194 ANIMAL LIFE 
begins life as a primitively simple vertebrate. It possesses 
in its larval stage a notochord, the delicate structure which 
precedes the formation of a backbone, extending along the 
upper part of the body, 
below the spinal cord. It 
is found in all young ver- 
tebrates, and is charac- 
teristic of the class. The 
other organs of the young 
tunicate are all of verte- 
bral type. But the young 
sea-squirt passes a period 
of active and free life as 
a little fish, after which 
it settles down and at- 
taches itself to a stone or 
shell or wooden pier by 
means of suckers, and re- 
mains for the rest of its 
life fixed. Instead of go- 
ing on and developing 
into a fish-like creature, it 
Fic. 122.—A sea-squirt, or tunicate. loses its notochord, its 
special sense organs, and 
other organs; it loses its complexity and high organiza- 
tion, and becomes a “ mere rooted bag with a double neck,” 
a thoroughly degenerate animal. 
A barnacle is another example of degeneration through 
quiescence. The barnacles are crustaceans related most 
nearly to the crabs and shrimps. The young barnacle just 
from the egg (Fig. 123, /) is a six-legged, free-swimming 
nauplius, very like a young prawn or crab, with single eye. 
In its next larval stage it has six pairs of swimming feet, 
two compound eyes, and two large antenne or feelers, and 
still lives an independent, free-swimming life. When it 
makes its final change to the adult condition, it attaches 
