ORGANS OF PROGRESSION. 163 



•tion of this investment cannot force the water back 

 through the same aperture ; it is expelled through the 

 funnel, and that with considerable force — so much so, 

 indeed, that it is, in a great measure, by the expulsion 

 of the water taken into the respiratory cavities, that the 

 animals propel themselves along. As the funnel (see the 

 figure of the onychoteuthis, p. 158) points towards the 

 head, and the water is thrown out in that direction, it 

 must be evident that they swim with the head back- 

 wards ; this is indeed their mode of aquatic progression, 

 the arms being either closed together and projected, so 

 as not to impede them, or acting as oars in unison with 

 the branchial cavities. Such species as have paddles, 

 or fin-like expansions of the mantle, are aided by them in 

 their aquatic movements ; but in those species which 

 have not these paddles, as the octopus, the arms of 

 which are connected together by thick basal webs, (see 

 figure, p. 153,) these arms, by their flapping, take an im- 

 portant part in the act of swimming, and enable the ani- 

 mal to shoot along with great velocity. Thus in the act 

 of swimming, they not only use the arms and the fins as 

 paddles, but they also employ a water-throwing ma- 

 chine, for the purpose of propelling them onwards. 



The cephalopods, as already stated, have organs of 

 vision ; on each side of the head an eye is seated, 



