206 THE DOCTRINE OF DESCENT, 
is exchanged for independent phases, signifies a rever- 
sion taking place systematically in all individuals to the 
once permanent condition of their progenitors. Of the 
Trematoda or Flukes, and Cestoda or Tapeworms, be- 
longing to the class of the Platelmintha Suctoria, the 
latter have diverged the most from their starting-point ; 
their adaptation to life within other animals has rendered 
the alimentary canal superfluous, and their generations 
and transformations hence point less to their progenitors 
than is the case with a number of other Trematoda, with 
which many anatomical characters prove them to be 
closely related. Both, moreover, share the characters of 
their class with the free-living Turbellaria. From such 
as these, that is to say, from forms approximate to the 
present Turbellaria, the Trematoda and Cestoda must 
be descended, and with this agrees the free roving phase 
which the larva of the Fluke (Distomum) undergoes as 
the so-called Cercaria, and previously as a rotating 
spherical body. 
Many of the ciliated Nematoids, or thread-worms, 
too,— the division which includes the Ascarides among 
others,—have in their infancy a stage of independent 
life, during which they cannot be distinguished from 
the infantine forms of their more numerous kindred, 
which never adopt a parasitic life, and chiefly inhabit 
the sea. The transition to parasitism, as recapitulated 
by ontogenesis, was nothing more than an extension 
to a new territory offering advantages of nutriment; 
and on this point it is highly instructive to compare 
the Nematodes with the systematic series of the leech- 
like Suctoria (Trematoda), so excellently described 
by Van Beneden. We here find all the transitions 
