398 PHYSIOLOGY OF I HE INVERTEBRATA. 



animal to submarine objects. This modification occurs in 

 Mijtilus^ Pinna, &c* 



In the Ptcropoda the function of swimming is performed 

 by the flapping epipodia, which are muscular expansions, but 

 it may be remarked that in these MoUusca "the rest of the 

 foot is always small, and often rudimentary, in correspondence 

 with the small size of the neural face of the body." 



The locomotive organs of the Cephalopoda are the tentacula, 

 which are arranged round the head, and furnished on their 

 inner surface with numerous sucking-cups, which enable the 

 animal to take a firm grasp of any object. By means of the 

 tentacula f the Cephalopoda, creep along the bottom of the sea 

 with the head downwards. These animals also swim rapidly 

 by the expulsion of the water from the branchial chamber. 



* For an account of the movements of various parts of certain bivalve 

 Molluscs, see the papers by D. M'Alpine in the Proc. Boy. Soc. Edinh., 

 •vol. 15, p. 173; vol. 16, p. 725. 



f The tentacula are also prehensile organs. 



