232 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



respiration, the branchial, is retained to-day by the 

 lowest of the Vertebrata, and gills are to be found in 

 all Fishes, in all Amphibia at some period of their lives, 

 and in some Amphibia throughout the whole course of 

 their existence. None of the Sauropsida or of the 

 Mammalia ever breathe oxygen dissolved in water, 

 but' are air-breathing forms with lungs ; though the 

 change of function has been completed, the remnants 

 of gill clefts are observable in the earlier stages of 

 development 



A most instructive series of gradations is to be 

 detected in Fishes; all adult 'forms have the gills in 

 pouches or recesses, but the young of some (Elasmo- 

 branchs, some Ganoids), like the tadpole at an early 

 stage, have protruding filaments or external gills ; 

 the lampreys have seven pairs of gill-clefts, as has the 

 shark Heptanchus ; Hexanchus, and most examples of 

 Myxine, have six ; most Elasmobranchs, five pairs ; 

 Chimsera has the first and fifth gill incomplete ; most 

 Teleosteans have four pairs of gills, but some have the 

 fourth incomplete ; the angler (Lophius) has three 

 pairs of gills, its ally Malthe has the third incomplete, 

 while in Amphipnous all the three pairs of gills are 

 more or less rudimentary. 



Where gill respiration ceases to be effective, the 

 blind outgrowth from the anterior portion of the 

 intestine (the air bladder) may take on respiratory 

 functions ; Amia has a single sac lying on the dorsal 

 surface of the intestine ; in Lepidosteus (the gar-pike), 

 the sac is divided internally, though it is single 

 externally ; in Ceratodus the opening into the single 

 sac lies to the left of the ventral surface of the 

 intestine, while in Polypterus the sac is double, and 

 opens on the middle line of the ventral surface. As 

 the air bladder becomes better developed, it becomes 

 better supplied with blood. (See page 202.) 



In the cyclostomatous Bdellostoma the ducts from 



