224 



HYDROZOA. 



The large zooids (" gastrozooids ") possess a mouth and digestive 

 cavity, and have from four to six short tentacles. On the other 

 hand, the smaller zooids (" dactylozooids ") have no mouth, and 

 possess short clavate tentacles on their sides : they are long and 

 slender, and perform the function of prehension for the colony. 

 Whatever may be the structure of the contained zooids, the zooi- 

 dal tubes are intersected internally by distinct transverse calcareous 

 partitions or "tabulae" (fig. 108, c, and fig. 109, b) ; but there are 

 no traces of radiating vertical partitions or " septa." The entire 

 ccenosteum shows a more or less evident composition out of thin 





d~ 



IP 



C 





mm 



r. te 





Fig. 108. — A, Portion of the ccenosteum of Millepora alcicornis, of the natural size ; b, Part 

 of the surface of the same enlarged, showing the mouths of the " gastropores " (g) and " dacty- 

 lopores " (d), and the intermediate ccenosteal tissue ; c, Vertical section enlarged, showing the 

 tabulate zooidal tubes. (Original.) 



concentric laminae, only a thin surface-layer being at any given 

 moment actually alive. The reproductive process in Millepora is 

 imperfectly known, but Mr Quelch has shown that in one species 

 of the genus the reproductive zooids are developed within special 

 globular cavities or " ampullae," which are contained within the 

 general spongy skeleton, and are covered by a thin porous layer. 

 The structure of Millepora has been dwelt upon at some length, 

 as throwing considerable light upon the relationships of the extinct 

 Stromatoporoids, but the genus itself is of little palaeontological im- 

 portance. The earliest known forms of the genus appear in the 



