240 THE MECHANICAL FUNCTIONS. 



It has been conjectured that this hydrostatic 

 apparatus is provided for the express purpose of 

 enabling the animal to descend or rise rapidly in 

 the water; the former of which it might effect by 

 injecting fluid into this extensible tube, so as to 

 compress into a smaller bulk the air contained in 

 each chamber, and thus increase the specific gra- 

 vity of the whole mass ; and the latter, by the 

 contrary action, of withdrawing fluid from the 

 tube.* 



Chapter IV. 



ARTICULATA. 



•^1. Articulated Animals in ge?ieral. 



From the Cephalopoda, the transition is easy to 

 the lowest order of vertebrated animals. But pre- 

 viously to pursuing the analogies which connect 

 these two divisions of the animal kingdom, we have 

 to pass in review a very extensive series of animal 

 forms, constructed on a peculiar system, and oc- 

 cupying, as well as the Mollusca, a place inter- 

 mediate between Zoophytes and the more highly 

 organized classes. 



We have seen that even in those Zoophytes 

 which are distinguished from the rest by a more 

 elaborate conformation of organs, the powers of 

 progressive motion are always extremely limited. 

 Nor are the Mollusca in general more highly 



* See Dr. Buckland's Bridgewater Treatise. 



