OF CEPHALOPODA. 211 



one on the right and one on the left of the body, half-way be- 

 tween the upper and lower sides, and extend from the heart 

 nearly to the anterior end of the liver. They are free from the 

 latter, but are attached along one edge to the mantle ; and thus, 

 during the contractions and expansions of the latter, the water is 

 brought in large quantities against the surface of these respira- 

 tory organs, and the commingled air is taken up and absorbed 

 by the blood within their numerous capillary vessels. This pro- 

 cess is called the aerification of the blood, and corresponds to the 

 breathing of air in the lungs of warm-blooded animals. 



The nervous system preponderates largely in the head ; and 

 we find here, as in all of the higher groups of animals, that the 

 tendency toward the head, cephalization, is a marked feature in 

 the organism. The great centre of this system may be likened 

 to a thick, broad, heavy ring, (fig. 125, g- 1 , g 2 , op,) lying immedi- 

 ately behind the jaws, and through which the throat, (th.) or gullet, 

 passes. It is situated pretty close to the lower side of the head. 

 Its principal regions are four in number, namely, 1, the so- 

 called optic lobe, (g 1 ,) which is a prominent bulging on the lower 

 middle line of the main mass ; 2, a pair of small lobes, one on each 

 side of, but slightly posterior to, the third; 3, another prominent 

 swelling (g- 2 ) on the upper median line ; and 4, the optic gan- 

 g-lions, (e 1 ,) two swellings, or rather expansions, one on each side 

 and between the first and third masses, and so wide in their 

 extent that they cannot be said to lie either above or below, but 

 rather on the equatorial line. It is equally clear, too, that the 

 optic nerve (fig. 125, op) for either eye springs from a point (fig. 

 124, op) exactly opposite the side of the throat (th). From the 

 foregoing it would seem to be pretty evident that the upper (g 2 ) 

 and lower (g 1 ) median lobes must share equally in the duty of 

 forming by their junction the ganglions which supply the organs 

 of vision, and that the so-called optic lobes, the first, are not ex- 

 clusively devoted to the eyes. The principal nerves which 

 originate from this centre are few, but easily demonstrable. In 

 the fore part of the head there are two sets : one above, known 

 as the throat, or cesophageal nerve and ganglion, and one below 



