792 



ZOOLOGY 



bursts and sets free the elongated hectocotylised arm. Spermato- 

 phores are taken by the arm from the genital opening, and in the 

 act of copulation 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 armed with hooks, sometimes replaced by hooks. In 

 many cases the arms are united by a web-like fold, the inter- 



Fio. 688. IiOligo vulgaris. 



A, entire animal, dorsal view ; B, horny internal shell or pen. 

 (From Keferstein.) 



brachial membrane (Fig. 691), 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 

 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. 



In all the Dibranchiata the funnel is a complete tube. In the 

 Nautilus, on the other hand, as we have seen, the folds which form 

 the funnel have their edges merely in apposition, and not united. 



