190 THE MECHANICAL FUNCTIONS. 



fish for giving an effective impulse to the water, are the ten- 

 tacula. 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 pro- 

 gress, 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 ne- 

 cessity 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 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 pro- 

 cesses 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 ex- 

 tremities of these long tentacula that are provided with 

 suckers, while the short ones have them along their whole 

 length. 



The other genera of cephalopodous Mollusca are, like the 

 Sepise, provided with tentacula attached to the head. They 

 comprehend animals differing exceedingly in their size: 

 some being very large, but a great number very minute, and 

 even microscopic* The shells of these animals are often 

 found to contain partitions dividing them into a number of 

 chambers; hence they have been termed ccnneraied, or mul- 

 tilocidar^ ov polythalamous shells. The Spirula (Fig. 124) 

 is a shell of this description, of which the cellular structure 

 and numerous partitions are rendered visible by making a 



* A particular account has been given of the shells of these microscopic 

 cephalopoda by M. D'Orbig-ny, in the Annales des Sciences Naturelles; vii. 

 96. 



