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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