114 HYDHOZOA. 



Family 5. Rhizophtsiad.e. 



Pneumatocyst small. Coenosarc filiform. 

 Nectocalyces and hydrophyllia absent. 

 Tentacida having branches, without sac- 

 culi. 



Family 6. Physaliad^. 



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^. 



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. 



y. Order 6: Ifledusidae. — 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 Physophoridce, but agree with the mem- 

 bers of the next order, Medusidce, Cuvier, in- 

 deed, associated the VelelUdce, Medusidce, and 

 free zooids of the Lucernaridce in a single group, 

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

 maining Physophoridce, together with the Caly- 

 cophoridce, being distinguished as " Acalephes 

 Hydrostatiques ". But the Velellidce, like all other 



