I 



94 THE AQUEOUS CURKENT IN THE SPTEACLE OF THE DOGFISH. 



to the test o£ experiment on ' the living animal. The only one o£ my 

 propositions, which was objected to, was that which referred to the passivity 

 of the gill-covers in the Dogfish as contrasted with the activity of these in 

 Rhina. Prof. Herdman, who disclaimed familiarity with the behaviour of these 

 structures in Rhina, said that from watching living dogfish on other occasions 

 he had come to the conclusion that the gill-covers were not passive but active 

 agents in determining the respiratory current. Dr. Masterman, however, 

 was in agreement with the view which I had expressed. After looking into 

 the matter again in the light of these criticisms, I have come to a conclusion 

 which reconciles these two apparently opposite views. The anterior half of 

 each gill-cover is supported by the gill-rays which are borne by the anterior 

 four epibranchial cartilages. If we look at the living dogfish, we see that the 

 anterior part of each gill-cover certainly does seem to move of its own accord, 

 so to speak, and not merely as the result of the outgoing rush of water. 

 Indeed, when we consider that the gill-rays, which support the anterior part 

 of the gill-cover, are attached to the branchial cartilages to whose movement 

 the expansion of the pharynx is largely if not wholly due, it is difficult to 

 imagine that it could be otherwise. On the other hand, there can be no doubt 

 that that part of the gill-cover which is not supported by the gill-rays is 

 an absolutely passive agent and that its movements, like those of any valve, 

 are solely determined by the direction of the current which plays upon it. 



Postscript. — After this paper had been sent to press, my attention was 

 called to an article in the ' American Naturalist^ (M^J 1907) by Herbert W. 

 Rand, " On the Functions of the Spiracle in the Skate." I am happy to find 

 that Mr. Rand's results, made on a species of Raja I have not examined, are 

 in complete accord with those of my own which refer to the Skate. 



