168 ARTHUR WILLEY. 
the north coast of the Gazelle peninsula. One of them showed 
a tendency to sink to the bottom, which it always performed 
very gradually. In this one I had accidentally punctured the 
mantle over the heart. The others remained floating and 
swimming about on the surface during the whole time of 
observation. They did not go far in one steady direction, but 
tended to go in circles, as in fact did another one whose 
siphuncle was uncut. If one of the individuals floating at the 
surface was forced down to the bottom with a hand-net, it 
would slowly rise to the surface again. This also often happens 
with a Nautilus that has not been operated on. 
The results indicated by the above experiments, which, it 
may be added, are worth repeating, may be summarised as 
follows: 
The cutting of the siphuncle (a) does not temporarily affect 
the vitality of the animal; (3) does not prevent it from making 
movements of translation ;! (y) does not prevent it from floating 
at the surface; (6) does not prevent it from sinking to the 
bottom. 
It still remains to be ascertained whether a Nautilus whose 
siphuncle has been cut, having sunk to the bottom of the sea 
in shallow water, will undertake a journey to the surface. 
My experiment of July 10th would seem to indicate that this 
" might be expected to occur. 
The above experiments do not appear to oppose the view 
which I expressed in a former communication—that the si- 
phuncle of Nautilus pompilius is, in some measure, of the 
nature of a vestigial structure. 
It might indeed be legitimate to suppose, on the principle of 
the correlation of organs, that in the Nautiloidea the course 
of evolution has led to a reduction of the siphuncle pari 
passu with an increase in the efficiency of the chambers as 
hydrostatic organs. 
1 Tn speaking above of progressive movements I mean, of conrse, in the 
usual backward direction. 
