118 BRAIN AND BODY OF FISH 



Gunther describes it as a deep-water fish ; it seems probable that 

 this fish has a wide range of movement just as the hake. The teeth 

 are smaller than those of the black scabbard fish, but it has three or 

 four large teeth at the anterior end of the upjDer jaw. This fish is also 

 known in the seas around New Zealand, and it is very sensitive to 

 cold ; it sometimes swims ashore in its thousands on frosty nights, 

 apparently " in a state of temporary insanity " (Norman). This 

 view does not commend itself to the writer, to whom it suggests 

 some movement very similar to the mass immolations of lemmings 

 and springbok. 



The coelho or rabbit fish (Promethicthys prometheus) differs 

 from the preceding, by having a large forked tail like a mackerel ; 

 it is silvery on the dorsal part of the body, while below the lateral 

 fine, which is very curved, the tint is greyish ; the dorsal fin has 

 spinous rays and is black in colour. The head is not so elongated 

 as in the two scabbard fishes and the jaws are shorter, but the canine 

 teeth of the upper jaw are curved and prominent. The depth to 

 which this fish is known to descend is from 100 to 300 or 400 fathoms. 



It will be wise to consider in some detail the lateral line organs 

 which are supphed by the laterahs nerve ; this, together with the 

 acoustic or eighth nerve ends in the acoustic tubercles or acustico- 

 lateralis lobes. They are pits developed in the skin and run along a 

 line on the sides of the fish ; each has a sensory organ of the same type 

 as is present in the ampullae of the semi-circular canals of man. 

 The linings of these pits are more sensitive than other parts of the 

 body to effects jsroduced by the flow of water or any change of 

 pressure. They can detect pressure produced by their own move- 

 ments and, though a fish does not produce vibrations rapid enough 

 to cause sound, yet it cannot move without causing movements of 

 water, and when water moves the pressure changes from place to 

 place and time to time. Special changes occur when it is passed 

 by another fish. 



Fish are, therefore, sensitive to both pressure and to imjDulses 

 from outside. They must also be sensitive to the pressure of the 

 column of water, which in the fishes which we are now considering, 

 must be very great. The effect of great pressure on physical 

 phenomena must be very difficult to estimate ; but we know from 

 the experience obtained in diving operations that talking in com- 

 pressed air gives the voice a high pitch and sounds squeaky. 



There are many other physical facts to be considered in relation 

 to the waves produced by the movement of a body in water and these 

 can be investigated in a ripple tank. To quote an article by Sir 

 Wilham Bragg, " If the water in such a tank is touched circular 



