QUEENSLAND AND THE NEW HEBRIDES. 325 



but they are not so numerous as in the ectoderm, and this rule obtains 

 throughout the polyp. 



Sphincter muscle (PL 45. fig. 19). — The sphincter muscle is single, meso- 

 gloeal and relatively weak, and lies throughout its length close to the 

 endoderm. In position it resembles the sphincter of G. multa sulcata 

 (Carlgren, 13), but is distinctly weaker. It consists of an irregular single 

 row of small mesogloeal cavities which are completely filled with muscle- 

 cells. These cavities are extremely small proximally ; distally they increase 

 in size. 



Tentacles. — In two polyps the tentacles numbered 56. The ectoderm is 

 several times thicker than either meso- or endoderm. Long, narrow nemato- 

 cysts occur in small numbers in the outer zone of the ectoderm, while the 

 inner zone is crowded with zooxanthellse. Little groups of sand-grains lie 

 in the ectoderm on the outer side only of the tentacles and towards their 

 tips. A weak ectodermal muscle is present. The mesogloea and endoderm 

 are both thin. The former contains numerous isolated cells. Zooxanthellse 

 occur in the endoderm, but not in sufficient numbers to fill the lumen. 



Disc. — The centre of the disc is smooth and flat. The mouth is oblong 

 and placed on a slight prominence. Peripherally there are a number of 

 ridges, produced by thickenings of the mesogloea, one corresponding to each 

 of the tentacles. In the centre of the disc both ectoderm and mesogloea 

 contain a thin sprinkling of minute sand-grains (PI. 45. fig. 20, lac.d.). The 

 ectoderm, which is very thick throughout and particularly so peripherally 

 (PI. 45. fig. 20), contains manj^ zooxanthellse. The mesogloea is thickest at 

 the central part of the disc. Numerous large isolated cells are enclosed in 

 this layer, and enter the bases of the tentacles at their point of junction with 

 the disc (PI. 45- fig- 20, ect.m.). These appear to be identical with the 

 ectodermal muscle-cells described by Hill and Whitelegge (12) as present in 

 the disc of G. Willeiji. The endoderm is thin and contains few zooxanthellse. 



(Esophagus (PI. 45. fig. 18). — The oesophagus has the usual well-marked 

 trimcated groove characteristic of the genus. Its ectoderm is deeply folded 

 into 10 ridges, the mesogloea passing partly into these. A good deal of 

 pigment is present in the clear nervous zone. In the sulcar groove the 

 ectoderm is smooth and thinner than elsewhere, while the mesogloea is 

 thicker. Its many cell enclosures are scattered irregularly and not placed 

 at the insertion of the mesenteries as in G. isolata, G. Musei, and G. cana- 

 riensis (3, 4, & 6). The endoderm is low with occasional zooxanthellse. 



Mesenteries. — The arrangement of mesenteries is brachycnemic. In one 

 polyp cut transversely, there are thirteen perfect mesenteries on one side and 

 fifteen on the other (PI. 45. fig. 18). In another there are fourteen perfect 

 mesenteries on each side. The sulcar directives are always much thicker 

 than the other mesenteries, but the degree of difference varies. The meso- 

 gloea is thick and the endoderm thin, containing from one to two rows of 



