No. 465.) ZVTERRELATIONSHIPS OF SPOROZOA. 609 
actually exceed other less specialized forms of life in their repro- 
ductive powers. Nevertheless the germs are produced in such 
numbers that if one in many thousands survive, the perpetuation 
of the line is assured. 
Examples in support of the statements given above are num- 
erous. Certain animals, particularly the small Nematodes, live 
indifferently either in the outside world or else upon or within 
higher forms. Such may be regarded as upon the threshold of 
the parasitic life. A second step is taken by those Flagellates 
which inhabit the recta of frogs and salamanders. These ani- 
. mals are morphologically like their free-living congeners nor is 
their life history known to-be more complicated. But turning 
to the flat-worms we find the parasitic habit in its most confirmed 
aspect. This phylum, moreover, furnishes us with a series of 
stages from typical free-living forms to typical parasites. It is 
divided into three classes: the Turbellaria, the Trematoda, and 
the Cestoda. The first class consists mainly of free-living ani- 
mals, and is characterized by an alimentary canal and a ciliated 
epidermis. The Trematoda retain the intestine, but have lost 
the cilia. The Cestodes go farther, and losing all trace of an 
alimentary canal, absorb their nutriment through the epidermis. 
There is thus a progressive morphological degradation. 
But along with this change in the individual, there is a strik- 
ing change in the life history. In the free-living Turbellaria and 
the ectoparasitic Trematodes, development is direct. That is, 
the eggs produce young which grow to be adults without meta- 
morphosis. On the other hand, the endoparasitic Trematodes 
and the Cestodes bear off the palm for a complicated life history. 
The organisms hatched from the eggs are wholly unlike the 
adults, and there is always at least one metamorphosis. In addi- 
tion, change of host is invariable. Thus, the liver fluke of the 
sheep spends a part of its existence in a water snail. Indeed, 
from a purely utilitarian standpoint, these animals appear to 
have overshot the mark, and their ontogenetic: development 
defies a rational explanation. The desired end could apparently 
have been reached quite as certainly by less devious methods. 
But while we cannot explain the raison d'étre for such processes, 
it is evident that they indicate a confirmed parasitism. These 
