. 4. In the membrane lining the 
calicles, in transverse sections, a peculiar structure (shown PI. I. fig. 3) is to be constantly 
observed. Stout offsets from the median connective tissue layer pierce the outer layer 
of connective tissue cells, and hang loose externally as flattened tags, which appear as if 
broken off, and are often somewhat curled up. I have been unable to determine the 
connection of these tags of tissue with the calicular wall (PI. I. fig. 3). Exactly similar 
structures occur in Tubipora, being specially developed around the lower part of the 
polyps. Beneath the uppermost tabulee scarcely any organic liming 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 fHeliopora, 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 undeealcified coral, but beneath is seen to be composed of a series of villous 
processes derived from the tubes with the bottoms of the calicular sacs appearing 
as tubercles amongst them. Since the tubes of the ccenenchym and_calicles 
have no lateral connections 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. 104, and shown in Plate II. fig. 7. These canals are mostly very short; they are cir- 
cular in section, and have the same three layers in their walls as which compose the sacs 
within the tubes. In older parts of the coral, where the calcareous tubercles on the 
surface are much developed and the mouths of the ccenenchymal tubes are consequently 
contracted, a series of open channels are to be observed in the corallum running at the 
bases of the tubercles, 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 deep canal system, through a system of large apertures shown in Plate IL. fig. 2. 
These apertures open in the intermesenterial spaces all around the summit of the calicle 
at its periphery, a single aperture 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 
