396 FISHES 



would extend, but it is evident that if a fish is affected by a 

 body falling in the water it must also perceive the movement 

 of another fish or animal in the water. We know that some 

 fishes seek their food by sight and some by smell, but experi- 

 ments have yet to be made to ascertain how far a fish without 

 sight or hearing would be able to perceive the presence of an 

 enemy in its neighbourhood. 



The lateral line organs are originally developed on the 

 surface of the skin, the tubes are formed by the closing in of a 

 groove, and communicate with the surface by a series of 

 pores. The tubes of the head, both the continuous ones of 

 Teleostei and the separate ones of Elasmobranchs, are devel- 

 oped in the same way. In many fishes groups of sensory cells 

 like the sense-organs of the lateral line occur on the free sur- 

 face of the skin, for instance in the cod ; and in addition to this 

 the skin has doubtless a general tactile sensibility due to nerve 

 endings not connected with special sense-organs. Experiment 

 has shown that the skin of fishes is sensitive not merely to the 

 contact of solid bodies, but also to the motion of the water in 

 surface waves and currents. A normal fish usually places itself 

 with its head towards a current and swims against it; it does 

 this instinctively or automatically, and it can be easily understood 

 that it must habitually do so in order not to be at the mercy 

 of every current it meets. Parker found that a fish in which 

 the lateral line nerves had been cut swam against a current in 

 the same way as a normal fish and also swam downwards out 

 of the reach of surface waves ; it was evident therefore that the 

 waves and the currents were perceived by the general skin with- 

 out the aid of the lateral line organs. 



The lateral line and the sensory tubes on the head form a 

 single system of sense-organs radiating from the region of the 

 auditory organ. 



There is good reason to believe from embryological re- 

 searches that the lateral line sense-organs were originally in 

 ancestral fishes confined to the region of the branchial clefts, 

 one organ belonging to each cleft ; and also that the auditory 

 organ is homologous with these organs, is in fact one of the 

 series which has been specially developed. The auditory organ 

 develops in the same way as the lateral line organs as an in- 

 vagination of the surface of the skin. We do not know that 



