112 THE BRAIN OF FISHES 



of Insects and other Arthropods which approaches most 

 closely to that of Fishes, inasmuch as they possess a 

 single or double ganglionated nervous cord running through 

 the body, which is fairly comparable with the spinal cord. 

 In Insects and their allies, however, this cord is situated 

 in the ventral region ; while the spinal cord of Verte- 

 brates lies above the alimentary canal in the dorsal region 

 of the body. No such structure exists or is needed 

 among Mollusks, because these organisms have no 

 articulated locomotor appendages, and are otherwise 

 notably different in form and organization ; yet it is true 

 that among the highest representatives of this latter 

 class (viz., the Cephalopods), we get the first approach to 

 the formation of a distinct brain case or ' cranium.' 



All the nerve-centres situated within the cranium have 

 been regarded as parts of the Brain in Vertebrates, whilst 

 those lying beyond it, and within the spinal canal, con- 

 stitute the Spinal Cord : the two together are sometimes 

 spoken of as the ' Cerebro- Spinal Axis.' 



But in addition to the Sensory Ganglia, and the Medulla 

 Oblongata, there are certain highly important supple- 

 mentary parts entering into the composition of the Brain of 

 the Fish. There is, for instance, a pair of bodies known 

 as the Cerebral Lobes ; whilst further back, in connec- 

 tion with the Medulla, we have another new nervous 

 ganglion, single, but having equal parts on each side of 

 the middle line, which is known as the Cerebellum. 

 That representatives of these parts (seemingly superadcled 

 to the brain of Fishes and other Vertebrates) are really 

 non-existent in the highest Mollusks and Insects it would 

 not be safe to affirm ; especially as ganglia, which have 

 been compared to Cerebral Lobes, exist in the Cuttlefish, 

 and even more distinctly in Ants, Bees and some Flies. 

 On the other hand, both the Cerebral Lobes and the 



