168 EMBRYOLOGY OF THE LOWEE VEETEBEATES ch. 



to form a kind of diaphragm perforated in its centre and capable of 

 being thrown into vibration by air being forced from one chamber 

 into the other so as to function as a sound-producing organ {e.g. 

 Gurnards). Other outgrowths may develop : thus for example in 

 many Siluroids numerous branched projections are formed along each 

 side of the air-bladder. 



The air-bladder rudiment is at its first appearance in some cases 

 approximately dorsal in position (Sahno). In Bhodeus Moser (1904) 

 has shown that the diverticulum is at first on the right side of the 

 alimentary canal. The same observer found that during the early 

 stages of development of the air-bladder the portion of alimentary 

 canal from which it springs undergoes a process of rotation about its 

 long axis in such a direction that a point on its dorsal surface is 

 carried towards the left side. 



Although the actual development has been worked out only in a 

 few cases, we may infer safely from the adult relations (Bowntree, 

 1903) that the amount of this rotation differs greatly in different 

 members of the group Teleostei. Thus in Siluroids and Cyprinodonts 

 the glottis or pharyngeal opening is in the adult still to the right of 

 the mesial plane ; in others such as the genera Osmerus, Clupea, 

 Ghirocentrus it is practically median ; in still others such as 

 Mormyrids, Characinids, Gymnotids and Cyprinids it has passed the 

 mesial plane so as to lie upon its left side, while in the case of the 

 Characinids Macrodon, Erythrinus and Lebiasina the glottis has come 

 to be completely lateral on the left side. This rotation of the gut in 

 the region of the glottis is of much morphological importance as will 

 be shown later. 



In the young Bhodeus, 7 mm. in length, Moser finds that a well- 

 marked diverticulum from the pneumatic duct is present (Fig. 93, C). 

 Later on it gradually disappears. A similar diverticulum occurs in 

 Salmo and in the Carp, and in all probability in numerous other 

 Teleosts : its morphological significance will be discussed later. 



Actinopterygian Ganoids. — In these fishes the development of 

 the air-bladder takes place on similar lines to that described for 

 Teleosts. In Amia the additional detail has been made out that the 

 rudiment is at first in the form of a longitudinally placed groove 

 which becomes constricted off from the alimentary canal from behind 

 forwards just as frequently happens in the case of the typical lung- 

 rudiment of air-breathing Vertebrates (Bashford Dean, 1896; 

 Piper, 1902). A rotation of the section of alimentary canal in the 

 region of the glottis takes place similar to that which occurs in the 

 Teleost. 



Lung-fishes. — In the adult Ceratodus an organ occurs which is 

 equally lung and air-bladder. It forms an unpaired sac lying dorsal 

 to the splanchnocoele just like a typical air-bladder, but the pneu- 

 matic duct, instead of opening directly into the alimentary canal 

 dorsally, passes round the right side and opens by a ventrally placed 

 glottis. In Zepidosiren and Protopterus the general arrangement is 



