190 THE MECHANICAL FUNCTIONS. 



(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 purpose for 

 which it was given to the animal ; enabling it to 

 steer its course by means of the loose edges, and 

 also of the tentacula, which extend from the 

 lower side of the body, and act as a rudder, 

 while the sail is impelled by the wind. 



A construction still more artificial is provided 

 in another family of the same order, denomi- 

 nated the Physalida, or Hydrostatic Acalepha. 

 They have attained this latter appellation from 

 their being rendered buoyant by means of 

 vesicles filled with air, which enable them to 

 float without the necessity of using any exertion 

 for that purpose. The Physalia, or Portuguese 

 Man-of-War, as it is called, (Fig. 85,) is fur- 

 nished 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 dis- 



