338 COMPARATIVE ZOOLOGY OF CHORDATES 



the gill-pouches are modified into sacs which pump water in 

 and out again. In the larvae of some fish (Polyp terus, 

 Lepidosiren), and in those of amphibia, external gills may be 

 developed in the form of tuft-like structures projecting out 

 from the body into the water, and which enable the blood to 

 be oxygenated before the gill-slits are pierced. The larval 

 amphibia afterwards develop ordinary gills on the outer faces 

 of the gill-arches, and their respiration is like that of the fish. 

 In all these cases the respiratory movements are brought about 

 by means of the contraction of visceral muscles, innervated by 

 dorsal cranial nerve-roots, and controlled by a centre in the 

 medulla oblongata. 



The first visceral cleft or spiracle is open in the Selachii, 

 but it is closed in all higher forms with the exception of 

 Polypterus and Acipenser (the sturgeon). There may be a 

 spiracular gill, which is called a pseudobranch because its 

 capillaries receive blood which has already been oxygenated 

 in the next posterior (true) gill. In the Tetrapods the cavity 

 of the spiracular cleft gives rise to the tympanic cavity and 

 Eustachian tube. 



The rays are Selachii adapted for living on the sea-bottom, 

 and they are of a flattened shape, with the gill-slits on the 

 under side. The spiracle is on the upper side, and serves to 

 admit water into the pharynx. In the Selachii, the gill-slits 

 are uncovered, but in the bony fish (Dipnoi and Teleostomes) 

 they are covered over and protected by an operculum. An 

 analogous operculum develops in the larva of the frog, and it 

 may be remembered that in Amphioxus the gill-slits are 

 protected by being enclosed in the atrial cavity. 



The gill-sacs of Petromyzon all open independently to the 

 exterior, whereas those of Myxine have a single joint opening 

 on each side. 



The number of gill-slits in Amphioxus is large (up to 180). 

 In Selachii, not counting the spiracle, it is five, except in 

 Heptanchus which has seven, and Hexanchus and Pliotrema 

 which have six. Five is also the number in bony fish. It is 

 important to remember that gill-slits or pouches are present 

 in early stages of development of all chordates up to and 



