Degeneration In almost every group of the animal kingdom there are forms that are recognized as degenerate. This degeneration is usually associated with the habitat of the animal. In many cases it can be shown with much probability that these degenerate forms have descended from members of the group that are not degenerate. We find there is a loss of those organs that are not useful to the organism in its new environment. The degeneration may involve nearly the whole organization (except as a rule the reproductive system), as seen in the tapeworm, or only certain organs of the body, as the eyes in cave animals. A few examples will bring the main facts before us. A parasitic existence is nearly always associated with degeneration. Under these conditions, food can generally be obtained without difficulty, at the expense of the host, and apparently associated with this there is a degeneration, and even a complete loss of so important an organ as the digestive 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 digestive 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 connection 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 rudimentary digestive tract. An interesting case of degeneration is found in the rudimentary mouth parts of the insects known as May-flies, or ephemerids. Some of these species live in the adult condition 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.