NERVOUS SYSTEM, SENSES, AND SENSE ORGANS 205 



of their tanks at night, and the suggestion has been advanced 

 that this is the reason why the trawlers make their best hauls 

 of marketable fish at night, since the commercial trawl does 

 not actually drag the bottom, but the lower edge passes a foot 

 or so above this, and in the day-time would miss the fishes 

 lying buried in the sand. Dr. Beebe records that a young Sole 

 (Achirus) may leave the bottom and on occasion actually float 

 at the surface of the sea at night. " It undulated to the surface," 

 he writes, "curved down to a saucer or cup-shape with the 

 circular fin-rays above the water, and floated until I captured 

 it. The fully expanded fins apparently made such intimate 

 contact with the surface film that, like a vacuum cup, it 

 remained suspended." Bat-fishes [Ogcocephalidae), which are also 

 normally bottom dwellers, likewise come to the surface in the 

 dark. 



Mr. Boulenger's observations on a number of young Grey 

 Mullet (Mugil) are of interest. During the hours of daylight 

 they were observed to swim about in a massed shoal, but at 

 night this broke up, every individual fish going to its own spot 

 on the bottom, the members separating and facing in all 

 directions. If disturbed, however, they rapidly returned to the 

 surface and again adopted the mass formation. 



Some indication has been already given as to the manner 

 in which the complex activities of the fish's body are co-ordinated 

 by the nervous system, and in concluding this chapter the 

 matter may be considered rather more closely. Sensory 

 impressions are received from the outside world by one or more 

 of the organs of sense, and messages in the form of nervous 

 impulses are flashed to the appropriate part of the brain, from 

 which motor impulses are promptly sent back to muscles, 

 glands, and so on, matters being at once adjusted by an appro- 

 priate movement or other activity. It is important to distinguish 

 between two very different types of behaviour, two replies, as 

 it were, to the impressions received from the sense organs. 

 There is the reflex action, or, more simply, the reflex, which 

 is quite automatic and independent of the mind. A familiar 

 example of this action in human beings is provided by the 

 drawing away of the hand or foot from a source of excessive 

 heat : the movement begins, not as the result of the pain, but 

 before consciousness of any pain has been experienced, and 

 follows almost instantaneously upon the application of the 

 stimulus. In the other type of action memory and consciousness 

 are involved, and this may also be illustrated by an example 



