CHAPTER XVI 

 PERCOMORPHI 



UBORDER Percomorphi, the Mackerels and Perches. 

 We may place in a single suborder the various groups 

 of fishes which cluster about the perches, and the 

 mackerels. The group is not easily definable and may con- 

 tain heterogeneous elements. We may, however, arrange 

 in it, for our present purposes, those spiny-rayed fishes 

 having the ventral fins thoracic, of one spine and five rays 

 (the ventral fin occasionally wanting or defective, having a 

 reduced number of rays), the lower pharyngeal bones separate, 

 the suborbital chain without backward extension or bony 

 stay, the post-temporal normally developed and separate from 

 the cranium, the premaxillary and maxillary distinct, the 

 cranium itself without orbitosphenoid bone, having a structure 

 not greatly unlike that of perch or mackerel, and the back- 

 bone primitively of twenty-four vertebrae, the number increased 

 in arctic, pelagic, or fresh-water offshoots. 



The species, comprising the great body of the spiny-rayed 

 forms, group themselves chiefly about two central families, 

 the ScombridfE, or mackerels, and the Serranidcc, the sea-bass, 

 with their fresh-water allies, the Percida, or perch. 



The Mackerel Tribe: Scombroidea. The two groups of Per- 

 comorphi, the mackerel-like and the perch-like, admit of no 

 exact definition, as the one fully grades into the other. The 

 mackerel-like forms, or Scombroidea, as a whole are defined by 

 their adaptation for swift movement. The profile is sharp an- 

 teriorly, the tail slender, with widely forked caudal; the scales 

 are usually small, thin, and smooth, of such a character as not 

 to produce friction in the water. 



In general the external surface is smooth, the skeleton 

 light and strong, the muscles firm, and the species are carniv- 



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