APPENDAGES OF MOLLUSCA. 



3^7 



all Dibranchiata. The arms of the Octopoda, like the similar ones in 

 the Decapoda, are connected together at their bases by a web, ex- 

 cepting the pair which are nearest to the sides of the funnel. This 

 connecting membrane extends farther in some Octopoda ; sometimes 

 over a few of the arms only (four in Tremoctopus), or over them all 

 (Histioteuthis, more completely in Cirroteuthis), and is continued 

 right up to the tips of the arms. 



The suckers are special structures found on the arms of the 

 Cephalopoda; they generally beset the 

 oral surface of the arms in two rows 

 (one in Eledone), and not unfrequently 

 they are carried on stalks. Their free 

 edge has often a cuticular thickening 

 which has the form of a chitinous rinsr, 

 and is sometimes toothed. Where a 

 particular tooth is largely developed the 

 sucker disappears, and is replaced by a 

 hook (Onychoteuthis). 



In many Cephalopoda certain of these 

 arms are peculiarly altered by function- 

 ing as copulatory organs ; even in 

 Nautilus the tentacles perform this func- 

 tion. It is not always the same arm 

 that is thus metamorphosed ; as a rule 

 it is one of those which belong to the 

 so-called ventral side of the animal. 

 The mode of metamorphosis varies 

 greatly in the different divisions ; it may 

 merely consist in the alteration of a part 

 of the base of the arm (Sepia), or a 

 more or less large number of the 

 suckers may be altered, or the tip of 

 the affected arm may be provided with 

 a spoon-like hollow process (Octopus, 

 Eledone). 



The highest grade of this adaptive 

 metamorphosis is seen when the arm 

 becomes greatly increased iu size, as 

 well as different in organisation inter- 

 nally (Argonauta, Tremoctopus). This 



" hectocotylised arm " is not developed, as are the others, by 

 a process of free gemmation, but it is formed in a vesicle, from 

 which it is not set loose till it is mature. The greatly-coiled 

 flagelliform end of the arm (Fig. 173, y), which is not set free till 

 the time of copulation, has a similar covering. This appendage, 

 with its investing membrane (x), corresponds to the modified end 

 of the arm in Eledone and Octopus. The more highly differen- 

 tiated copulatory arms may continue to live within the mantle- 

 cavity of the female for some time after they are broken off; this 



Fig. 173. Mule of Tremoc- 

 topus carenas. t l Superior; 

 t~ Second pair of arms, t 3 Third 

 left arm. i 4 Inferior pair of 

 arms, h Hectocotylus. x Its 

 terminal vesicle, y Filamen- 

 tous appondage, set free from 

 the terminal vesicle, i Funnel. 



