RESPIRATION 41 



and contraction of their muscular walls. The related Hag- 

 fish (Myxine) possesses still more singular habits, and bores right 

 into the fishes it attacks. Under these conditions the current of 

 water is inhaled through the single nostril situated on top of the 

 head, and after entering the pharynx passes out through the 

 gill-sacs. 



It has already been pointed out that a* Skate {Raia) is essen- 

 tially adapted for a life on the sea floor, and it is of interest to 

 note that the method of breathing has also been modified for 

 this end. While swimming or crawling about it is able to 

 breathe in the normal manner, but when resting on the bottom 

 there is a grave danger of taking in sand with the stream of 

 water and thus clogging up the delicate gill-filaments. In all 

 the members of this group of Selachians the mouth and external 

 gill-openings are on the under side of the head, but the spiracle 

 remains on the upper surface, and is represented by a com- 

 paratively large opening situated immediately behind the eye 

 and pro\ided with a movable valve (Fig. 14, b.i). To avoid 

 the danger of introducing foreign particles into the gills, the 

 Skate inhales water by way of the spiracles, expelling it through 

 the gill-openings in the usual manner. 



The respiratory modifications found in fishes normally 

 inhabiting rapid streams or mountain torrents, where they are 

 in the habit of fixing themselves to stones and other objects to 

 avoid being swept away by the current of water, are dealt 

 with in a subsequent chapter {cf. p. 239). 



The Trunk-fishes {Ostraciontidae), in which the head and body 

 form a firm bony box, are obliged to keep up the flow of water 

 over the gills by a series of rapid panting movements, as many 

 as 180 per minute having been counted in a resting fish. Here 

 the pectoral fins are used to assist respiration, and by their 

 constant motion fan a current of water through the gill-openings. 

 Professor Goode writes: "when taken from the water one of 

 these fishes will live for two or three hours, all the time solemnly 

 fanning its gills, and when restored to its native element seems 

 none the worse for its experience, except that, on account of 

 the air absorbed, it cannot at once sink to the bottom." 



The famihar phrase "to drink like a fish" is based on a 

 complete misconception; it is assumed that the constant and 

 regular opening and shutting of the mouth is a proof that the 

 fish is drinking, whereas, as has been explained, this is really 

 nothing more than an outward sign of the act of breathing. 

 It is very doubtful whether the fish drinks at all, and, in any 



