Origin of Different Kinds of Adaptations 353 
even a complete loss of so important an organ as the diges- 
tive tract. Thus the tapeworm has lost all traces of its 
digestive tract, absorbing the already digested matter of its 
host through its body wall. Some of the roundworms, that 
live in the alimentary tracts of other animals, may have their 
digestive organs reduced. In Trichina, this degeneration has 
gone so far that the digestive tract is represented, in part, 
by a single line of endoderm cells, pierced by a cavity. The 
digestive organs are also absent in certain male rotifers, 
which are parasitic on the females, and these organs are 
also very degenerate in the male of Bonellia, a gephyrean 
worm. A parasitic snail, Extoscolax ludwigit, has its diges- 
tive apparatus reduced to a sucking tube ending in a blind 
sac. The rest of the tract has completely degenerated. The 
remarkable parasitic crustacean, Sacculina carcini, looks like 
a tumor attached to the under surface of the abdomen of a 
crab. It has neither mouth nor digestive tract, and absorbs 
nourishment from the crab through rootlike outgrowths that 
penetrate the body. From its development alone we know 
that it is a degenerate barnacle. 
“There seems to be in all these cases an apparent connec- 
tion between the absence of the digestive tract and the 
presence of an abundant supply of food, that has already 
been partly digested:by the host. Put ina different way, we 
may say that the presence of this food has furnished the 
environment in which an animal may live that has a rudi- 
mentary digestive tract. 
An interesting case of degeneration is found in the rudi- 
mentary mouth parts of the insects known as May-flies, or 
ephemerids. Some of these species live in the adult con- 
dition for only a few hours, only long enough to unite and 
deposit their eggs. In the adult stage the insects do not 
take any food. In this case the degeneration is obviously 
not connected with the presence of food, but apparently 
with the shortness of the adult life. 
2A 
