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- 



