CEPHALOPODA. 421 



nauta, Tremoctopus, Philonexis) it is entirely detached from the 

 male and left in the mantle-cavity of the female, where it was found 

 by Cuvier, who mistook it for a parasitic worm and called it 

 Hectocotylus. , 



The suckers are stalked in Decapoda, sessile in Octopoda. Further, in Deca- 

 poda, they have a horny ring, which may be smooth or denticulated in some cases ; 

 on certain parts of the arms the suckers may be replaced by hooks, and on the 

 tentacular arms of Onychoteuthis a large retractile hook arises from the centre 

 of each sucker. The suckers consist of a disc-like surface, in the centre of which 

 is a pit ; the depth of this pit can be increased by the retraction of its bottom, 

 to which muscle-fibres are attached. 



FIG. 335. Argonauta argo (female), the paper Nautilus, swimming. 



In the Tetrabranchiates (Nautilus) there are no arms, but in their place we 

 find lobe-like prolongations of the margin of the head, which bear tentacles. 

 The tentacles are retractile into sheaths, which are possibly comparable to the 

 suckers of the Dibranchiates. The lobes are as follows (Fig. 336) : an external 

 annular lobe (J), the anterior part of which forms the hood, into which the coil 

 of the shell fits, while the posterior part is much reduced ; this lobe carries 

 19 tentacles on each side ; within the annular lobe are in the female three tenta- 

 culiferous lobes, a posterior (ventral) and two lateral (d, c) ; in the male the 

 posterior lobe (d) is reduced to a paired group of lamellae and bears no tentacles, 

 and the right and left inner lobes are divided into two parts, a larger and a 

 smaller. On the posterior inner lobe of the female there is a lamellated organ (?i), 

 and behind it, on the inner side of the annular lobe, another lamellated 

 organ (m) to which the spermatophores of the male appear to be affixed (Kerr). 

 In the male the smaller of the two parts into which the left inner lobe 

 is divided ends in imbricated modified foliaceous tentacles (p) ; it is the 

 spadix, and has been regarded as the lobe used in sperm transference and 



