278 FOOT-NOTES TO EVOLUTION. 
tors, are reduced to motionless sacs, buried in the sand 
or anchored to rocks or wharves. The evidence of their 
origin is found in the fact that the young Tunicate is 
tadpole-shaped, with a rudimentary back- 
bone, and has the motions and in large 
degree the structure of the fish. With the loss of power 
of locomotion the structures on which locomotion de- 
pends also disappear. 
Still more marked is the degeneration of parasites. 
It is a universal rule that all creatures dependent on 
others for support lose their power of 
self-help. Parasitic insects lose their 
wings and are confined to the bodies of 
those unwillingly made their hosts. Parasitic worms 
are the simplest of their kind. Insects feeding on the 
juices of plants which they suck without moving be- 
come reduced to mere living scales. 
Perhaps the most remarkable example of the degen- 
eration of parasitism is that seen in the crustacean 
called Sacculina, This creature appears 
as a simple sac attached to the body of 
the crab, into which its root processes or blood vessels 
extend. When it is hatched from the egg it is similar 
in form to a young crab, independent and free-swim- 
ming. It soon attaches itself to some adult crab, into 
the body of which it extends its processes. It loses its 
power of locomotion, and the limbs all disappear. Liv- 
ing at the expense of others, self-activity is not de- 
manded, and its position protects it from competition to 
which free-swimming crabs are subject. It becomes 
degraded into a parasitic sac, with no organs except 
a nervous ganglion, its ovaries, and root processes. 
This is the female Sacculina, and parasitic upon this is 
the smaller and still more degraded male of the same 
species. 
Tunicates. 
Parasitical 
animals. 
Sacculina. 
