3 6 



THE HYDROMEDUSAE 



(cyclosystem) may be protected by a calcareous flange (Cryptohelia) ; 

 in some cases calcareous laminae between the dactylozooids of a 

 cyclosystem simulate the arrangement of septa in an Anthozoan 

 theca (Allopora). In branching forms the whole thickness of the 

 branch is often permeated by coenosarcal tubes ; in massive forms 

 the living tissues are confined to the circumference, and by secreting 

 plates of coenenchyme behind them as they grow peripherally 

 outwards, give rise to tabulae below the zooids. Calcareous brush- 

 like styles rise in some instances from the tabulae of both gastro- 

 pores and dactylopores (Stylaster), or in the gastropores only 



FIG. 43. 



42. Portion of the calcareous corallum of Millepora nodosa, showing the cyclical arrange- 

 ment of the pores occupied by the hydroids. Twice the natural size. (From Moseley.) 



43. Enlarged view of the surface of a living Millepora, showing five dactylozooids surround- 

 ing a central gastrozooid. (From Moseley.) 



(Distichopora). Special pits for the reception of the gonophores 

 may occur in the coenenchyme, and are termed ampullae. 



The coenosarc is covered by a superficial sheet of ectoderm 

 which is provided with very large nematocysts. This sheet, which 

 is perhaps composed of two layers, rests partly on spines of the 

 skeleton, partly on the blind ends of the coenosarcal tubes, and in 

 retraction is continued downwards as a lining to the pores ; here 

 it becomes continuous with the ectoderm of the zooids, and appears 

 to form a circular operculum over them when retracted completely. 

 Elsewhere than in this sheet, ectoderm, mesogloea, and endoderm 

 bear to one another the relations usual in Hydrozoa. 



The hydroids (Fig. 43) are of two kinds. The gastrozooids, the 



