Psychical Nervous Centres 107 



to respire while clinging on to another fish and 

 sucking its blood. Lampreys pass from the 

 sea up rivers during the summer months for 

 the purpose of spawning. Sharks form another 

 genera of cartilaginous fishes; their instinctive 

 behaviour is somewhat more pronounced than 

 that of the lamprey, for if attacked they will 

 defend themselves with great determination; 

 and it is well known they follow the same ship 

 at sea for many days, swallowing almost any- 

 thing thrown overboard. Many sharks are 

 ovoviviparous, while others produce eggs 

 having a long filament attached to them, which 

 is apt to become entangled in sea-weed and 

 held there until the egg is hatched ; the parents 

 do not appear to take any interest in their 

 young. 



The brain of bony fishes affords a favourable 

 field for the study of the development of cere- 

 bral nervous structures, there being a marked 

 tendency for its living matter to become, as it 

 were, sorted out so as to perform distinct kinds 

 of work, and thus to reach a higher standard 

 of organisation than that attained by car- 

 tilaginous fishes. This tendency is marked by 



