350 LECTURE XXIV. 



Nautilus. In the Octopus, the brachial nerves pass along the inner 

 surface of the base of the arms before they penetrate them. As soon 

 as the acetabula begin to be developed, a series of closely approx- 

 imated ganglions are formed upon the brachial nerves, of which some 

 of the longitudinal fibres have been observed by Dr. Sharpey to pass 

 over the ganglions. Before the ganglionic enlargements commence, 

 each brachial nerve in the Octopus gives off two large branches, which 

 traverse the fleshy substance of the base of the arms to join the two 

 corresponding branches of the contiguous nerves which are thus as- 

 sociated together for consent of action by a nervous circle. 



The infundibular nerves arise behind the origin of the brachial 

 ones. Posterior to these the small acoustic nerves are given off from 

 the sub-oesophageal mass. The delicate motores oculi arise from the 

 upper part of the lateral connecting bands of the infra- and supra-ceso- 

 phageal masses, which may be compared with the crura cerebri, as the 

 nerves in question obviously answer to the third pair in the Vertebrata. 

 The nerves which correspond to those of the shell-muscles of the 

 Nautilus, form a single large pair (m, »?), which arises from the pos- 

 terior and lateral angles of the sub-cesophageal mass, extend outwards 

 and backwards, perforate the shell-muscles, and form the large stel- 

 lated ganglions (n, ?i), from which the nerves of the mantle are prin- 

 cipally derived. In the Octojjoda this may be described as the ter- 

 mination of the nerve ; but in the Decapoda, in which lateral fins are 

 superadded to the trunk, the great nerve previously divides into two 

 branches, of which the outer one expands into the ganglion, whilst 

 the inner branch, having been joined by one of the rays of the gan- 

 glion (o, o), pierces the fleshy substance of the mantle, and ends in a 

 diverging series of twigs appropriated to the muscles of the fin. 



In proportion as the trunk of the Cephalopod is attenuated and 

 elongated, the pallial nerves become more parallel in their course, 

 more dorsal in their position, and more similar to a rudimental spinal 

 chord, of which the two lateral columns have retained their primitive 

 embryonic separation. 



The two large visceral nerves (p) arise from the interspace of the 

 origin of the paUial pair ; after distributing filaments to the muscles of 

 the neck they descend parallel and close to one another behind the 

 vena cava, give off" the small filaments which constitute the plexus upon 

 that vein and around the oesophagus, then diverge from each other 

 towards the root of each gill, where they divide into three principal 

 branches : one of these dilates into an elongated ganglion (g) and 

 penetrates the fleshy stem of the branchia ; the second descends to 

 the generative organs ; the third passes to the middle or systemic 

 heart. The oesophageal plexus unites into a ganglion (r), attached 



