HYDROMEDUSAE. 



125 



mec- 



tentacles, while the attached end often spreads itself into a discoidal root 

 (Fig. 109), the hydrorhiza.* This is the first polyp. The free upstanding 

 portion is correctly 

 termed the hy- 

 dranth, though the 

 word polyp is some- 

 times loosely ap- 

 plied to it. The 

 hydranth elongates 

 and begins to bud, 

 and two parts be- 

 come distinguish- 

 able in it a ter- 

 minal part called 

 the polyp-head or 

 hydrocephalis with 

 the mouth and ten- 

 tacles, and a lower part with 

 the buds, the stem or coeno- 

 sark. The buds lengthen, 

 remain attached to the parent, 

 and themselves become differ- 

 entiated into hydrocephalis and 

 coenosark. The whole colony 

 of hydranths thus formed is 

 called the hydrosoma. The 

 term hydropJiyton seems to be 

 applied to the coenosark plus 

 hydrorhiza, while hydrocaulus^ 

 appears to be synonymous with 

 coenosark. In some forms the 

 perisark does not extend on to 

 the polyp-head but stops short 

 at its base ; in others it is con- 

 tinued round the polyp- head 

 as a cup the hydrotheca (Fig. 

 114), which, however, stands 

 off from the polyp, being only 

 connected with it by pseudo- 

 podial prolongations of the 

 ectoderm. The polyp-head can 

 shrink into the hydrotheca for 

 protection. Some of the buds 

 formed by the colony develop 



gth- 



FIG. 110. Gonangium with hedrioblasts of Gono- 

 thyraea loveni (from Chun, after Allman). U 

 blastostyle ; gth gonangium (gonotheca) ; c oper- 

 culum of gonangium ; go 1 -^ budding gonophores 

 (hedrioblasts) ; mec 1 hedrioblast with planulae 

 pi; mec 2 hedrioblast with ova in manubrium. 



* The hydrorhiza has some- 

 times the form of a branching 

 and anastomosing tube, from 

 which the hydranths arise by 

 budding. 



f The coenosark or hydrocaulus is said to be fascicled or polysiphonic when it 

 is composed of several adherent tubes, monosiphonic when consisting of a single 

 tube. 



