188 LECTURE VIII. 



Fishes. If, on the other hand, we compare the Cyclostome and 

 Plagiostome Cartilaginous Fishes, in reference to their modes and 

 powers of locomotion, we shall find a contrast which directly accoi'ds 

 with that in their cerebellar development. The Myxine commonly 

 passes its life as the internal parasite of some higher organised fish : 

 the Lamprey adheres by its suctorial mouth to a stone, and seldom 

 moves far from its place : neither fish possesses pectoral or ventral 

 fins. The Shark, on the contrary, unaided by an air-bladder, by 

 vigorous muscular exertion of well-developed pectoral and caudal 

 fins, sustains itself at the surface of the sea, soars, as it were, in the 

 upper regions of its atmosphere, is proverbial for the rapidity of its 

 course, and subsists, like the Eagle, by pursuing and devouring a 

 living prey : it is the fish in which tlie 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 indentation 

 instead of transverse laminre. 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, liowever, 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. 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 the Polypterus the cerebellum 

 hardly more developed than in the Lepidosiren, whilst in the somewhat 

 more active and predacious Lepidosteus it is the smallest of all the 

 segments of the brain. In the grovelling Sturgeons the cerebellum 

 offei's an intermediate grade of development between those that 

 characterise the above-cited Ganoids. Finally, amongst the normal 

 Osseous Fishes, the largest 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 i)eculiarly 



