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ANIMAL LIFE AND HUMAN PROGRESS 



become modified in relation to new methods of breathing 

 and feeding ; it is of some interest to compare with it a fish 

 that has retained its apparatus for breathing and feeding 

 unmodified, but has completely changed its form. 



The Blennies are little shore-fishes, very characteristic of 

 rock-pools. One Indo-Pacific genus Petroscirtes is a typical 

 Blenny in most respects, and is characterised chiefly by having 

 the gill-opening reduced to a small hole and by the presence 



FIG. 1. Petroscirtes elegans. 



FIG. 2. Xiphasia setifer. 



of a pair of canine teeth in each jaw, the lower pair being 

 strong and curved, in comparison with the size of the fish 

 almost as large as the canines of the Sabre-toothed Tiger. 



Now Petroscirtes, like other Blennies, is moderately elongate, 

 has about 30 to 40 vertebrae, about half of which belong to 

 the tail, and has a well-developed tail-fin and broad-based 

 pectoral fins. There is another Indo-Pacific genus, Xiphasia, 

 evidently very closely related to Petroscirtes, for it has the 

 same curious dentition and the same small gill -open ings. 

 But Xiphasia is very elongate in form eel - shaped ; the 



