CHAPTER IV 
FISHES 
54. Vertebrates. — All the animals thus far studied are 
grouped together under the name of Invertebrates, because 
they have no backbone. We are now to study the Verte¬ 
brates, animals with a backbone, such as fishes, frogs, snakes, 
birds, and mammals. 
The presence of a backbone in vertebrates is their most 
conspicuous characteristic. The formation of the backbone 
Figure 59. — Perch, an Important Food-fish Common in Nearly All 
Freshwater Ponds. 
is always preceded by the growth of an embryonic group of 
cells that do the work of a skeleton. This embryonic group 
of cells forms a structure which is called the notochord (no'to- 
kord: Greek, notos, back; chorda, cord). In all the true 
vertebrates (such as fishes, frogs, etc.), the notochord is 
gradually absorbed and the backbone takes its place, but 
between the vertebrae it remains as cushions. But in the 
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