90 Introduction to A nimal Morphology. 



contents, elongating, developing tentacles, and be- 

 coming a hydranth. Allman saw the same in Cordy- 

 lophora. Blastostyles may pass into hydranths, as in 

 Hydractinia polyclinia, where the blastostyle has a 

 mouth, but no tentacles ; in some Eudendria, the 

 young blastostyle has tentacles, but loses them when 

 the gonophores are developed. 



The ova and spermatozoa are produced usually in 

 different gonophores ; the former by modifications of 

 the interstitial* endodermal, the latter of the ectoderm 

 cells (except in some Geryonidae). The ova are im- 

 pregnated within their sacs, and undergo segmentation 

 as usual, and the embryo resulting is either cylindrical, 

 ciliated, and infusorium-like (planula. Fig. 9, A), or 

 polypoid, free, non-ciliated (actinula). The planula 

 becomes flask-shaped, its wide end forming a hydro- 

 rhiza ; the narrow end rises into a hydrocaulus, and 

 finally develops a hydranth. The actinula swims for a 

 few days by the contraction of its body, then settles 

 down, forms a mouth, and becomes a hydranth. 

 Sometimes a planula develops directly into a medusa 

 with no hydroid trophosome (Cunina, M'Crady ; 

 Carmarina, ^gineta, and ^ginopsis, Metschnikoff ; 

 Lizzia, Claparede). 



The alternation of generations may be binary 

 (hydranth, gonophore, + hydranth, gonophore, &c.), 

 or ternary (hydranth, blastostyle, gonophore, + h., b., g., 

 &c.), or quaternary (hydranth, blastostyle, blastocheme, 

 gonocheme, + h., b., b., g., &c.) ; or even more complex 

 if the hydranths be heteromorphic. The succession 

 is, as a rule, constant in each species ; and no subse- 



* The testis in Hydractinia and Tubularia is certainly ectodermal 

 {Kleinenberg and E. Van Beneden), while the eggs are endodermal. 



