92 MR. A. D. DARBISHIRE ON THE DIRECTION OF THE 



with ' no current/ Carmine liberated near the mouth entered it also in 

 a uniform stream. 



The uniformity of the stream is evidently due to the manner in which the 

 respiratory current is brought about. It is effected by the undulation of 

 the gill-covers, which expels the water underneath them to the exterior. 

 Water enters the mouth and spiracles because these are the only channels 

 through which it can enter the pharynx to replace that which has been 

 expelled by the undulation of the gill-cover. The uniformity of the current 

 is accounted for by the fact that by the time a wave has reached the hinder 

 end of the gill-cover another has started at its anterior end. 



It is evident how different is the method by which water is drawn in, 

 in Rhina on the one hand and Scyllium and Raja on the other. For in these 

 latter, as we have seen, this is effected by the expansion of the whole 

 pharyngeal region accompanied by the simultaneous closing of the gill-slits 

 and opening of the mouth and spiracle. 



Water is drawn into the mouth and spiracle of Rhina in the same way as 

 air is drawn into the door of a room which has a revolving ventilator in its 

 roof. Water is drawn into these apertures in the Dogfish and Ray in the 

 same way that smoke is drawn into the mouth from a tobacco-pipe or 

 cigarette. In the first instance, the lumen of the chamber into which water 

 (or air) is drawn is not enlarged ; in the second, the lumen of the chamber 

 into which water (or smoke) is drawn is enlarged. 



We have said that when a living Rhina is examined no movement can 

 be detected in the walls of the spiracle. The spiracle is, however, sometimes 

 found open and sometimes shut ; but from the fact that I have never seen 

 the slightest movement on the part of the spiracle, although I examined it 

 repeatedly whilst it was gradually but imperceptibly closing, I imagine that 

 it never closes rapidly. In the case of the adult Angel which was examined 

 in the large tank at Plymouth, the spiracle was always closed when the fish 

 was undisturbed — for example, when I first visited it in the morning ; but 

 if it was disturbed and made to swim about, the spiracle gradually opened. 

 But in the case of the two younger Angels which were confined to a shallow 

 glass vessel in the laboratory in London, the spiracle was permanently open 

 for a space of two days. 



Judging the normal behaviour of the spiracle from the observations made 

 in the large tank behind the Laboratory at Plymouth, we find that it is 

 curiously different from that of the spiracle in the Ray. For whilst in the 

 undisturbed Ray the water is drawn solely through the spiracle and not 

 through the mouth at all, in the undisturbed Angel the spiracle is shut and 

 water enters solely through the mouth. 



The spiracle closes by the backward movement of its anterior border. This 

 I was able to determine on a fish that had just died. One of the reflex move- 

 ments executed as a result of pressing the back of the fish was the shutting of 

 the spiracle, which was brought about in the manner described. 



