26 THE VOYAGE OF H.M.S. CHALLENGER. 



occurs only through the action of reagents. It seems probable that it does occur in the 

 living animal, since by its means the masses of large thread-cells are brought as a 

 protection directly between the zooid and the exterior. Just as in one small portion of 

 the coral the zooids died in the expanded condition, so more often, in certain specimens, 

 they die and are preserved with the superficial ectodermal layer not closed in over the 

 mouth of the calicle, but with the calicle open, and their retracted tentacles remaining 

 fully exposed to view from above. In Plate XIV. figs. 2 and 3, both zooids figured are 

 shown in this latter condition. The connection between the superficial ectodermal layer 

 within the cabcle and the adjacent vascular network of the ccenosarc was not made out. 

 The layer is probably merely the largely developed ectodermal layer of that part of 

 the network, but the connection not having been seen is not indicated in Plate 

 XIV. fig. 2. 



The superficial layer of the ccenosarc being a special development of the ectodermal 

 cells of the vascular network, and the interspaces in this network being occupied by cal- 

 careous trabecular, it follows either that the tips of the trabecular at the surface of the 

 ccenosarc must be directly exposed, or that the superficial ectodermal cells of the network 

 must close in over them. The latter arrangement seems to occur ; and in vertical sections 

 of the decalcified ccenosarc numerous spaces left by removed calcareous structures are 

 seen in the superficial ectodermal layer (see PL XIV. figs. 2 and 8), with the ectodermal 

 cells arching over to cover them. I should have had no doubt in this matter had I 

 not observed that in the living Millepora the soft parts of the ccenosarc appear to 

 be retracted below the surface of the ccenosteum when the zooids are in their retracted 

 condition. It can, however, hardly be the case that any part of the ccenosteum is 

 directly exposed to the water. It is probably always covered everywhere by the super- 

 ficial layer of the ectoderm, which, however, is in the recent condition so transparent 

 as to escape observation. The calcareous tissue of the ccenosteum must obviously be 

 deposited by the ectoderm, with which alone it is in contact. It spreads by extension 

 of the trabecular at the surface ; and since there it is seen to be often in contact only 

 with the cells of the superficial layer, it seems that these cells must have the power of 

 producing it. The calcareous network undergoes thickening in the deeper parts of the 

 living lamina, as must necessarily be the case, because of the formation of the tabulae 

 and lines of growth. In these parts no doubt the fusiform nucleated cells of the ectoderm 

 are the instruments of the deposition of the carbonate of lime. No special calciferous 

 tissue was observed, such as exists in Heliopora ccerulea. 



Beneath the layer of ectodermal cells in the vascular canals composing the ccenosarc 

 lies a layer of apparently homogeneous membrane, which appears to form everywhere a 

 wall to the vessels and canals. The cavities of the vascular network are lined by, and 

 in many places nearly filled with, cellular elements of two kinds — pigmented cells and 

 small transparent globules. The pigmented cells (PI. XIV. fig. 9) closely resemble those 



