178 THE MECHANICAL FUXCTIONS. 



ments, or cilia, proceed : the whole apparatus is 

 highly sensitive and irritable, and on the slightest 

 touch the filaments are thrown into spiral coils, 

 and retract rapidly within the body. They thus 

 act the part of tentacula, or delicate organs both of 

 touch and of prehension.* It was observed by 

 Otto Fabricius, that when a Beroe is cut into many 

 pieces, each piece continues to live, and to swim 

 about by the action of the cilia, which still continue 

 their vibratory motions. 



In two other genera of Acalephse, the Porpila 

 and the Velella, provision is made for the mecha- 

 nical support of the soft gelatinous mass, by means 

 of an internal cartilage. In the former, this car- 

 tilage 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 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, denominated the 



* See a description of the Beroe pileiis, by Dr. Grant, in the 

 Transactions of the Zoological Society of London, vol. i. p. 9, and 

 also a Memoir on the Beroidece, by R. P. Lesson, Ann. So. Nat. 

 serie 2, v. 235. 



