•28S ANATOMY OF VEkTEBkATES. 



unaided by an air-bladder, sustains itself at the surface of the 

 : ea, by vigorous muscular exertion of well-developed pectoral 

 and caudal fins, soars, as it were, in the upper regions of its 

 atmosphere, is proverbial for the rapidity of its course, and sub- 

 sists, like the Eagle, by pursuing and devouring a living prey : 

 it is the fish in which the instruments of voluntary motion are 

 best developed, and in which the cerebellum presents its largest 

 size and most complex structure. And this structure cannot be 

 the mere concomitant of a general advance of the organisation to 

 a higher type, for the sluggish Rays, that grovel at the bottom, 

 though they copulate, and have in most other respects the same grade 

 and type of structure as the more active Squaloid Plagiostomes, 

 yet have a much smaller cerebellum, with a mere crucial in- 

 dentation instead of transverse lamina?. A more decisive instance 

 of the relation of the cerebellum to the power of locomotion is 

 given by the Lepidosiren in which, with a more marked general 

 advance of organisation than in the Ray or Shark, the cerebellum 

 has not risen above the simple commissural condition which it 

 presents in the Lamprey ; the generative system, however, of the 

 Lepidosiren is as complex as in the Plagiostomes, and is more 

 extensive : but the fins are reduced to mere filaments, and the 

 fish is known to pass half the year in a state of torpid inactivity. 

 The cerebellum is large in the Chimaera, fig. 179, c. In the heavy 

 laden ganoid fishes, the cerebellum is smaller than in the ordinary 

 Osseous Fishes : the imbricated armour of dense enamelled bony 

 scales must limit the lateral inflections of the tail ; so we find in 

 Polypterus the cerebellum hardly more developed than in Lepido- 

 siren, whilst in the somewhat more active and predaceous Lepi- 

 dosteus it is the smallest of all the segments of the brain. In 

 the grovelling Sturgeons the cerebellum offers a grade of develope- 

 ment above that in the Lepidosiren. Finally, amongst the normal 

 Osseous Fishes, the lamest and highest organised cerebellum has 

 been found in the Tunny, whose muscular system approaches, in 

 some of its physical characters, most nearly to that of the warm- 

 blooded classes. 



If we could enter the sensorium of the fish, and experience 

 the kind of sensations and ideas derived from the inlet of their 

 peculiarly developed and enormous eyes, we might be enabled to 

 understand the office of the peculiar complexities of their 

 large optic lobes : without such experience, we can at best 

 only indulge in vague conjecture from the analogy of our own 

 sensations. We find, when Nature reduces the organs of 

 sight to such minute specks as can give but a feeble idea of 



