CHAPTER XIV 



THE FISHES 



146. General characters. In a general way the name fish- 

 is applied to all vertebrates which spend the whole of their 

 life in the water, which undergo no retrograde metamor- 

 phosis, and which do not develop fingers or toes. Of other 

 aquatic chordates or vertebrates the ascidians undergo a 

 retrograde metamorphosis, losing the vertebral column, and 

 with it all semblance of fish-like form. The amphibians, 

 on the other hand, develop jointed limbs with fingers and 

 toes, instead of paired fins with fin rays. A further com- 

 parison of the animals called fishes reveals very great dif- 

 ferences among them differences of such extent that they 

 cannot be placed in a single class. At least three great 

 groups or classes must be recognized : the Lancelets, the 

 Lampreys, and the True Fishes. The general characters of 

 all these groups will be better understood after the study 

 of some typical fish, that is one possessing as many fish-like 

 features as possible, unmodified by peculiar habits. Such an 

 example is found in the bass, trout, or perch. In either fish 

 the pointed head is united, without any external sign of a 

 neck, to the smooth, spindle-shaped body, which is thus fitted 

 for easy and rapid cleaving of the water when propelled by 

 the waving of the powerful tail (Fig. 97). A keel also has 

 been provided, enabling the fish to steer true to its course. 

 This consists of .folds of skin arising along the middle line of 

 the body, supported by numerous bony spines or cartilaginous 

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