AQUEOUS CUKRENT m THE SPIRACLE OF THE DOGFISH. 89 



The existence o£ pigment on the anterior wall o£ the spiracle underneath 

 the valve and the rich nerve-supply from the palatine branch of the 7th 

 nerve to the spiracle itself, coupled with the above-mentioned facts relating 

 to the respiratory current in this form, all point to the greater differentiation 

 of the spiracle in the Ray than in Scyllium. 



The observation of the exit of the water from the gills was not so easy a 

 matter in the case of Baja as in that of the Dogfish. For, in the tirst place, 

 Raja is much more easily made restless by the inhalation of carmine ; and in the 

 second, the water is not, so to speak, ejected directly to the exterior as in the 

 case of the Dogfish, but to an intermediate closed chamber whose floor is the 

 base of the vessel and whose roof and sides are the Aentral surface of the fish. 

 So far as I was able to observe, however, carmine liberated near the spiracle 

 emerged chiefly from the foremost four gills ; and, as in the Dogfish, carmine 

 entering at the left side of the mouth emerged through the left gills, but also, 

 though to a less extent, through the right ones. 



The Hay occasionally did what I never observed Scyllium to do, namely, 

 ejected water from the spiracle at each respiratory act for a considerable number 

 of times. This regular and repeated reversal of the current in the spiracle was 

 a very different thing from the occasional violent spasmodic expulsion of water, 

 to which reference has already been made. The former barely disturbed the 

 surface of the 2 inches of water in which the fish lay ; the latter sometimes 

 sent a spout of water half an inch from the surface. 



It will be noted that in the foregoing account I have referred to that part 

 of the anterior wall of the spiracle which moves backwards at each expiratory 

 phase as the spiracular valve. I regard the behaviour of this structure sufficient 

 justification for this name. But it must be pointed out that this valve is, 

 certainly in the Ray, and, as far as I can discover, in the Dogfish too, identical 

 with what is known as the pseudo-branch. It is not my intention to discuss 

 the question whether this structure really represents a rudimentary gill or 

 not : I am here merely concerned in pointing out the relation of its move- 

 ments to the respiratory currents. 



Rhina squatina. 



Rliina differs from both Raja and Scyllium in the following characters : — 

 The mouth is terminal. The spiracle is much more elongate transversely 

 even than in the Skate ; it is, as shown in fig. 1, a considerable distance 

 behind the eye ; whereas in the Dogfish it is close behind it, and in the Ray 

 it is even overlapped by the upstanding eye *. The spiracle possesses no 



* The closeness of the eye to the spiracle in the Skate is the cause of a curious belief 

 which, according to Dr. Masterman (to whom I am indebted for my acquaintance with it), 

 exists among fishermen. They believe that the spiracle enables the fish to look into its 

 pharynx and satisfy itself as to the quality of the food which it has swallowed. 



The eye is certainly capable of a considerable degree of rotation. 



