OCEAN LIFE. 23 



This part consists of an outer thin and dense membrane, of an 

 inner thicker membrane beset with long cilia, and of an air- 

 bladder, which at one point is attached to the above membranes, 

 where there is a small constricted aperture, at least in the outer 

 membrane. This membrane is developed into a kind of crest 

 along its upper part." "The air-bladder is endowed with a con- 

 siderable power of contraction, and when carefully examined, 

 two orifices are observable, one at each extremity, through which, 

 upon pressure, the contained air readily escapes, a provision for 

 enabling the creature to regulate its specific gravity at pleasure, 

 and, when alarmed, at once to lessen its buoyancy by diminish- 

 ing the capacity of its swimming-bladder, and to sink into the 

 waves." " The opinion that these animals are able to expel the 

 air from the air-bladder at will, was rendered doubtful, as a 

 general rule, by Olfers, who could find no opening in the large 

 bladder of Physalia. [Subsequent observations, however, have 

 determined that Physalia is the only one of the Physsophoridse 

 whose bladder does really communicate with the external air. 

 But, though there be no such communication in the rest, Leuckart 

 states that in many of them (and he believes it to be true of all,) 

 the air may be readily caused to pass from the cavity of the 

 bladder into that of the common stem, by the expansion of the 

 upper extremity of which the air-bladder is in all cases sur- 

 rounded." "Quatrefages has described the action of the sphincter 

 muscle, and the connexion of both bladders with the aperture ; 

 he also caused the air contained in the interior bladder to be 

 analyzed, and found that it contained less of oxygen than atmo- 

 spheric air, by about three per cent. ; the animal appeared to be 

 able to expel the air voluntarily at intervals, and to distend the 

 bladder again after a short time ; it would therefore seem to be 

 a respiratory organ for the colony. The air-bladder is surround- 

 ed on all sides by the external bladder or envelope, which is, in 

 fact, the expanded stem of the colony ; with the under surface 

 of this the various appendages are connected, and into its cavity 

 the cavities of them all open more or less directly. The bladder 

 in Physalia did not appear to Quatrefages to be merely a passive 

 organ, for besides the power of emptying and distending it, the 

 animal seemed to be able to direct the fluid contained in the 



