VELELLA. 145 



soft gelatinous mass, by means of an internal cartilage. In 

 the former, this cartilage is of a circular form; in the latter 

 (Fig. 84,) it is oval, and bears upon its upper edge a thin 

 pellucid membrane of a triangular shape, which extends the 

 whole length of the upper surface of the body. As this 

 membrane is connected with the cartilage at its middle part 

 only, while its edges are loose and floating, it is peculiarly 

 adapted, when above the surface of the water, to catch the 

 wind and act as a sail. Such, indeed, appears to be the pur- 

 pose for which it was given to the animal ; enabling it to steer 

 its course by means of the loose edges, and also of the tenta- 

 cula, which extend from the lower side of the body, and act 

 as a rudder, wdiile the sail is impelled by the wind. 



A construction still more artificial is provided in another 

 family of the same order, denominated the P/iysalida, or 

 Hydrostatic Acalepha. They have attained this latter ao- 

 pellation from their being' rendered buoyant by means of ve- 

 sicles filled with air, which enable them to float without the 

 necessity of using any exertion for that purpose. The Phy- 

 salia, or Portuguese Man-of-War, as it is called, (Fig. S5,) 

 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 thl?y float on the surface of the sea, rearing their 

 purple crests, and. appearing at first like large air bubbles, 

 but distinguishable 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 ani- 

 mals, quietly sailing in the tropical seas. Whenever the 

 surface is ruffled by the slightest wind, they suddenly absorb 

 the air from their vesicles, and becoming tlius 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 Fhys- 



VoL. I. 19 



