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



Structure of the Hard and Soft Parts. 

 Coenosteum of Millepora. 



The structure of the ccenosteurn is illustrated ou Plate XIII. The coenosteum 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 coenosteum 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 coenosteum, 

 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 coenosteum, and in parts of some specimens almost every 

 system appears to have its separate small prominence. In some regions of the coenosteum 

 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 ccenenchymal 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 trabeculse of dense calcareous matter, 

 which forms a spongy-looking mass traversed in all directions by tortuous canals. In 

 some species of Millepora the coenosteum is much more dense than in the Tahiti an 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 coenosteum, 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 coenosteum was of this 

 nature. The appearance presented by a thin section of its coenosteum is shown in Plate 

 XIII. fig. 7. In Millepora alcicornis and in the Tahitian species the canal systems and 

 trabeculse 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 ccenosarc. 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 



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