4O J. E. DUERDEN. 



glyph along the ventral extremity of the stomodaeum, as com- 

 pared with the two opposite grooves (dorsal and ventral) usually 

 present in hexactinian polyps. The single groove in zoanthids is 

 usually well-developed, and its walls often project for some little 

 distance within the ventral directive entocoelic chamber, and are 

 frequently continued into the polypal cavity beyond the rest of 

 the stomodaeal wall. It is strongly ciliated and is concerned 

 chiefly in the circulation phenomena of the polyp, sometimes re- 

 maining open when the mouth is otherwise closed by the approx- 

 imation of the lips. 



The occurrence of the single gonidial groove gives a marked 

 contrast to the two extremities of the stomodoeum of zoanthid 

 polyps, and a decided bilateral character to transverse sections. 

 Manifestly it is the presence of just such a structure which we 

 should expect to result in a diminution of the axial septum were 

 a skeleton formed beneath it ; indeed, with such a stomodaeum, 

 having a groove at one end and not at the other, we could 

 scarcely expect that the two extremities of a corallite would be 

 alike. In the absence of the rugose polyp itself, no surer proof 

 of the relationship of the group to the zoanthids could to my 

 mind be adduced than that which admits of the correlation of the 

 simple cardinal fossula with a ventral stomodaeal groove. 1 



On the above explanation it may reasonably be asked why no 

 corresponding axial fossulae are found in the calices of recent 

 corals. In answer to this it can be affirmed that as yet we have 

 no certain instance of the presence of a gonidial groove in the 

 stomodseum of modern coral polyps. In the polyps of over 

 thirty species of West Indian corals which I have examined 

 there was no evidence of such a groove, and the same can also be 

 said of a like number of Pacific corals. The stomodaeum of coral 

 polyps is comparatively short, and presents the same histological 

 structure all round, showing there is very little if any physiologi- 



1 Other evidences of bilaterality and axial differentiation are sometimes presented 

 by zoanthid polyps. In living examples of Zoanthiis the axial tentacle over the ven- 

 tral directive entoccele is frequently larger than any of the others ; sometimes the 

 coloration along the dorso-ventral axis is different from that elsewhere ; and I have 

 also found the disc to be actually grooved or depressed along the axis. The Ceriantheas 

 are also characterized by the possession of a single siphonoglyph, but the manner of 

 addition of the mesenteries in this group precludes any relationship with the Rugosa. 



