PROTRACTILE MOUTHS 



the maxillae. To distinguish them we call one the 

 upper and the other the lower. These two pieces, 

 although they seem to be unified, are made up of 

 several parts connected and joined together. These 

 are not completely coherent, and can consequently 

 be more easily seen in fishes than in other groups. 

 The skeleton of the upper jaw is made up of many 

 parts. In mammals, in spite of its compactness and 

 unity, it is made up of four parts, two on each side. 

 These are the maxilla itself, and in front of it at the 

 middle of the mouth, the premaxilla. Occasionally, 

 as in man, the premaxillae and maxillae unite to com- 

 pose a single bone, although formed separately during 

 the embryonic stage of life. But typically the four 

 bones are joined only by sutures. There are cases, 

 however, where the premaxillae are defectively joined 

 together and to the maxillae. When there is no 

 cohesion, there remains a slit at the place where it 

 should have been, and when this happens in man, we 

 have the unpleasant and distressing malformation 

 known as hare lip. 



In the bony fishes, the premaxilla and the maxilla, 

 instead of being always bound together in a single 

 mass, fairly often remain independent, distinct to some 

 extent, and partially set one before the other. So 

 varying according to family, they come together at 

 different stages, bordering the mouth, holding up the 

 jaw, and supporting the teeth. Sometimes the upper 

 edge of the mouth is formed by the one, sometimes 

 by the other, sometimes by both together. Usually 

 both are shaped like drumsticks curved in accordance 

 with the contour of the mouth, in which, on each side, 

 the premaxilla is situated above and to the front, the 

 maxilla below and towards the back. This is the 

 general arrangement, and we find it in the John Dory, 

 but in this fish it is accompanied by a more peculiar 

 conformation, on an entirely different plan. 



I put side by side, for purposes of comparison, the 

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