MOLLUSCA 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and can be withdrawn into pouches at their base; they do not 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 
Fic. 125. 
a. Internal skeleton of Sepia ornata. Rang. 
6. Internal skeleton of Histioteuthis Bonelliana. D’Orb. 
c. Internal skeleton of Spirula fragilis. Lamarck. 
d, Animal of Spirula Peroniti. 
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- 
