AMER. ZOOL., 19:1183-1184 (1979).
Introduction: How to Know a Parasite
REINO S. FREEMAN
Department of Microbiology and Parasitology,
University of Toronto, Toronto, Canada M5S 1A8
The title of this symposium is "Contemporary Methods in Systematic Parasitology." From it comes the obvious title of this
introduction "How to Know a Parasite."
Many biologists would in turn recognize
this as a take-off on the familiar "How to
Know . . ." series initiated by Jaques and
published by Wm. C. Brown since the early
1940s. Numerous volumes now cover organisms belonging to a variety of taxa, and
on parasites the series includes "How to
Know the Trematodes" by Schell (1970)
and "How to Know the Tapeworms" by
Schmidt (1970). These valuable little publications follow existing systems of classification and by using key characters are of great
assistance in the taxonomy of the taxa concerned.
Presumably up to this point I have not as
yet lost anyone. Nevertheless I am certain
that some of us are already traveling along
differing channels of thought, because we
assume our definitions of the words used
are common knowledge, or facts. Do we for
example agree on: What is a parasite, what
is taxonomy, classification, or systematics,
or for that matter what is a trematode or
a tapeworm? According to Blackwelder
(1967) systematics is ". . . the study of the
kinds and diversity of organisms, their distinction, classification and evolution." I assume that such a definition of systematics is
in your minds as well as those of our contributors.
As scientists we must live by definitions if
we expect to communicate meaningfully
with each other. We all have, however, our
own vocabulary based on our own comprehension of these various concepts. This
comes about because no matter how careThis symposium was organized and arranged by
Drs. W. W. Moss and D. R. Brooks. I am most grateful
to them for inviting me to be chairman.
fully the original concept was defined, continued use and increasing knowledge have
stretched or compressed, twisted or even
dismembered it. New words are proposed
as guides for revised or supposedly new
definitions. Old words slowly or quickly, intentionally or inadvertently, come to take on
new meanings. Many terms go into limbo,
or the original concept may even be totally
lost, but the term involved may remain in
use in another context. For example is there
anyone who does not think of the tapeworm
holdfast or "head" in connection with the
word scolex? Yet, Scolex was initially put
forward as a genus, and even today is used
in the generic sense, e.g., Scolex pleuronectis
Mueller, 1788 (see Wardle and McLeod,
1952), for a group of marine metacestodes,
i.e., juvenile cestodes. Such confusion of
terms is inevitable in the biological sciences
where living, reproducing, changing organisms, the species, are involved. Quite
likely there is even less unanimity as to the
definition of the term species than the
terms mentioned earlier. This should become evident as the symposium progresses.
Parasitology obviously is the study of
parasites, and certainly all of us have a concept of what parasites are. Yet again, with
our diverse backgrounds, I doubt that we
could arrive at a single definition of parasite
agreeable to us all. Often a subject about
which one knows little is easier to define
than one about which one knows a great
deal. That is the situation with parasites.
Many of us now synonymize parasite with
symbiont, because there are species which
apparently are pathogens at one time,
commensals at another, and conceivably
even mutuals at other times. Yet even here
we run into difficulty with organisms such
as mosquitoes and bedbugs. Are they ectoparasites, intermittent parasites, micropredators, or what? And if mosquitoes and
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REINO S. FREEMAN
similar blood-sucking forms are not parasites, then how do they differ from fleas,
which nearly every textbook considers to be
ectoparasites? Thus are the meanings of
definitions stretched or compressed until
almost meaningless, or definitions divided
and subdivided until the whole is no longer
evident.
I have spent over 30 years investigating
the developmental biology, morphology
and systematics of the Cestoda. Yet in some
respects I am more confused today as to
what cestodes really are than when I first
saw these live, wriggling, fascinating creatures. At that first sight I knew immediately
that they were cestodes. Yet today, I have
difficulty in pointing to a single cardinal
feature which invariably identifies a cestode! Since near the beginning of this century cestodes have been included in the
Phylum Platyhelminthes, usually with trematodes, turbellarians and the like (Liihe,
1910). According to Hyman (1951), whom
most English speaking workers in the field
follow, members of this phylum are "acoelomate bilaterians," hence by definition triploblastic. To my knowledge, however, no
one has ever demonstrated that cestodes
have true entoderm, either such as derived
from an archenteron, or any other way.
Nor do I, on the other hand, know of any
serious proponent of the position that cestodes are diploblastic. Presumably, therefore, adult cestodes have an interior mass
of both entoderm and ecto- and entomesoderm as postulated for the turbellarian
order Acoela (Hyman, 1951). Although
adult cestodes lack any type of body cavity,
during early development the oncosphere
of numerous species of the more recently
evolved cyclophyllideans does develop a
prominent cavity. Earlier workers referred
to this as a "primitive lacuna," but primary
lacuna would seem more appropriate (Freeman, 1973). This cavity one must assume is
in the mesoderm. Its early postlarval development resembles development of the blastocoel or segmentation cavity of true coelomates. This cavity can, however, become a
highly functional structure, as in the taeniid
bladderworm, but it is not considered homologous to true coelom.
It is equally difficult to establish whether
cestodes are truly bilaterally symmetrical or
not. For example is the adult holdfast the
true "head," hence the anterior end? Are
adult cestodes dorso-ventrally flattened as
usually stated? If so which side is dorsal and
which ventral? More importantly do adult
cestodes really function as dorso-ventrally
oriented animals in the normal intestinal
habitat? At the present time answers to
most of these q uestions are largely based on
arbitrary definitions. There is need for new
data, and fresh thought on this whole matter. Still, most of us have ideas as to what
cestodes are. The single feature I consider
most characteristic of true cestodes is the
larval 6-hooked oncosphere (Freeman,
1973). No established system of cestode
classification has yet taken these larvae or
the resultant juvenile stages into account,
although a suggestion that this might prove
fruitful has been made (Freeman, 1973).
Will any of the contributors refer to larval
orjuvenile stages when considering the systematics of their particular taxa? We shall
have to wait and see.
REFERENCES
Blackwelder, R. E. 1967. Taxonomy. A text and reference
book. John Wiley and Sons, New York.
Freeman, R. S. 1973. Ontogeny of cestodes and its
bearing on their phylogeny and systematics. In B.
Dawes (ed.), Advances
in parasitology,
Vol. 1 1 . p p .
481-557. Academic Press, New York.
Hyman, L. H. 1951. The invertebrates: Platyhelminthes
and Rhynchocoela the acoelomate bilateria. Vol. 2.
McGraw-Hill Book Company, New York.
Liihe, M. 1910. Parasitische Plattwiirmer. II: Ces-
todes. In Brauer (ed.), Die Siisswasserfauna Deutschlands. Heft 18, Gustav Fischer, Jena.
Schell, S. C. 1970. How to know the trematodes. Wm. C.
Brown Co. Publishers, Dubuque, Iowa.
Schmidt, C D . 1970. How to knoxu the tapeworms. Wm. C.
Brown Co. Publishers, Dubuque, Iowa.
Wardle, R. A. and J. A. McLeod. 1952. The zoology of
tapeworms. University of Minnesota Press, Minneapolis.