CLASSIFICATION AND ADAPTATION 31 



of the head. The subcaudal flaps were perfectly 

 motionless and tightly pressed between the base 

 of the tail and the surface of support, so that any 

 movement of them was impossible. The question 

 arose, however, whether the tail and these flaps 

 acted as a sucker which aided in the adhesion. 

 The flaps were therefore cut off with scissors an 

 operation which caused practically no pain or injury 

 to the fish and it adhered afterwards quite as well 

 as when the fin-flaps were intact. The subcaudal 

 prolongations of the fins are therefore not necessary 

 to the adhesion, nor to the pumping action, of 

 the muscles and fins, which went on as before. It 

 seemed probable, therefore, that the pumping action 

 was itself the cause of the adhesion. But the 

 difficulty in accepting this conclusion was that there 

 was a distinct though gentle respiratory movement 

 of the jaws and opercula ; and if the pumping of 

 the water from beneath the body caused a negative 

 pressure there, and a positive pressure on the outer 

 side of the body, it seemed equally certain that the 

 respiratory movement must force water into the 

 space beneath the body and so cause a positive 

 pressure there which would tend to force the fish 

 away from the surface with which it was in contact. 

 Examination of the currents of water around the 

 edges of the fish, by means of suspended carmine, 

 snowed that water passed in at the mouth and out 

 at the lower respiratory orifice, but also into the 

 space below the body at the upper and lower edges 

 of the head, without passing through the respiratory 

 channel. It was thus proved that the rate at which 

 water was pumped out at the sides of the tail was 

 greater than that at which it passed in by the 



