XII 



PHYLUM MOLLUSCA 



739 



the entire arm is detached, and left in the mantle-cavity of 

 the female. In other cases the arm is not detached. The 

 suckers are sometimes stalked, sometimes sessile, sometimes 



FIG. t,4s. Argon auta arpo, showing the relations of the animal to the shell in the living 

 state, the arrow showing the direction of movement. /. funnel ; in. mouth, with jaws project- 

 ing ; sh. shell, with arms as seen through it ; wa, webbed arm clasping the shell. ^(From 

 Cooke, after Lacaze-Duthiers.) 



armed with hooks, sometimes replaced by hooks. In many cases 

 the arms are united by a web-like fold, the interbrachial mem- 

 brane (Fig. 650), which may reach nearly to 'their extremities. 



In the Tetrabranchiata the series of groups of slender, ringed, 

 sheathed tentacles,- situated on lobes of the foot surrounding the 



Fio 049 Octopus lentus, male specimen, showing the structure of the hectocotylised'arm 

 (h. a.). (From Cooke, after Yen-ill.) 



mouth, take the place of the arms, and suckers are not present. 

 In the males the spadix probably represents, functionally at least, 

 the hectocotylised arm of the Dibranchiata. 



Nautilus is the only Cephalopod that has any head-appendages 



SB 2 





