126 



HYDEOZOA. 



bladder bears a crest, c, of a beautiful purple colour, that, presenting a 

 broad surface to the wind, acts as a 

 sail, by the assistance of which the 

 creature scuds along with some ra- 

 pidity. The air-bladder is endowed 

 with a considerable power of con- 

 traction, and when carefully ex- 

 amined, two orifices are observable, 

 one at each extremity, , 6, 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 buoy- 

 ancy by diminishing the capacity of 

 its swimming-bladder, and to sink 

 into the waves. The nature of the 

 air with which the little voyager dis- 

 tends its float has not been accurately 

 determined ; but it is undoubtedly a 

 secretion furnished at pleasure when 

 at a considerable distance from the 

 surface, although the mode of its pro- 

 duction is still unknown. 



(327.) CIKEIGEADA. The Cirri- 

 grade Acalephaa form a very remark- 

 able family, peculiarly distinguished by the possession of an internal solid 

 support, or skeleton, secreted in the substance of their soft and deli- 

 cate bodies. In Porpita 

 (fig. 62), this consists of 

 a flat plate of semicar- 

 tilaginous texture (fig. 

 62, 2), evidently depo- 

 sited in thin secondary 

 laminae, which gradually 

 increase in size as the 

 animal advances in 

 growth, the inferior be- 

 ing the largest and last formed. When examined after its removal from 

 the body, this fragile skeleton is seen to be extremely porous or cel- 

 lular ; and the pores being filled with air, it is specifically lighter than 

 water a circumstance that may contribute to the buoyancy of the 

 creature even when alive. 



(328.) The lower surface of Porpita is furnished with numerous 

 appendages called cirri, whereof some appear to be organs of prehension, 



Physalia. a b, vesicular float ; c, crest ; 

 d, orifice ; e, nucleus; //, inferior append- 

 ages. 



Porpita. 



