226 



HYDROZOA. 



condition are occupied by prolongations of the ccenosarc, and which 

 place the different zooids of the colony in direct connection. The 

 zooids are of two kinds, differing in size, structure, and function. 

 The larger zooids ("gastrozooids") are provided with a mouth and 

 stomach-sac ; while the others (" dactylozooids ") are elongated and 

 destitute of a mouth, thus coining to represent tentacles in form. 

 The gastrozooids occupy, as in Millepora, the large tubes of the 

 skeleton, and the dactylozooids are lodged in the small tubes. 

 Hence, when the dactylozooids are arranged in definite "cyclo- 

 systems " round the gastrozooids, then each of the large apertures in 

 the skeleton comes to be surrounded by a circle of smaller elongated 

 pores, which are only separated laterally by thin partitions, and which 



Fig. 1 10. — A, Portion of the skeleton of Stylaster sanguineus, of the natural size ; b, Small 

 portion of a branch of the same, enlarged, showing the calices and ampullae. Living, in the 

 Australian seas. (After Milne-Edwards and Haime.) 



thus give rise to the appearance of a central " calice " surrounded by 

 radiated "septa" (fig. in, c). In certain forms of the Stylasterids, 

 as in Allopora, the tubes lodging the gastrozooids are occupied in- 

 feriorly by a reticulate calcareous style or "columella" (a similar 

 structure occasionally existing in the dactylopores) ; and transverse 

 calcareous partitions or " tabulae " are sometimes present, though 

 usually in small numbers. Lastly, reproduction in the Stylasterids 

 is effected by means of reproductive zooids developed within special 

 sac-like cavities or " ampullse," which at certain periods communicate 

 with the exterior by means of minute pores. 



As regards their distribution in time, a few species of Stylasterids 

 have been recognised in the Tertiary deposits, the recent genera 



