37 



enveloped in a tough membrane, or dura mater. The two anterior ganghons 

 (2. 2. fig. 1. pi. 7.), which are analogous to those termed pes anserinus in the 

 Cuttle-fish, are of a flattened elongated form, are lodged in the groove on the 

 anterior part of the cephalic cartilage, and lie on either side the oesophagus : 

 they become gradually smaller towards the ventral aspect, and are there united 

 so as to encircle the alimentary tube. They give oiF the nerves to the tentacles 

 and parts surrounding the mouth. The middle and superior pair of ganglions 

 (3. fig. 1. pi. 7.) are analogous to the reniform ganglions of the Cuttle-fish, and 

 have the same peculiar fibrous structure ; but they are of an oval form, and 

 exhibit a diminution of volume corresponding with that of the central com- 

 missure or brain, being only three lines in the long, and two in the short dia- 

 meter. They are continued from the central commissure more directly than the 

 other ganglions, are connected with it by short narrow pedicles, and appertain 

 exclusively to the eyes. The posterior ganglions (4. 4. fig. 1. pi. 7.) resemble 

 in form the anterior, and encircle the oesophagus in a similar manner, becoming 

 narrower as they approach to coalesce on the ventral aspect. They give off' the 

 nerves to the muscles of the shell and to the viscera. 



The double collar thus formed is not, however, pecuhar to Nautilus, but exists 

 in other Mollusks, as, for example, in the genus Aplysia ; where, however, the 

 suboesophageal ganglions being more remote, the filaments that connect them to 

 the common centre above are necessarily longer. Nor is the similarity of dis- 

 position to be overlooked in the nervous system of the higher Cephalopods, 

 although obscured by a greater degree of concentration in the larger masses. 

 In Octopus, for example, a double chord extends from the brain on either side the 

 oesophagus, but the ganglions are united into one mass below ; from which mass 

 the nerves analogous to those suppUed by the separated suboesophageal ganglions 

 of Nautilus are given off. In Sepia also, the nerves of the arms are derived from 

 the anterior suboesophageal ganglions, as represented at 5. 5. fig. 3. pi. 7, which 

 figure has been added both for the purpose of comparison with the ners'ous 

 system of Nautilus, and also on account of the errors which, as Cuvier has 

 pointed out, exist in the figures previously given of this organ*. 



In Nautilus the nerves which arise from the ganglions of the anterior circle 



* See Scaqia, " de Auditu et Olfacto," tab. iv. fig. 7. 10. 11. and M. TOesius in " Beitrage fiir die 

 Zergliederungskunst," von H. F. Isenflamm, B. 1. Heft 2. tab. ii. fig. 6. 



