ON NAUTILUS AND SOME OTHER ORGANISMS. 169 
2. Some Features in the Arterial System of N. 
pompilius, as determined by Injection. 
(1) Cireulus Pallialis.—After successful injections a sin- 
gular feature in the circulatory system is to be observed. The 
marginal pallial artery, which I described and figured in a 
former note, is found to pass on each side, dorso-laterally, into 
a branch of the dorsal aorta, so that a complete arterial circuit 
is produced. 
I have even partially injected the marginal pallial arteries 
from the dorsal aorta itself, but the injection fluid did not 
proceed very far in this centripetal direction, owing no 
doubt to the resistance it met with from the action of the 
heart. 
For this remarkable arterial circuit, produced by the con- 
fluence of the marginal pallial arteries, which arise ultimately 
as branches from the so-called “lesser aorta,” with a pair of 
branches! from the great aorta, I propose the above name of 
“circulus pallialis.”” It is illustrated in the accompanying 
sketches (Figs. 16 and 17). 
In Fig. 16 the posterior convex extremity of the body is 
supposed to be somewhat tilted up, in order to show the whole 
outline of the septum-producing area of the mantle. This out- 
line is very distinct in fresh specimens, and the region of the 
mantle enclosed by it is distinguished from the surrounding 
portions of the mantle by its greater thickness and opacity. 
. As already stated, this is the portion of the mantle which 
manufactures the septa, and it has an abundant arterial supply 
through the ramifications of the two main branches of the 
posterior pallial artery. These ramifications may be grouped 
together as the pallio-septal arteries; and it is surprising 
to see how rigidly they are confined within the septal 
contour. 
It will be noticed that the latter makes on each side a 
1 Yor reasons which will presently appear, these may be called the pallio- 
nuchal arteries. 
