NERVOUS SYSTEM AND LOCOMOTION OF CEPHALOPODA. 343 



is more complicated than that of other Mollusks; and the different 

 ganglia, situated around the oesophagus, tend more to unite in a 

 single mass. The nervous collar thus formed is composed of a 

 pair of cephalic ganglia, whence originate the optic nerves ; of a 

 pair of ganglia, situated more in front, but beneath the oasopha- 

 gus, and furnishing the tentacula with nerves ; lastly, of a pair 

 of thoracic ganglia, supplying the mantle with nerves, and 

 sending backwards two cords on each side, which are themselves 

 furnished with ganglia, whence proceed nerves that supply the 

 mantle, gills, heart, c. It is evident from this description, 

 that the nervous system of the Cephalopods approaches that of 

 the lower Fishes in many particulars ; especially in the almost 

 complete concentration of the nervous centres in the region of the 

 head, and in the presence of the rudiments of a cerebral mass. 

 If the cords that proceed backwards from the sub-cesophageal 

 ganglion had been united on the central line, instead of diverging 

 from each other, they would have presented a strong analogy in 

 position, as well as in function, to the Spinal Cord of Vertebrata. 

 883. Cephalopods are almost exclusively marine in their 

 habits; only a few of them, such as the Octopus or Poulp 

 (Fig. 545), ever quitting the water to prowl along the shore in 

 search of food. When thus moving, they walk in what may be 

 considered an inverted position ; the mouth being downwards, 

 and the opposite extremity of the body being directed upwards. 

 When swimming, the Poulp moves backwards through the 

 water, propelling itself by the alternate contractions and exten- 

 sions of the circular fin, which unites the bases of the arms. But 

 the long slender-bodied Calamaries (Fig. 540), in which the arms 

 are short, swim much more after the manner of Fishes ; striking 

 the water, by means of the fin- like expansions of the mantle 

 along the back and abdomen, with such force as occasionally to 

 raise themselves out of the water. The Nautilus (Fig. 548) 

 seems more limited in regard to its means of locomotion ; since 

 its arms are not long enough to serve as efficient instruments for 

 this purpose, and it has no other means, except a fleshy disc, 

 which resembles that of the Gasteropods, and which enables the 

 animal to crawl along solid surfaces in an inverted position. 



