LUNG-FISH 



381 



Elasmobranch, play any part in suspending the lower jaw. The upper 

 part of the cartilaginous mandibular arch becomes incorporated in the 

 cranium, forming a support to which Meckel's cartilage articulates 

 directly. Here we have probably the most nearly primitive mode of 

 attachment of the lower jaw to the cranium and we may speak of it as 

 the protostylic arrangement. The palatopterygoid cartilage or primitive 

 upper jaw skeleton has practically disappeared from development (Agar), 

 and we may probably attribute this to the precocious development of 

 the bony trabeculae connected with the tooth bases, which spread rapidly 



Gills 



s.a. 



Tissues 



Tissues 



mv. ■ 



Fig. 165. 



Diagram illustrating the essential differences between tlie " single " (A) and " double " (B) types 

 of circulation. A^ Atrium; c, conus arteriosus; L.A, left auricle; L.V, left ventricle ; m.v, main 

 vein returning blood to heart ; ^.a, pulmonary artery ; p.v^ pulmonary vein ; R.A, right auricle; 

 fi.K, right ventricle ; s.a, systemic aorta ; s.v, sinus venosus ; V, ventricle. 



and form a bony upper jaw continuous with the tooth base, so that a 

 cartilaginous upper jaw is no longer necessary. 



In the Dipnoi we find for the first time what is known as a double 

 circulation, i.e. one in which a special circuit is arranged for taking the 

 blood through the respiratory organ instead of this latter being simply 

 intercalated in the course of the general circulation. 



In a single circulation such as that of a fish the blood passes from the 

 ventricle (Fig. 165, A, V) towards the respiratory organs, in this case 

 the gills, and after traversing these and becoming oxygenated it is 



