220 



BONY FISHES 



vii. 17- 



the fish. This, however, still leaves open the question of what agency 

 initiates movement of the fluid in the canals during life. It has been 

 shown that when small streams of water are directed against the side 

 of the tail some fish make escaping movements, but that these no 

 longer appear when the lateral line nerve has been cut. The lateral 

 line organs thus provide signals when agitation of the water causes 

 pressure changes. In fact they provide the animal with a kind of 

 water touch, though it is not certain whether this is their only func- 



/^v Lat.d. 

 gust. 



max 



Lat.v. 

 pelv. pect. 



Fig. 137. Dissection of whiting to show the cranial nerves, and especially the 

 nerves for the taste-buds. 



an. anal fin; gust, gustatory branch of facial; lat.d. and lat.v. dorsal and ventral lateral line 



nerves of vagus; mand. and max. mandibular and maxillary divisions of trigeminal; op. 



ophthalmic; pect. pectoral fin; pelv. pelvic fin; VII, hyomandibular branch of facial; IX, 



glossopharyngeal; X.visc. visceral branch of vagus. 



tion. Why this type of receptor should need such a peculiar apparatus 

 of canals, rather than a system of nerve-fibres in the skin, innervated 

 by the spinal dorsal roots, is not clear, nor do we know the significance 

 of the pattern of lines on the head. The lateral line system must cer- 

 tainly be of great importance in aquatic life, for it is found in all types 

 of fishes and also in the early Amphibia and in the aquatic larvae of 

 modern members of that group. The distant touch receptors could 

 obviously be used in many ways, not only to locate moving objects 

 and water currents but to serve for echo-location, by computing the 

 time relation of reflected waves set up by the fish itself. 



18. Chemoreceptors. Taste and smell 



As in all vertebrates, there are two separate chemical senses, taste 

 and smell. The former serves mainly to produce appropriate reactions 

 to food near the body, such as snapping, swallowing, or movements 

 of rejection. Smell, on the other hand, is a 'distance sense', by which 

 the whole animal is steered. The distinction between the two types 

 of receptor is somewhat obscured in bony fishes by the fact that taste- 

 buds are not restricted as they are in mammals to the tongue and 



