ACALEPH;^:. 170 



Physalida, or Hydrostatic Acalephce. They have 

 obtained this latter appellation from their being 

 rendered buoyant by means of vesicles filled with 

 air, which enable them to float without the neces- 

 sity of using any exertion for that purpose. The 

 Physalia, or Portuguese Man -of- War, as it is called, 

 (Fig. 85.) is furnished with a large air-bladder, of 

 an oval shape, placed on the upper part of the body ; 

 and also with a membrane of a beautiful purple 

 colour, which, as in the Velella, serves as a sail. 

 These Zoophytes are met with in great numbers in 

 the Atlantic Ocean, and more especially in its 

 warmest regions, and at a considerable distance 

 from land. In calm weather they float on the sur- 

 face of the sea, rearing their purple crests, and 

 appearing at first like large air bubbles, but distin- 

 guishable by the vivid hues of the tentacula which 

 hang down beneath them. Nothing can exceed 

 the beauty of the spectacle presented by a numerous 

 fleet of these animals, quietly sailing in the tro- 

 pical seas. Whenever the surface is ruffled by the 

 slightest wind, they suddenly absorb the air from 

 their vesicles, and becoming thus specifically 

 heavier than the water, immediately disappear, by 

 diving into the still depths of the ocean. By what 

 process they effect these changes of absorption and 

 of reproduction of air yet remains to be discovered. 

 Other genera, as the Pliyssophora^ have several of 

 these air-bladders ; but in other respects resemble 

 the ordinary Medusae, in having no membranous 

 crest. 



The ActinicB are a tribe of Zoophytes, which 

 from the general resemblance of their forms to those 

 of Polypi, are by most naturalists included under 



