ORGANIC DEPENDENCE AND DISEASE 
39 
ILLUSTRATIONS OF PRIMITIVE PARASITISM 
The Case of the Annelids 
One group of animals, the worms or annelids, is of prime 
interest in these considerations. The worms occur in vast 
variety in the existing fauna and their derived or secondary 
expressions are abundant. It is not with these that we are 
concerned. The primitive or archetypal worm is conceived 
as a simple fore-and-aft segmented structure in which the 
innervation is repetitive by segments and the alimentary 
and distributive organs simple and continuous. The worm 
has led a long career of ideal independence and it has been 
the architectural model for the higher creation. In the 
judgment of many morphologists there is, as we have al- 
ready intimated, a convergence backward into the past 
toward the archetypal worm, of great differentiated stocks 
like the brachiopods and the echinoids, while we recognize 
in all segmented creatures the normal continuous progeny 
of the annelid prototype. 
Worms, we may restate, were common enough in the 
Cambrian fauna, known both by their trails and burrows 
and by some highly specialized bodies ; and it is probable 
that such evidences of their existence will not long be lack- 
ing in the Precambrian. The worm, however, had a soft 
body; its acquisition of a cover or shell which would en- 
able its preservation was a secondary development. So we 
are confronted in all the early rocks by few actually fossil- 
ized worms but with a great abundance of their trails in 
the soft muds. The worm buried itself halfway or wholly 
in the mud; encased itself, at times, in tubes of its own 
making; thus ensuring a protection against adversaries. 
But it retained an active, vibratile vitalism and an en- 
tire freedom from attachment to its tube. The rocks of 
these formations are often filled with vertical worm tubes, 
and the surface of the same beds may be marked by fes- 
