ABSTRACT
Amphistomida is one of the natural groups of the digenetic trematodes whose representatives have successfully colonized each grade of vertebrates. The tempo of the higher level differentiation of amphistomes seems to be slower than that of their definitive hosts. The amphistomes of the lower vertebrates and that of some of the mammals have fundamentally the same general plan in their structure, indicating that amphistomes have their own phylogeny. The amphistome type of parasites has supposedly developed from Rhabdocoele-like ancestors which, in the prevertebral geologic time, existed in the form of mollusk-turbellarian relationships. The amphistome parasites of the submammalian vertebrates bear ancient morphological and ecological characters, and their intercontinental distributional pattern of today is a reflection of the conditions that existed before the breakup of the Pangaea. The habitat of the lower digestive tube is regarded to be a plesiomorphic ecological trait. The amphistomes, living in this part of the digestive tube, are characterized by plesiomorphic states.
