VERTEBRATES THAT BREATHE IN WATER 383 



and such water would then be regularly taken in for respiratory 

 purposes instead of passing in casually with the food. Though, 

 judging from analogy with some Crustacea and other animals where 

 the last part of the intestine is pressed into the service of breath- 

 ing, there may have been a stage before gill-slits were evolved, 

 in which breathing-water was taken into the pharynx at regular 

 intervals and as regularly ejected from the mouth after doing its 

 work. Leaving the realm of speculation, we come to the breathing 

 arrangements found among the chief groups of Fishes, and among 

 the simply-organized forms known as Protochordates. 



FISHES AS BREATHERS IN WATER 

 LAMPREYS AND HAGS (CYCLOSTOMES) 



One of the most interesting forms as regards breathing organs 

 In this very ancient and primitive group is the Californian Hag- 

 Fish (Bdellostoma) of the Pacific coast. This is an elongated 

 eel -shaped creature with suctorial mouth, devoid of anything 



Fig. 512 



A, Californian Hag-Fish (Bdellostoma], reduced, showing external apertures of the gill-pouches. NA, Unpaired 

 nostril. B, Diagrammatic cross-section of same, showing two gill-pouches connected with the gullet and the 

 exterior. Course of the breathing-water indicated by arrows. 



comparable to a lower jaw. On each side of the body are to be 

 seen, a little distance behind the head, some seven to fifteen 

 small round holes, one behind the other (fig. 512). These are 

 the external openings of a corresponding number of gill-pouches. 

 Dissection shows that the pouches communicate internally with 

 the digestive tube by similar apertures, and that the lining of 

 each pouch is raised into a large number of thin folds, arranged 

 almost like the leaves of a book, and presenting a very large 

 surface for purification of blood. The heart contains nothing 

 but impure blood, loaded with carbonic acid gas and deficient 

 in oxygen. This it pumps to the gill-pouches, where dissolved 

 oxygen is taken up from the surrounding water, which receives 



