VERTEBRATES THAT BREATHE IN WATER 



385 



V.AO. 



A very interesting and curious specialization is found in the 

 Common Hag- Fish (Myxine), a form which abounds on the east 

 coast of Scotland. For here there are only 

 two external openings, situated rather far 

 back on the under side of the body. On 

 dissection we find that there are six pairs 

 of gill-pouches, opening directly out of the 

 digestive tube, but those of each side com- 

 municate externally with a canal which 

 opens to the exterior by one of the two 

 openings already noted (fig. 515). This is 

 probably connected with the habit this fish 

 has of attacking cod, &c., in a moribund 

 condition, and eating its way into their 

 bodies. This practice would interfere with 

 breathing if the gill openings were arranged 

 as in a Lamprey. A further interesting 

 point about the Hag is the existence of a 

 useless seventh gill-passage behind the 

 pouches of the left side, and this vestige 

 clearly indicates that the fish is descended 

 from forms possessing more numerous 

 gill-pouches than it does itself. Such 

 a conclusion is borne out by the facts A > auricle > v > ventricle; V.AO. ventral 



aorta taking impure blood to the gill- 



already mentioned regarding the number pouches; PA. pharynx; a and G6, 



. ' 1 . . , f. | first and last gill-pouches of left side; 



of gill -pouches in other members ot the G ?, vestige of seventh gm-pouch on 



left side ; X X, the two external gill 

 grOUp. apertures 



Fig SIS . _ circu i atory Breathing 



SHARKS AND RAYS (ELASMOBRANCHS) 



A description has already been given (vol. i, p. 257) of the 

 structure of the Spotted Dog- Fish (Scy Ilium canicula), which is 

 in effect a small shark. In this creature, as in most of its kind, 

 five gill-clefts are readily seen on each side of the body a little 

 way behind the head, and these are the external openings of a 

 corresponding number of gill-pouches which communicate inter- 

 nally with the pharynx (fig. 516). Projecting into each pouch are 

 a large number of folds, collectively presenting a large surface for 

 purification of the blood. The breathing-pouches of a Lamprey 

 are supported and prevented from collapsing by a basket-work of 



VOL. II. 57 



