STKUCTUKE AND RELATIONS OE CERTAIN CORALS. 
Ill 
somewhat sparsely, very small finely ramified nucleate corpuscles. In the walls of the 
zooid- and polyp-cavities there is to be seen a transverse fibrillation of a part of their 
mesodermic layer ; and the wall, when seen in section on edge, shows a layer next the 
cavity of the polyp or zooid consisting of true transversely directed fibres. 
The sclerites or spicules are imbedded in the thick layers of the sarcosome intervening 
between the zooid- and polyp-cavities and between the tubes composing the stem, 
besides being found in the tentacles. The growing tips of the spicules project up 
amongst the ectodermic cells (Plate 9. fig. 8), carrying with them their investment of 
connective-tissue. When the spicules are removed by acid, corresponding cavities are 
left in the mesoderm. A transparent membrane can be distinctly seen investing closely 
each spicule ; no structure, however, could be seen in the membrane. The polyp- and 
zooid-cavities, and the whole of the canal-system, is invested as usual by an entodermic 
layer, consisting of spherical cells with yellow contents exactly like those of Heliopora- 
In the zooid-cavities, at their summits, around the top of the stomach, masses of these 
cells were always observed to be accumulated. Possibly the accumulation of these in 
this situation is consequent on action taking place on the death and contraction of the 
colony when placed in spirits. 
Sarcophyton is an extremely favourable subject for the examination of the vascular 
system. In sections from alcoholic specimens preserved in glycerine jelly the whole 
ramifications of the vessels are most clearly displayed. Owing to the pigmentation of 
their lining entoderm, the canals show out dark and defined in the perfectly transparent 
connective-tissue. The arrangement of the canals is shown in Plate 8. fig. 2. 
Two systems of canals are to be distinguished — the transverse and vertical systems. 
The transverse canals run parallel to the surface of the colony and to one another in 
each interspace between the polyp-cavities. They take the most direct courses to con- 
nect the cavities of the polyps with those of the surrounding zooids and with those of 
the adjacent polyps. They commence to be given off laterally from the polyp-cavities 
at their very summits, forming there communications with the zooid-cavities. They 
continue to be given off at tolerably regular intervals, crossing now to a closely situate 
zooid, now to a distant one. Deeper down in the colony the canals make long stretches 
to join the next adjacent polyp-cavity, and become shorter and shorter as the polyp, 
cavities converge below. Similar short canals connect the zooid-cavities with one 
another. Punning in a general vertical direction between these transverse canals are 
the vertical canals, distinguished by their more undulatory course. The chief stems of 
this system of canals are the direct prolongations of the zooid-cavities. In connexion 
with these canals is an irregular meshwork, by which the whole deep connective-tissue 
is permeated, and through the meshes of which the transverse canals pass. Offsets of 
the vertical canal-system pass between the zooid-cavities, and between the polyp-cavities 
and zooid-cavities give off transverse connecting branches. 
There is no surface network of canals present in the superficial layer of mesoderm 
directly beneath the ectoderm as there is in Ileliopora. 
