,THE GILLS. 161 



in length the transverse diameter of the head. Each of the two 

 anterior gills consists of from five to seven main lobes, decreasing 

 in size from above downwards ; and each main lobe gives off 

 minor lobes along its posterior border. The third or posterior 

 gill (Fig. 77) is much smaller than the other two, and only 

 slightly subdivided. 



The external gills are usually carried projecting outwards 

 and backwards from the head, at an angle of about 45 with the 

 -axis of the body. Each gill has, however, muscles of its own, by 

 means of which the entire gill or its individual lobes can be 

 moved freely and independently. 



The course of the circulation in the external gills can be 

 well studied in the living animal. Each main lobe, and each 

 of its minor lobes, contains two blood-vessels, afferent and 

 efferent, which lie alongside each other and are directly con- 

 tinuous at the tip of the lobe ; the afferent vessel being posterior, 

 and in part ventral to the efferent vessel (Fig. 77, AF and EF). 



Before the mouth opens, the opercular folds arise, as a pair of 

 folds of skin from the posterior edges of the hyoid arches, which 

 soon become continuous with each other across the ventral 

 surface of the head. Shortly after the formation of the mouth, 

 the opercular fold begins to grow back rapidly, covering over the 

 gills like a hood. The posterior border of the fold fuses with 

 the body wall along the right side, and across the ventral 

 surface : on the left side of the body it remains free, and is 

 prolonged backwards as a short tubular spout (Fig. 71, OP), 

 through which the opercular cavity opens to' the exterior. 

 After completion of the opercular fold the external gills rapidly 

 shrink up, those of the left side persisting longer than those 

 of the right side, and often protruding for a time through the 

 opercular spout. 



External gills occur in the adult or in the larval stages of 

 most, though not of all Amphibians. Their morphological value 

 has been much discussed, and it is commonly held that they are 

 to be regarded as secondarily acquired or larval organs, essen- 

 tially different in their nature to the internal gills. 



The internal gills. In tadpoles of from 9 to ] mm. length 

 the mouth opening is formed, by perforation of the oral septum 

 {p. 148, and Fig. 64, DS) ; and about the same time the gill-clefts 



II 



