236 THE MECHANICAL FUNCTIONS. 



Mhich are supported by slender bones, called rays;' 

 but no structure of this kind exists in the fins of tiie 

 Cephalopoda. 



In swimming, the organs principally employed 

 by cuttle-fish for giving an eff^ective impulse to the 

 water, are the tentacula. These they employ as 

 oars, striking with them from behind forwards ; so 

 that their effect is to propel the hinder part of the 

 body, which is thus made to advance foremost, the 

 head following in the rear. They also use these 

 organs as feet for moving along the bottom of the 

 sea. In their progress, under these circumstances, 

 the head is always turned downwards, and the body 

 upwards, so that the animal may be considered as 

 literally walking upon its head. The necessity of 

 this position for the feet arises probably from the 

 close investment of the mantle over the body ; for 

 although the mantle leaves an aperture in the neck 

 for the entrance of water to the respiratory organs, 

 yet, in other respects, it forms a sac, closed in every 

 part, except where the head, neck, and accompany- 

 ing tentacula protrude. 



In the Calamary, as well as in the common 

 Sepia, two of the arms are much longer than the 

 rest, and terminate in a thick cylindrical portion, 

 covered with numerous suckers, which may not 

 unaptly be compared to a hand. These processes 

 are employed by cuttle-fish as anchors for the 

 purpose of fixing themselves firmly to rocks, dur- 

 ing violent agitations of the sea ; and accordingly 

 we find that it is only the extremities of these long 

 tentacula that are provided with suckers, while the 

 short ones have them along their whole length. 



The other genera of Cephalopodous Mollu;sca 



