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



appear to be distinct from the membranous layer of the zooid, though in close relation 

 with it. These bundles of longitudinal fibres are plainly to be seen in preparations of all 

 kinds, and no doubt it is to their presence that the zooids owe their power of almost 

 instantaneous retraction. In several osmic acid preparations an appearance indicating 

 the existence of a set of circularly-directed fibres lying externally to the longitudinal 

 fibres, or possibly of circular fibrillation of the membranous layer, was seen, but the 

 existence of such structure was not determined with certainty. 



Ccenosarc. — The canals and spaces within the calcareous ccenosteum are occupied, as 

 has already been stated, by a network of soft tissue. This, together with the superficial 

 layer of ectoderm, constitutes the ccenosarc. Only a thin layer at the surface of the 

 coral is living. This layer separates from the underlying dead matter when the coral is 

 decalcified in acids, and appears as a soft membrane about - 5 mm. in thickness. When 

 the entire ccenosteum is dissolved away there remains besides this membrane only a 

 greenish gelatinous mass, which consists of the mycelium and spores of the parasitic 

 organisms, which were the sole bving occupants of the deep parts of the ccenosteum. The 

 living part of the Hydroid seems to be entirely confined, as is the case in Heliopora 

 ccerulea, to the region superficial to the last-formed tabulas. 



The ccenosarc consists of a series of ramifying canal-systems, which occupy in the 

 recent condition the canals already described as existing in the ccenosteum. The branches 

 and secondary branches of the canals are joined by a complex network of smaller vessels, 

 which join in all directions the body-cavities of the zooids (PI. XIV. fig. 4), and thus 

 maintain a vascular connection of the freest character between the various zooids of 

 the colony. In some cases, comparatively large tertiary branches of the canals join the 

 zooid-cavities directly. The large main canals run sometimes for long distances, and in a 

 species of MiUepora, obtained at Samboangan, Philippines, their corresponding channels 

 in the hard tissue are plainly visible to the naked eye on the surface of the corallum, 

 extending sometimes for as great a distance as 1^ inch. The ramifications of the 

 ccenosarc are best seen on the under surface of the superficial living film decalcified in 

 chromic acid and viewed by reflected light. The appearance presented in such a pre- 

 paration is accurately represented in Plate XIV. fig. 4. The appearance of the ccenosarc, 

 as seen in vertical section, is shown in Plate XIV. fig. 2. In the more superficial region 

 of the living layer, the elements of the network take a direction more or less vertical 

 to the surface. The horizontally directed main canals and their branches lie near the 

 under surface of the layer on a level with the bases of the zooids. 



The histological structure of the ccenosarc is shown in Plate XIV. fig. 8. The canals 

 and vessels forming the network are composed of an ectodermal layer, with a mem- 

 branous layer developed beneath it, and an endodermal lining. 



The ectodermal layer consists, in the greater part of the network, of fusiform cells 

 with a finely granular appearance and a well-defined oval nucleus, but with the cell- 



