264 THE MECHANICAL FUNCTIONS. 



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 

 movijig along the bottom of the sea. In their 

 progress, under these circumstances, the head is 

 always turned downwards, and the body up- 

 wards, so that the animal may be considered as 

 literally walking upon its head. The necessity 

 o£ 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 respi- 

 ratory organs, yet, in other respects, it forms a 

 sac, closed in every part, except where the head, 

 neck, and accompanying 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, during violent agitations of the 

 sea ; and accordingly we find that it is only the 

 extremities of these long tentacula that are pro- 

 vided with suckers, while the short ones have 

 them along their whole length. 



