MOLLUSCA 
N 
N 
wo 
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 Architeuthis, 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 Mollusca, 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. 
