18 THE VOYAGE OF H.M.S. CHALLENGER. 
STRUCTURE OF THE Harp AND Sort Parts. 
Ccenosteum of Millepora. 
The structure of the ccenosteum is illustrated on Plate XIII. The ccenosteum has a 
widely spread encrusting base covering rocks, dead corals, &c., and at its surface presents a 
series of projecting, short, irregular tubercles and lobules, which never rise to any con- 
siderable height. Fig. 3 represents the appearance of two lobules of the ccenosteum and 
a portion of a third, enlarged two diameters. The surface of the lobules is uneven and 
covered with slight rounded elevations. The pores of the zooids are dispersed over the 
entire surface both of the lobules and of the flatter encrusting portions of the ccenosteum, 
being absent only at the tips of some of the lobules, which are possibly those that are in 
rapid growth. The pores are disposed in irregularly circular groups, a larger gastropore 
being in the centre of each group or system with usually from five to eight smaller 
dactylopores arranged around it. These systems of pores often occupy small rounded 
prominences on the surface of the ccenosteum, and in parts of some specimens almost every 
system appears to have its separate small prominence. In some regions of the ccenosteum 
the systems are scarcely defined, the calicles appearing irregularly placed; but such an 
arrangement is only exceptional in the present species. An entire system of calicles has 
been accurately drawn for me by Mr J. J. Wild and is represented in Plate XIII. fig. 4, 
enlarged eighty diameters. The outlines of the pores are seen to be extremely irregular ; 
their cavities are encroached upon in all directions by projections of the contorted trabe- 
cular ecenenchymal tissue of the coenosteum. The larger central gastropores of the systems 
measure about 1°5 mm. in diameter. 
The main mass of the coenosteum is composed of trabeculze of dense calcareous matter, 
which forms a spongy-looking mass traversed in all directions by tortuous canals. In 
some species of Millepora the ccenosteum is much more dense than in the Tahitian one, and 
in these might rather be described as a compact mass in which a series of tortuous channels 
are excavated for the reception of the soft structures. In such species of Millepora, in 
finely-ground sections of the ccenosteum, the tortuous canals become filled with opaque 
debris, and show out, when the section is viewed by transmitted light, dark on a light 
ground. In a species of Millepora obtained at Samboangan the ccenosteum was of this 
nature. The appearance presented by a thin section of its ccenosteum is shown in Plate 
XMM. fig. 7. In Millepora aleicornis and in the Tahitian species the canal systems and 
trabecul of calcareous matter seem to form equally complex interpenetrating meshworks. 
The canal systems correspond to, and in the recent state contain, the ramifications of the 
soft parts of the coenosare. The canals form regular branching systems with main trunks 
which give off numerous branches from which arise secondary branches and from these 
again smaller ramifications. The whole canal-system is connected together by a freely 
anastomosing meshwork of smaller vessels, and communicates freely by numerous offsets 
