36 



THE HYDROMEDUSAE 



{cydosystem) 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 ialulae below the zooids. Calcareous brush- 

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

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



42.— Portion of the calcareous corallnm of MilU-pnm ncilo^^n, showinR the cyclical airaii(;e- 

 meiit of the pores occupieil by the hydroiils. Twice the natural size. (Froui Mdseley.) 



43.— Enlarged view of the surfaceof a living Millepora, showing live liactylozooids surrouml- 

 iug a central gastrozooid. (From Moseley.) 



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

 may occur in the coenenchyme, and are termed amjniUac. 



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 }>ecomes continuous with the ectoderm of the zooids, and appear.s 

 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. 13) are of two kinds. The gastrozooids, the 



