REPORT ON CORALS — HYDROCORALLINvE. 49 



of an upper cylindrical portion (PI. VIII. fig. 2, Z), and a wider saucer-shaped basal 

 region, to join the margins of which the lower part of the cylindrical portion gradually 

 widens out inferiorly. Canals are given off from the margin of the basal saucer of the 

 zooid all round, and pass to join the general ccenenchymal meshwork; but no canals 

 at all are given off from the rounded under surface of the zooid. The upper surface of 

 the cylindrical portion of the zooid is nearly flat, and is occupied by the mouth, which 

 is a cruciform slit bounded by elongate gastric endoderm cells, closely similar to those 

 described as existing in Sporadopora. 



The dactylozooids are simple elongate-conical bodies devoid of mouths, with a minute 

 structure closely similar to that of the corresponding zooids of Sporadopora. In the 

 retracted condition they are thrown into a series of transverse folds which are indicated 

 by flue transverse lines in the figure (PI. VIII. fig. 2, TZ, TZ). The zooids appear to 

 be retracted directly within their sheath, and not to be attached on one side of their 

 base. 



The gonophores are contained in ampulke, which are often sunk deep within the 

 ccenosteum; and it is not apparent by what means the large mature planulas find their way 

 to the exterior. I have not had sufficient material at command to determine whether 

 the ampullae, as they enlarge, come gradually to communicate with the surface of the 

 ccenosteum by means of absorption of the intervening hard tissues. It seems probable 

 that they may do so. 



The ova are solitary, one only being developed in each growing ampulla. Each ovum 

 is developed within the cup of a cup-shaped spadix (PI. VIII. fig. 2, 0). The ovum is 

 provided with a germinal vesicle and spot. It is covered by a fine layer of ectoderm, 

 which is reflected over it from the surface of the spadix. It is not patent how fertili- 

 sation takes place — that is to say, how the spermatozoa find their way to the sometimes 

 deeply-seated ova. As the ovum advances in development and increases in size, the 

 spadix enlarges with it (PI. VIII. fig. 2, G). Subsecpiently, however, in later stages, 

 the spadix appears not to increase further ; and when in relation with a nearly fully- 

 developed planula appears proportionately small. 



The nearly mature planula (PL VIII. fig. 2, P) is a large object of an ovoid form, 

 with a long diameter greater than the extreme width of the gastrozooid. Its ectoderm 

 and endoderm are plainly differentiated. The endoderm is composed mainly of oil-cells, 

 but contains also a few fully formed nematocysts of the larger variety. The ectoderm, a 

 thick layer, shows the characteristic striation vertical to the outer surface of the planula, 

 the striae being composed of granules and nuclei arranged in linear groups. As far as 

 could be ascertained from the scanty material at command, it appeared that the ectoder- 

 mal layer is formed in development by the process of delamination. No trace of an 

 invagination in the embryo was observed. 



In very advanced stages the planulaj become folded to a slight extent, as in the case 



(ZOOL. CHALL. EXP. — PART VII. — 1880.) G 7 



