REPORT ON CORALS — HYDROCORALLIN^E. 19 



with the cavities of the pores. In Plate XIV. fig. 4, part of one of these canal-system s is 

 shown, being there drawn from a decalcified specimen, and thus representing the soft 

 tissues which in the recent state occupied corresponding calcareous canals. In Plate XIII. 

 fig. 6, a secondary branch of one of the canals is seen to communicate with a pore cavity, 

 C. The course of the smaller vessels being tortuous, only short lengths of them are 

 exposed in the remainder of this section. Similar secondary branches are seen in vertical 

 section in Plate XIII. fig. 5, B, B. 



Where a Millepora encrusts foreign bodies, the investing film of ccenosteum formed is 

 usually extremely thin. At Bermuda, Millepora alcicornis is frequently found encrust- 

 ing glass bottles thrown into the harbours. The film of ccenosteura can, in such specimens, 

 easily be detached in flakes from the glass, and does not measure more than from l-8th to 

 l-5th of a millimeter in thickness. In the same manner at Bermuda the dead fans of a 

 Gorgonia are found entirely encrusted with a thin film of Millepora, so thin that the 

 fenestrations of the horny meshwork of the Gorgonia are not obliterated. Such thin 

 encrusting films, if obtained in the living condition, would, no doubt, be excellently 

 adapted for the study of the soft parts of Millepora, since they are thin enough to 

 transmit a considerable amount of light. When dead and dry they show extremely well 

 the ramifications of the canal-systems and their connections with the pores. In such films 

 the dactylopores and gastropores are fully developed, though necessarily very shallow ; 

 and it is evident that such a thin film of ccenosteum is all that is absolutely necessary for 

 the existence of the Millepora, and, in fact, in all Milleporidse it is such a thin film only 

 which is actually living, covering the surface of the ccenosteum. In a Millepora forming 

 tubercular or ramified masses a superposition of a series of such films takes place arid 

 constitutes the coral mass. 



In the films encrusting bottles the under surface in contact with the glass is perfectly 

 continuous and highly polished, and is exactly moulded on the surface of the glass, repro- 

 ducing casts of the most minute splinterings or scratchings. 



In homology with this continuous layer, layers more or less continuous occur in the more 

 massive ccenosteum appearing in vertical sections as lines of calcareous matter running 

 parallel to the surface of the ccenosteum and indicating successive stages of growth, and 

 the tubercles of which the mass of the Tahitian Millepora is made up, when cut through 

 vertically to the surface, show a series of such lines of growth following the contour of 

 the surface. It is in connection with these layers that are developed the successive 

 transverse laminae or tabula? which divide the cavities of the calicles into a series of 

 chambers (PI. XIII. fig. 5). As the ccenosteum is extended in growth at certain intervals, 

 possibly after each period of generative activity, a tabula is formed, reducing the depth 

 of the calicle and shutting off the living tissue from the abandoned dead structures below. 

 The larger canals and their branches ramify in planes parallel to the surfaces of the 

 ccenosteum, being confined within each successively added thin layer of the ccenosteum, and 



