MOLLUSC A 



213 



and can be withdrawn into pouches at their base ; they do not 

 bear their suckers scattered uniformly over their inner surface, 

 but the suckers are all aggregated in a swollen pad at the free 

 end of the arm (Fig. 126). The suckers are very remarkable 

 organs^ they are cup-like structures whose rim is strengthened 

 by a toothed horny ring. A retractor muscle can deepen the 

 cavity of the cup, so that when the edge of the cup is applied 

 to any object and the muscle contracts, the sucker adheres to 

 the object by the pressure of the surrounding medium. In 



Fio. 125. 



a. Internal skeleton of Sepia ornata. Rang. 



b. Internal skeleton of Sistioteuthis BoneUiana. D'Orl). 



c. Internal skeleton of Spirvla fragilis. Lamarck. 



d. Animal of Spirula Peronii. 



Sepia, as in other Decapods, the suckers are stalked. In the 

 male the fifth arm on the left side has lost some of its 

 suckers, and this is termed the hectocotylised arm, vide p. 222. 

 That portion of the foot which is modified to form the 

 sucker-bearing arms is homologous with the fore-foot or pro- 

 podium of other Mollusca (Fig. 122). The mid-foot or 

 mesopodium has become converted into the siphon, a funnel- 



