1875.] Structure and Relations of certain Corals. 63 



senterial filaments longer than the rest. The zooids have eight short 

 mesenteries, four of which, the " dorsal " and " ventral," are deeper than 

 the rest. They have two mesenterial filaments, the dorsal only. They 

 have no tubercles and no generative organs. They have a simple globular 

 stomach, communicating by a short tube with the exterior, and lined with 

 long cilia. A sarcosome of transparent homogeneous connective-tissue, 

 containing small ramifild nucleate corpuscles, connects the polyp- and 

 zooid-cavities ; these cavities are connected by vertical and horizontal 

 systems of canals. The vertical canals are continuous with the bottoms 

 of the zooid-cavities ; they form networks] of canals in the sarcosome. 

 The sarcosome contains elongate tuberculate spicules of the usual form, 

 which are largest and most thickly set in the stem of the Sarcophyton. 

 Smaller spicules are present in the tentacles of the polyps. The spicules 

 show a special sheath of transparent tissue, in which structure was not 

 seen. The " Dorsalf acher " of the polyps and zooids have a general direc- 

 tion towards the central axis of the stem and ceutre of the pileus ; but both 

 polyps and zooids are often more or less twisted on their axes. 



On the Structure of Millepora. — The examination of Millepora is beset 

 with serious difficulties ; the present notes are merely preliminary. The 

 calcareous ccenenchymal tissue of Millepora differs extremely from that 

 of Heliopora in being reticulate, not tubular : in histological structure 

 it is similar to Heliopora. The coral has only a thin superficial layer of 

 soft living tissue, composed of a network of canals filled with cells re- 

 sembling those of the canals of Alcyonarians, and covered externally with 

 nematocysts. Two kinds of nematocysts, small and large, are present : 

 the small ones are confined to the tentacles. Two kinds of polyps are 

 present, large and small. Tentacles are present in both kinds ; they 

 appear to be four in number and compound. They are simply retracted 

 by means of muscular fibres, which are arranged round the base of the 

 cylindrical stomach radially, but, as far as has yet been seen, without any 

 disposition in definite groups. No mesenteries have been seen. 



On the Structure of Pocillopora (P. acuta). — The corallum is very 

 dense and composed of definite prisms of calcareous matter, which show a 

 transverse banding, somewhat like that of striped muscular fibres. The 

 polyps have twelve tentacles, six large and six small, and twelve mesen- 

 teries with long mesenterial filaments coiled up. A very thin layer of 

 living tissue covers the corallum ; it is devoid of canals. 



On the Structure of Stylaster. — A Stylaster dredged in 500 fathoms was 

 found to have the tentacles disposed betiveen the calcareous septa, as was 

 shown to be the case in Allopora oculina by Sars (Torh. Selsk. Chr. 1872, 

 p. 115). The septa are twenty-two in number, and the tentacles also 

 twenty-two. The stomach has a conical projecting mouth or proboscis, 

 as seen by Sars in Allopora oculina. It has apparently no inferior out- 

 let. There are no well-defined mesenteries, and no mesenterial filaments. 

 A very open network of soft tissue surrounds the stomach and tube 



