324 MllS. L. J. WILSMORE ON ZOANTHEiE FROM 



Colour. — Sand-coloured in spirit-specimens. 



Dimensions. — Vary considerably in the difEerent polyps, and are inde- 

 pendent o£ position of polyp in the colony. Height 5-14 mm.^. average 

 y-10 mm. ; diameter 4-7 mm., average 5-G mm. 



Locality. — Collected at Masthead Island, Queensland^ by Mr. C. Hedley's 

 expedition in 1804. Numerous specimens. 



The specific name arenacea has reference to its markedly incrusted 

 character. No species of this genus hitherto described is so thickly incrusted 

 with calcareous sand-grains. 



Column-ivall (PL 45. figs. 17-18). — The incrustations of the column-wall 

 are chiefly calcareous sand-grains with a sprinkling of siliceous spicules. 

 A certain amount of selection is shown, the sand-grains in the distal part 

 of the capitulum being more regular in size and much smaller than those 

 in the column. Still smaller grains are found in the disc and tentacles. 

 The occasional siliceous spicules are distributed irregularly throughout, and 

 not confined to the lower column-wall as described by von Heider in 

 G. variabilis (11). The incrustations extend through the whole of the 

 ectoderm and occupy from one-half to four-fifths of the mesogloea also. 

 They penetrate to a varying and irregular depth in different parts of the 

 same column- wall. Peripherally the sand -grains are so closely packed that 

 only small portions of the original tissue are seen between them. More 

 centrally they may be very scattered. Occasionally a grain comes into 

 contact with the endoderm, and pushing it inwards forms a projection into 

 the ccelenteron. In the capitulum the incrustations do not pass through the 

 sphincter muscle. 



The ectoderm is of the continuous type, and no cuticle is present. It is 

 much broken up and its structure difficult to determine owing to numerous 

 grains of sand imperfectly enveloped which project through the surface. 

 Zooxanthellse are everywhere very numerous. In the capitulum the ectoderm 

 is much less broken, and the zooxanthellse, massed together, occupy all the 

 tissue free from sand-grains. In this region the ectoderm presents very 

 rregular internal limitations as in G. variabilis, Duerden (8). It is here 

 much thicker than in the column, reaching an average width of 2 mm. 

 and in the ridges 3"5 mm. The mesoglcea measures about 2 mm. in the 

 capitulum, and attains its greatest breadth (8 mm.) in the proximal part of 

 the column. The innermost portion of the mesogloea, which is free from 

 incrustations, contains no sinuses, but many cell islets and lacuna^, are 

 scattered irregularly through its substance. They are filled with zooxan- 

 thellfe, and are much more numerous proximally. Single cells and long- 

 radial fibres stretching from the endoderm through the mesogloea are very 

 common, and the endodermal surface of the mesogloea is much plaited. 

 A considerable amount of pigment is present, chiefly confined to the 

 cell islets and lacuntc. The endoderm is thin and ' contains zooxanthellse, 



