AQUEOUS CURRENT IN THE SPIRACLE OF THE DOGFISH. 93 



Summary of Results. 



1. Water is drawn into the mouth and spiracle o£ the Dogfish by the 

 expansion of the whole pharyngeal region. Water is prevented from entering 

 the gill-slits by their automatic closure. The gill-covers are passive agents 

 in determining the respiratory current. 



2. The respiratory current in the Ray is brought about in essentially the 

 same way as in the Dogfish. 



3. The differences between the Dogfish and Ray in this respect all relate to 

 the flat shape and bottom-living habit of the Ray. (a) In the former the 

 greater part of the inhaled water enters through the mouth ; in the latter, 

 through the spiracle, and indeed, when the fish is at rest, solely through the 

 spiracle ; in the Dogfish, water never enters solely through the spiracle. 

 (h) The Dogfish is capable of occasionally ejecting water from the spiracle ; 

 the Ray can not only do this with much greater vehemence, but is capable 

 of definitely reversing the current for a considerable number of respiratory 

 acts, (c) In correspondence with the greater control which the Ray has over 

 its spiracle, this aperture is both much larger than it is in the Dogfish and 

 provided with a more efficient valve. 



4. In Rliina the water is drawn into the mouth by an entirely difi'erent 

 mechanism from that which obtains in the Dogfish and the Ray ; namely, by 

 the undulation of the gill-covers themselves. The gill-covers in this fish 

 therefore are active agents in determining the respiratory current. The 

 current entering the mouth and spiracle is uniform and not rhythmical ; this 

 is the result of the method by which the water is drawn in. The spiracle in 

 RJiina is only capable of slow and imperceptible opening and closing ; it does 

 not open and shut rhythmically, as in the case of the Ray and Dogfish. 



5. The profound difference between the respiratory mechanism in Rhina 

 on the one hand, and Scyllium and Raja on the other, makes strongly against 

 the view, which Regan * opposes on anatomical grounds, that Rhina is a 

 connecting link between the Sharks and Raj^s. At the same time, this 

 mechanism in RJiina is as different from that in the Dogfish as it is from that 

 in the Ray. Our attitude to the conclusion arrived at by Regan, that 

 Rliina is a Shark and not a Ray, depends on the significance we attribute to 

 physiological characters of this kind, in classification. 



An abstract of this paper was read in the meeting of the Linnean Society. 

 The experiments with carmine were demonstrated on two Rays, two Dogfish, 

 and two Angel-fish, sent up from Plymouth for the occasion ; so that any of 

 my statements which did not carry conviction could be immediately subjected 



* P. Z. S. 1906, p. 7r)l. 



LINN. JOUEN. ZOOLOGY, VOL. XXX. 8 



