00 MR. A. D. DARBISHIRB ON THE DIRECTION" OF THE 



by expelling a considerable volume o£ water from it. This only happens after 

 a number of trials with carmine, and is evidently a means of ridding the 

 pharynx of unpleasant matter. 



The carmine which is drawn into the spiracle emerges from the first three 

 gills, but especially from the first one, or two. It will be remembered that 

 water which entered the mouth emerged from the posterior two or three gills. 

 It seems reasonable to suppose that this difi'erence between the course of the 

 oral and spiracular currents is, in some way, connected on the one hand 

 with a slight difi'erence between the degree of oxygenation of the water 

 composing the two currents (like that between systemic and carotid blood in 

 the Frog), and on the other with that aggregation of the roots of the afferent 

 branchial vessels which aff'ects the first two in Scyllium, and the last three as 

 well as the first two in Raja. 



The observations made on the just-hatched Dogfish at the Royal (college of 

 Science were not conclusive, partly because of the smallness of the spiracle, 

 which made it very difficult to determine the direction of the current ; and 

 partly because in two of the three youngsters no trace of an ingoing current 

 could be detected, whilst in the third carmine was most distinctly drawn 

 in. Whether this apparent difference between the fish was due to difficulties 

 in observation or to real differences between the fish, I am not prepared 

 to say definitely, though I am inclined to attribute it to the latter. 



To return to the observations made at Plymouth. In Sc. catulus the 

 amount of disturbance in the cloud of carmine, caused, as we have supposed, 

 by the shutting of the spiracular valve, is greater than in the case of Sc. cani- 

 cula. This was particularly noticeable in some experiments I made with a 

 large Nursehound over 3 feet long. 



Raja. 



The observations were made on fairly young specimens of R. batis 

 measuring less than one foot from snout to tip of tail. The respiratory act in 

 this fish is, in its main features, the same as in the Dogfish. In correspondence 

 with the very much greater size of the spiracle and the greater differentiation 

 of its valve (which has a serrated edge and is strengthened by the spiracular 

 cartilage), the rapidity of the current and the volume of water drawn in at 

 the spiracle is considerably greater than in Scyllium. Another difference 

 between the two lies in the fact that when a Skate is resting; he inhales 

 water solely through his spiracle : it is only when he becomes restless and 

 crawls or swims that water enters the mouth. A Skate clears his spiracle 

 more forcibly and more often than a Dogfish does under the same treatment. 

 A sleeping Dogfish will inhale suspended carmine for an indefinite time ; 

 but a sleeping Raja wakes and moves away after a very few trials with the 

 carmine. 



