THE HYDROMEDUSAE 45 



are attached side by side, leaving an incomplete hydroecium between 

 them (Praya); or the hydroecium is altogether absent (Galeolaria). 

 It serves essentially as a protective canal, into Avhich the coenosarc 

 may be withdx'awn. The coenosarc is extremely long, tubular, and 

 contractile ; its endoderm is continued upwards beyond the hydroe- 

 cium as the blind somatocijst (acrocyst), the upper end of which 

 usually secretes an oil globule, presumably of hydrostatic function 

 (oleocyst). The coenosarc carries either a cormidimn, or numerous 

 cormidia at regular intervals separated by free internodes ; they 

 are aggregations of individuals, which may in some cases become 

 freed fi'om the colony. They generally appear under one or other 

 of two main forms — Eudoxomes, which consist typically of hydro- 

 phy Ilium, gastrozooid Avith tentacle, and one or more medusoid gono- 

 phoi'es ; or Ersaeomes, in which typically a nectophore is added to 

 the persons which occur in the Eudoxome. In some cases hjalro- 

 phyllia are absent ; in others more than one gastrozooid is present 

 in each cormidium (Apolemia). 



(c) In the Physonectae (Fig. 45) the coenosarc is generally long 

 and tubular, and carries at its apex a small pneumatophore ; below 

 this generally occur series of nectophores followed by series of 

 hydrophyllia ; but either may be developed without the other ; 

 these are followed by the cormidia. There may be only a single 

 gastrozooid (Athoria) ; generally they are numerous. Dactylo- 

 zooids are generally present, each provided Avith a simple palpacle ; 

 sometimes they have an oral opening, and appear to serve for excre- 

 tion (ci/stons). The cormidia are generally ordinate, with free 

 internodes, but are rarely scattered irregularly along the stem 

 (Forskalea). Each cormidium is composed typically of a gastro- 

 zooid with a branched tentacle, one or more hydrophyllia, blasto- 

 styles, gonophores, and cystous. 



((/) In the Auronectae (Figs. 48rt, iSb) a small and highly 

 modified sub-order, the coenosarc is short and very thick, and is 

 traversed by anastomosing canals. It is covered above by a large 

 pneumatophore, provided " dorsally " with an aurophore ; below 

 this lies a corona of nectophores. Thje lower part of the coenosarc 

 is covered by cormidia more or less ordinate in arrangement, each 

 consisting primarily of a gastrozooid with tentacle, a branched 

 gonodendron, and a palpon (dactylozooid). 



((?) In the Cystonectae (Fig. 49) a large pneumatophore is also 

 developed, but the family is distinguished by the complete al)sence of 

 nectophores and hydrophyllia. The coenosarc is long and tubular 

 (Rhizophysa), or short and wide (Physalia) ; in the former case 

 the cormidia are generally ordinate, in the latter they are arranged 

 in a multi))le series along the ventral side of the trunk ; they con- 

 sist typically of one or more tentaculate gastrozooids, of gono- 

 dendra, and dactylozooids. 



