394 FISHES 



such an agreement in the structure of so complicated and special 

 an apparatus can only be the result of descent from a common 

 ancestor. All these families are fresh-water fishes with open 

 air-bladders and soft fin-rays, and therefore belong to the more 

 primitive Teleostei in their general characters, although this 

 particular adaptation is anything but primitive. It has been 

 suggested that the close connection between the air-bladder 

 and the auditory organ may have nothing to do with hearing, 

 but may serve to enable the fish to perceive the variations of 

 pressure in the air-bladder due to ascent or descent in the 

 water. Even without the aid of consciousness or voluntary 

 action, sensations of this kind might be related to reflex action 

 affecting the blood-glands which as we have seen regulate the 

 amount and pressure of oxygen in the bladder. It seems more 

 probable, however, that the secretion and absorption of oxygen 

 are regulated automatically by the pressure itself, and the con- 

 nection of the air-bladder with the ear would be then really of 

 auditory function. This view is supported by the fact that the 

 fishes which possess Weberian ossicles are all inhabitants of 

 rivers, or at least of fresh water, in which the range of depth is 

 not very great, and such fishes may be held to be more in need 

 of a delicate sense of hearing, than of special adjustments of the 

 size of the air-bladder, or of the pressure of the gases with- 

 in it. 



It has long been agreed that the lateral line organs are 

 sense-organs, but it has been always very difficult to discover 

 to what particular stimulations they were sensitive. There is 

 no evidence that they are affected by light, heat or electricity. 

 Experiments have been made to decide whether the organs re- 

 sponded to chemical stimulation, such as oxygen, the salinity 

 of the water, the dissolved substances of food, etc., but the results 

 were negative. Mr. G. H. Parker has recently carried out a 

 systematic series of experiments on these organs at the Govern- 

 ment Biological Laboratory at Woods Hole, Massachusetts. 

 The fish which he principally used was Fundulus heteroditus, 

 which is known in America as the kilifish or mummichog, but 

 experiments were also made on seven other species, including 

 the smooth dog-fish, Mustelus canis, a skate, Raja erinacea, and 

 a flat-fish, P seurfopleuronectes americanus . The method of ex- 

 perimenting was to cut the nerves supplying the lateral line and 



