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. 
Cenosarc.—The canals and spaces within the calcareous coenosteum 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 living 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 
cerulea, to the region superficial to the last-formed tabulee. 
The coenosare 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 (Pl. 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 Millepora, 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 14 inch. ‘The ramifications of the 
coenosare 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 coenosare, 
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 le 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- 
