114 



HYDROZOA. 



Family 5. Ehizophtsiad^. 



Pneumatocyst small. Camosarc filiform. 

 Nectocalyces and hydrophyllia absent 

 Tentacula having branches, without sac- 

 culi. 



Family 6. Physaliad^e. 



Pneumatocyst occupying almost the whole 

 of the thick and irregularly fusiform coenosarc, 



Nectocalyces and hydrophyllia absent. 



Tentacula with basal sacs, but no lateral 

 branches. 



Family 7. Velellid^e. 



Pneumatocyst flattened, divided into cham- 

 bers by numerous concentric partitions, and 

 occupying almost the whole of the discoidal 

 coenosarc. 



Nectocalyces and hydrophyllia absent. 

 Tentacula short, clavate, simple or branched 

 submarginate. 



A single central, principal polypite. 



? 



7 



Hednsidae 



If the phyogem- 



maria of Velella and Porpita be regarded as 

 gonoblastidia, the hydrosoma of these genera may 

 then be said to present not more than one true 

 polypite ; a character in which they differ from 

 all other Pkysophoridce, but agree with the mem- 

 bers of the next order, Medusiclce. Cuvier, in- 

 deed, associated the Velellidce, Medusiclce, and 

 free zooids of the Lucernaridce in a single group, 

 under the name of « Acalephes Simples " ; the re- 

 maining Physophovidm, together with the Calf 

 cophoridce, being distinguished as "Acalephes 



Hydrostatiques ". But the Velellidce, like all other 



