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 which the coenosarc 

 may be withdrawn. The coenosarc is extremely long, tubular, and 

 contractile ; its endoderm is continued upwards beyond the hydroe- 

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

 usually secretes an oil globule, presumably of hydrostatic function 

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

 cormidia at regular intervals separated by free internodes ; they 

 are aggregations of individuals, which may in some cases become 

 freed from the colony. They generally appear under one or other 

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

 phyllium, gastrozooid with tentacle, and one or more medusoid gono- 

 phores ; or Ersaeomes, in which typically a nectophore is added to 

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

 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 with a simple palpacle ; 

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

 tion (cystons). 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 cystons. 



(d) .In the Auronectae (Figs. 48a, 486) 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. The 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). 



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

 developed, but the family is distinguished by the complete absence 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 multiple series along the ventral side of the trunk ; they con- 

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

 dendra, and dactylozooids. 



