STRUCTURE AND MECHANISM OF FISHES 13 



jaw which lies on the surface outside of the primary upper jaw. This 

 secondary upper jaw is composed on each side of two pieces, called the 

 premaxilla and maxilla; these at first form a single curved piece which 

 together with its fellow of the opposite side forms a A shaped arch 

 above the mouth. Gradually, however, the front piece (premaxilla) 

 grows downward in front of the second piece (maxilla) and finally ex- 

 cludes it from the margin of the mouth. Meanwhile the maxilla de- 

 velops a movable joint on the front part of the skull; finally it acts as a 

 lever or handle for the premaxilla and is attached to the lower jaw in 

 such a way that when the lower jaw is lowered the maxilla is pulled down- 

 ward and the premaxilla is pushed forward. It is by means of this 

 arrangement that many fishes such as carps and bass can suddenly 

 shoot out their secondary upper jaw, or bony lips. 



The secondary lower jaw consists of bony plates in the skin around 

 the primary lower jaw, which these plates ensheath, both inside and out- 

 side the mouth, but not at the back part. The joint of the lower jaw is 

 still formed from the back part of the primary upper and lower jaws. 

 The skin on the roof of the mouth also gives rise to bony plates which 

 form a bony palate and unite more or less closely with the underlying 

 primary upper jaw. 



The primary, or internal, upper jaw of higher fishes is attached in 

 front to the lower part of the skull and in the rear through the enlarged 

 hyomandibular bone very much as it was in the sharks, but it receives 

 much additional support and bracing from the front part of the gill-cover. 

 The whole jaw-gill apparatus is movably jointed so that the food can be 

 engulfed and swallowed. On the inside of the upper joints of the gill- 

 arches in the back of the throat are a pair of clusters of teeth. Similar 

 clusters of teeth are developed in the floor of the throat, supported by the 

 lower parts of the gill-arches. These teeth, called "pharyngeal teeth," 

 assist either in pushing the food down into the stomach or in crushing the 

 food in the throat. Numerous forms of throat teeth occur in different 

 fishes. In the carps they take the form of finger-like projections; in the 

 drums, the dinners, parrot fish and others, the lower throat teeth be- 

 come consolidated into stout crushing masses. 



The pectoral fins of sharks are supported by a stout arch of cartilage 

 called the primary pectoral girdle. In typical fishes bony plates in the 

 skin become attached to the primary pectoral girdle and greatly strength- 

 en it. This secondary, or outer, girdle is connected above with the back 

 of the skull; it curves around tiie back of the gill-cover and runs forward 

 on the lower surface of the throat behind the lower segments of the gill- 



