420 MOLLUSCA. 



In our descriptions the words upper and lower will be discarded, 

 and we shall use only anterior and posterior, dorsal and ventral, 

 in the sense described above. 



The head is sometimes described as being placed in the centre 

 of the foot, and the arms are looked upon as the frayed-out margins 

 of the foot ; on this view the name Cephalopoda is justified anatomi- 

 cally as well as functionally. On the other hand, this view is not 

 taken by all anatomists, many of whom regard the funnel as the 

 sole representative of the footof other Gastropods. We shall 

 consider this question further on. 



The head is well-developed, and carries on its anterior sides a pair 

 of conspicuous and generally elaborate eyes. Immediately behind 

 (ventral to) the eyes there is often on each side a pit supposed to be 

 olfactory. In certain Oeyopsida the eyes are stalked; in Nautilus 

 they are also stalked, and there are on each side two cephalic 

 tentacles in relation with the eye, the preocular and the postocular 

 tentacles (Fig. 338, Pr.o, Pt.o). 



The margins of the head are produced into exceedingly muscular 

 sucker-bearing arms in the Dibranchiates, and into tentacle-bearing 

 lobes in Nautilus. Of the former there are always four pairs (Odo- 

 2Joda), and sometimes five (Decapoda) ; they are arranged in a circle 

 round the head, and the anterior arm of each side is called the first, 

 while the posterior arm, viz., that nearest the funnel, is called the 

 fourth. The arms of the fifth pair, which are found in the Decapoda, 

 are longer than, and attached somewhat internally to, the others 

 between the third and fourth ; they are often inserted into pits, into 

 which they may be retractile (Sepia, Sepiola, Rossid), and are called 

 the prehensile or tentacular arms. The tentacular arms have suckers 

 only near their free, club-shaped end, but the other arms are suckered 

 all along their internal (oral) surface. 



The basal parts of some or all of the eight arms of the Octopoda 

 may be united by a membrane ; in Tremoctopus the four anterior, 

 in Histioteutliis the six anterior, in Alloposus and CirroteutMs all the 

 arms are so united along their whole length. The umbrella so formed 

 assists in locomotion by its alternate contraction and expansion. 



In the female Argonauta (Fig. 335) the terminal parts of the two 

 anterior (dorsal) arms are expanded into thin membranes (vela), which 

 secrete the unilocular spiral shell. 



Finally, in the males one of the arms is more or less modified for 

 the purposes of sperm-transference in reproduction. The arm so 

 modified is said to be Jiectocotylized, because in some species (Argo- 



