MOLLUSC A 223 



lobes, which differ in their arrangement in the two sexes. The 

 lobes bear at their edges cylindrical tentacles, which can be 

 retracted into muscular sheaths. Probably the tentacles cor- 

 respond to the suckers in the Dibranchiata. In some species 

 of cuttle-fish the suckers are replaced by hooks, or both may 

 coexist ; the arms in the Octopoda are usually connected by a 

 fold of skin forming a web, which is no doubt of use in swim- 

 ming. The arms of Architeuihis, a gigantic form, sometimes 

 attain the length of 40 feet, and the total length of the body 

 and arms may measure 60 feet. 



The beak which guards the mouth is calcareous in Nautilus, 

 and horny in other Cephalopods. The possession by Nautilus 

 of two pairs of auricles which open into the single ventricle 

 is correlated with the two pairs of ctenidia. There are in 

 this same animal two pairs of nephridia; this repetition of 

 parts is almost unknown in MoUusca, the only other case 

 being the gills and shells of Chiton, and it is therefore par- 

 ticularly interesting. 



.The chief nerve ganglia in Nautilus are band-like, and 

 hardly to be distinguished from the commissures which con- 

 nect them. The nerves to the mantle are numerous, and are 

 not aggregated into one stout cord as in the Dibranchiata. The 

 same animal is provided with a pair of osphradia, situated at 

 the base of the anterior ctenidia ; these organs have not yet been 

 discovered in other Cephalopods. The eye of Nautilus is one 

 of the most remarkable organs found in the order. It has the 

 shape of a kettledrum ; the tense membrane, which is external, 

 being pierced at its centre by a minute hole, which leads into 

 a dark chamber lined by the retina. The latter is bathed by 

 sea water, which enters through the minute pore. The mechan- 

 ism by which images must be formed on the retina resembles 

 that of a pin-hole camera. 



