PHYSONECTAE. 



nectocalyces at their ends) 

 are usually numerous. 

 They have four radial 

 canals, a circular canal, a 

 velum, and sometimes 

 ocelli. The cormidia are 

 rarely dissolved : i.e., the 

 parts are rarely scattered 

 along the stem, but gener- 

 ally ordinate, i.e., in groups 

 (Fig. 126). Each cormi- 

 dium consists of one siphon 

 (sometimes two or four) 

 with tentacle ; several pal- 

 pons each with a tentacle 

 called a palpacle ; several 

 bracts which may even in 

 the forms with ordinate 

 cormidia occur in the in- 

 ternodes of the stem (Fig. 

 126); two gonostyles, one 

 bearing male gonophores 

 and the other female, and 

 very often a cyston. A 

 cyston is a structure like 

 a palpon but Avith a ter- 

 minal opening : it acts as 

 an anus to the colony, ex- 

 pelling fluid and crystalline 

 excretions through its aper- 

 ture. 



The batteries of the 

 tentilla of the tentacles 

 are enveloped in an invo- 

 lucruni or fold of ectoderm 

 arising at their proximal 

 end. The female gono- 

 phore produces only one 

 egg- 



Pn 



FIG. 126. Halistemma tergestinum. Pn pneumatophore ; S nectocalyx ; 

 P siphon ; D hydrophyllium ; A r fc group of nematocysts on tentacles. 



