Patterns of association of digenean families and their mollusc and vertebra
te hosts are assessed by way of a new database containing information on ov
er 1000 species of digeneans for lift-cycles and over 5000 species from fis
hes. Analysis of the distribution of digenean families in molluscs suggests
that the group was associated primitively with gastropods and that infecti
on of polychaetes, bivalves and scaphopods are all the results of host-swit
ching. For the vertebrates. infections of agnathans and chondrichthyans are
apparently the result of host-switching from teleosts. For digenean famili
es the ratio of orders of fishes infected to superfamilies of molluscs infe
cted ranges from 0.5 (Mesometridae) to 16 (Bivesiculidae) and has a mean of
5.6. Individual patterns of host association of 13 dipenean families and s
uperfamilies are reviewed. Two, Bucephalidae and Sanguinicolidae. are excep
tional in infecting a range of first intermediate hosts qualitatively as br
oad as their range of definitive hosts. No well-studied taxon shows narrowe
r association with vertebrate than with mollusc clades. The range of defini
tive hosts of digeneans is characteristically defined by eco-physiological
similarity rather than phylogenetic relationship. The range of associations
of digenean families with mollusc taxa is generally much narrower. These d
ata are considered in the light of ideas about the significance of differen
t forms of host association. If Manter's Second Rule (the longer the associ
ation with a host group, the mure pronounced the specificity exhibited by t
he parasite group) is invoked, then the data may suggest that the Digenea f
irst parasitised molluscs before adopting vertebrate hosts. This interpreta
tion is consistent with most previous ideas about the evolution of the Dige
nea but contrary to current interpretations based on the monophyly of the N
eodermata. The basis of Manter's Second Rule is. however, considered too fl
imsy for this interpretation to be robust. Problems of the inference of the
evolution of patterns of parasitism in the Neodermata al-e discussed and c
onsidered so intractable that the truth may be presently unknowable. (C) 20
01 Australian Society for Parasitology Inc. Published by Elsevier Science L
td. All rights reserved.