590 



STRUCTURE OF VERTEBRATES 



The front surface of the spiracle usually has a small gill, called 

 a pseudobranch, which is probably functionless. In the tetrapods 

 the spiracle persists as the Eustachian tube (p. 468) but the 

 other slits have no more than a temporary existence — relatively 



lengthy and functional in the larvae 

 of Amphibia, transitory in the 

 amniotes. 



Where the oxygen concentration is 

 low, it seems that the internal gills 

 of the gill slits are inadequate to 

 obtain all that the animal needs, and 

 various fishes living under such circum- 

 stances have developed two types of 

 accessory respiratory organ. The first 

 is the external gill, a vascular out- 

 growth of some other part of the body, 

 analogous to similar structures in 

 many other phyla. In modern fishes 

 they are found only in the larvae of 

 Polyptertis, Lepidosiren sindProtopterus 

 (p. 546) but they are found in all larval 

 Amphibia and so were presumably 

 present in the ancient crossopterygians 

 from which these were derived. In 

 the perennibranchiate urodeles, which 

 do not metamorphose, they persist 

 throughout life, and are the only 

 respiratory organs, apart from the 

 skin, in the adult. 



The other accessory respiratory 

 organ, the lung, was much more 

 revolutionary, since its value depended on the changed habit 

 of coming to the surface to breathe air, and it led to the 

 evolution of the tetrapods. The lungs were almost certainly 

 in their origin ventral outgrowths of the pharynx ; such a con- 

 dition is found in the actinopterygian Polypterus, but in other 

 fish either the lung by itself has moved dorsally, or its opening 

 has changed its position also. A functional lung is found amongst 

 fishes in the Dipnoi, and in Polypterus, Amia, and Lepidostetts 

 of the actinopterygians. In adult Amphibia it is the usual and 

 in amniotes the only respiratory organ, and its functional evolu- 



FiG. 458. — Transverse sections 

 through gill arches of a 

 dogfish (on the right) and 

 a cod, showing how Elasmo- 

 branchii differ from Actino- 

 pterygii and Choanichthyes 

 in respect of these organs. — 

 From Sedgwick, after R. 

 Hertwig. 



a. Afferent branchial artery ; b, 

 branchial arch of skeleton ; bl^ 

 and bl^, gill lamellas ; h, skin of 

 the side of the body between the 

 openings of two gill clefts in the 

 shark ; r, cartilaginous gill-ray 

 supporting the septum between 

 two gill pouches in the same ; 

 V, efferent branchial arteries, 

 double in the shark, single in the 

 cod ; z, small tooth-like tubercle 

 (in some Teleosteans elongated as 

 a ' gill raker '), one of a double row 

 on the branchial arch of the cod. 



