On the Structure and Relations of certain Corals. 151 



canals in the sarcosome. The sarcosome contains elongate tuber- 

 culate spicules of the usual form, which are largest and most 

 thickly set in the stem of the liarcoplujton. 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 centre of the pileus ; 

 but both polyps and zooids are often more or less twisted on their 

 axes. 



0)1 the Structure of Millepora. — The examination of Millepora is 

 beset \inth. serious difficulties ; the present notes are merely pre- 

 liminary. The calcareous coenenchymal tissue of Millepora differs 

 extremely from that of Hdiopora in being reticulate, not tubular : 

 in histological structure it is similar to Helioj^ora. The coral 

 has only a thin superficial layer of soft living tissue, composed of 

 a network of canals filled with cells resembling 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 mesenteries 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 between the 

 calcareous septa, as was shown to be the case in Allopora oculina 

 by Sars (Forh. fSelsk. 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 outlet. There 

 are no well-defined mesenteries, and no mesenterial filaments. A 

 very open network of soft tissue surrounds the stomach and 

 tube leading to it from the circle of the tentacles. Suspended in 

 this reticulate tissue are the testes, large sacs filled with spermatic 

 cells disposed sometimes in one, sometimes in two vei'tical rows ; 

 they occupy the interior of the ampullae. These corals are dia'cious. 

 Cnjptohelia resembles Stylaster most closely in structure, and is 

 also dioecious. 



Vegetable Parasites. — The corallum of both Millepora and Pocil- 

 lopora is permeated by fine ramified canals, formed by parasitic 

 vegetable organisms of the same nature as (hose described by Dr. 



