58 ADAPTATIO^^ OF THE FISH'S SKULL 



tlie limbs of fishes are prehensible ; the moutli may be 

 propelled by them to the food, but the act of taking it 

 must be performed by the jaws ; these can, accordingly, 

 be not only opened and shut, but can be protruded and 

 retracted. The division of the long tympanic pedicle into 

 several partly-overlapping pieces adds to its strength, 

 and by a slight elastic yielding diminishes the liability to 

 fracture. The tongue, to judge by its structure, seems to 

 serve little as an organ of taste, but the arch sustaining it 

 has much to effect in the way of swallowing; for this 

 action relates not merely to food ; the mechanical part of 

 breathing is a modified, habitual, and frequent act of deglu- 

 tition. The hyoid arch is the chief support of the bran- 

 chial arches and gills ; and the branchiostegal membranes, 

 stretched out upon the diverging rays of the hyoid arch, 

 regulate the course and exit of the respiratory currents. 



By the retraction of the hyoid arch the opercular doors 

 are forced open, and the branchial cavity is widened, 

 whilst all entry from behind is prevented by the branchi- 

 ostegal flaps, which close the external gill-openings. The 

 water, therefore, enters by the gaping mouth, and rushes 

 through the sieve-like interspaces of the branchial arches 

 into the branchial cavity; the mouth then shuts, the 

 opercular doors close upon the branchial and hyoid 

 arches, which again swing forwards; and the branchio- 

 stegal membranes being withdrawn, the currents rush out 

 at the gill- openings. Thus the mechanical functions of 

 the haemal arches of the thorax of the higher air-breath- 

 ing classes are transferred to the haemal arches and ap- 

 pendages of the skull in fishes. 



The persistent gills and gill-arches in fishes have been 

 compared with the same parts which are transitory in 

 frogs, and with some traces of branchial organization in 



