144 



THE MECHANICAL FUNCTIONS. 



denominated the Beroe, (Fig. 82 and 83,) which is remark- 

 able for its organs of progressive motion. Its body is either 

 globular, or oblong, and it swims with its aicis in a vertical 

 position. Eight longitudinal bands or ridges, which^have 



been sometimes compared to ribs, extend down its sides, 

 like those of a melon; and along each of these is attached a 

 set of little membranes, extended horizontally, and support- 

 ed on radiating fibres; so that they bear a pretty exact re- 

 semblance to the fin of a fish. Their action is not unlike 

 that of the wings of a bird; for they are made to flap up and 

 down, striking the water vertically, and communicating an 

 ascending impulse to the body. This animal is also pro- 

 vided with two very long and slender processes, which come 

 out from the sides of the body, and from these a great num- 

 ber of still finer filaments, or cilia, proceed: the whole ap- 

 paratus 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 tenta- 

 cula, or delicate organs, both of touch and of prehension.* It 

 was observed by 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 vibrato- 

 ry motions. 



In two other genera of Acalepha, the Porpita and the 

 Velella, provision is made for the mechanical support of the 



* See a description of the Beroe pikusy Lam. by Dr. Grant, in the Trans- 

 actions of the Zoological Society of London, vol. i. p. 9. 



