12 



THE HYDROMEDUSAE 



colonial (Bougainvillea) ; but may be replaced by filiform pro- 

 cesses (Corymorpha) ; it is, of course, absent in the motile 

 forms (Hydra). In the colonial forms it gives rise to one or 

 more simple (Perigonimus) or branching (Bougainvillea) hydrocauli. 

 The coenosarc of both hydrocaulus and hydrorhiza generally forms 

 a single tube. 



FIG. 16. 



FIG. 17. 



16. Diagram showing possible modifications of persons of a Gymnoblastic Hydromedusa. 

 a, hydrocaulus (stem) ; 6, hydrprhiza (root) ; c, coelenteron ; d, endoderm (thick black line) ; 

 e, ectoderm (hatched); /, perisarc (thin black line); g, hydroid expanded; g 1 , hydroid con- 

 tracted ; ft, hypostome, bearing mouth at its extremity ; k, degenerate medusoid (sporosac) 

 springing from the hydrocaulus ; K, sporosac springing from m, a modified hydroid (blastostyle) ; 

 thegenitalia are seen surrounding the spadix ; I, medusoid ; m, blastostyle. (After Allman.) 



17. Diagram showing possible modifications of the persons of a Calyptoblastic Hydro- 

 medusa. Letters a to k same as in Fig. 16. i, the horny cup or hydrotheca of the hydroid ; I, 

 medusoid springing from m, a modified hydroid (blastostyle) ; TO, the horny case or gonotheca 

 enclosing the blastostyle and its buds. This and the hydrotheca i give origin to the name 

 Calyptoblastea. (After Allman.) 



The tubes of the hydrorhiza are generally distinct from one 

 another, although they are often connected by cross-tubes into a 

 loose meshwork. In Podocoryne, however, such a meshwork occurs 

 only at the growing points of the colony ; in the more central parts 

 the tubes increase in number and anastomose so freely as to appear 



