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, Entoscolax ludwigii, 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 in a 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. 



