HYDEOZOA. 63 



float ; there are no swimming-bells or covering-pieces. PhysaUa Q^g^ 3^ 

 pela^ica (Model, Case 3) has one long stout main tentacle and Upright part, 

 numerous lesser ones ; GaraveUa * (Model, Case 3) has numerous 

 large tentacles. The float is borne wholly above the surface, and is 

 carried along by the breeze with the broad end foremost and the 

 tentacles trailing behind. Prof. Agassiz saw specimens with tentacles 

 over fifty feet in length, Physaha is notorious for its dangerous 

 stinging properties.^ 



Rhodalia miramla* is a deep-sea Siphonophoran, wonderfully 

 adapted for living at great depths ; the depressed oval air-sac is 

 followed by several circles of swimming-bells each attached by a 

 broad vertical lamella to the stem. 



A curiously modified swimming-bell, the aurophore, allows of 

 communication between the air-sac and the water ; by emptying or 

 secreting the air or gas the animal can sint or rise within certain 

 limits without using its swimming-bells. The " stem " is not a 

 delicate tubular siphon as in Physo])hora, but forms a thick mass 

 permeated by canals, the feeding polyps with tentacles and generative 

 buds being attached to its lower surface. 



Most of the swimming-bells and all the tentacles have become 

 detached in the specimen, the red colour of which is artificial ; 

 but the air-sac and massive stem are well preserved. The specimens 

 were obtained by the Challenger from a depth of GOO fathoms in 

 the South Atlantic. 



Diphyes * (Fig. 2-4 a, b), which is without an air-sac, has two 9*^^® ^' 

 swimming-bells, and below these groups of covering-pieces, feeding ^"^ 

 polyps, and generative buds situated along the stem. 



In VeUIla* or " By-the-wind Sailer " (Case 3), a vertical semi- 

 circular " sail " is attached diagonally across the upper surface of 

 an oblong disk ; attached to the lower surface of the latter are one 

 large central feeding polyp and circles of smaller feeding polyps, 

 generative buds, and a marginal fringe of tentacles. Fleets of 

 Velella sailing along in the breeze are more commonly seen in 

 warm latitudes, but specimens, both of Velella and PhysaUa, have 

 been found off the south-west coasts of England, 



Porpita* consists only of a circular disk with its dependent polyps 

 and tentacles. 



' Mrs, David mentions in her book on Funafuti that the natives are more 

 afraid of Pln/salia than they are of tlio sharks. 



