trans

NEUROTRANSMITTERS IN PLATYHELMINTHS 385
organisms like C. elegans to provide an added
knowledge base. A further difficulty relating to
the study of neurotransmitters in the study of
flatworms is a greater diversity in the detailed
neuronal arrangements of the different flatworms.
For these reasons we know less about
the structure and function of cestodes and
trematodes and consequently less about the
neurotransmitters of these parasites. Indeed,
no single candidate that meets the classical
criteria for inclusion as a neurotransmitter in
flatworms has been identified. As with nematodes,
we will consider first the organization of
the nervous systems of the trematodes and
cestodes.
The structure of the nervous system
of trematodes and cestodes
The flatworm nervous system can be thought
of as a melding of a complex central nervous
system with a peripheral system of more
primitive nerve nets, or plexuses. The primary
features of the central nervous system are a
bilobed anterior ganglion and two corresponding
‘main’ longitudinal nerve cords (Figure
15.19). Although a variable number of anteriorto-posterior
nerve tracts can be present, there
is most often a pair of predominant, or ‘main’
nerve cords. As the main nerve cords progress
posteriorly, they are connected by a series of
commissures, creating a ladder-like ‘orthogonal’
structure. The peripheral nervous system
is composed of a number of nerve net-like
plexuses, each a flat meshwork of nerve fibers
confined to a limited area in the animal. Subepithelial,
sub-muscular and intra-epithelial
plexuses are present, as well as distinct plexuses
associated with the suckers, the gut, the pharynx,
and the reproductive structures. The
cytology of the nervous system is consistent
with this depiction of a complex brain joined
to more primitive peripheral nerve nets, as the
peripheral nervous system consists exclusively
of multi- and bi-polar cells which are thought
to be more ancient, while the central nervous
system ganglia often feature more specialized
unipolar cells.
The body structure of flatworms is without
celomic cavities and does not contain a circulatory
system, such as blood vessels or
lymphatics. The distribution of regulatory
hormones that stimulate growth and generalized
tissue responses is presumed to occur via
the nervous system. The nervous system
therefore serves a neurosecretory function as
well as the role of direct and more rapid control
of tissue responses.
It is remarkable how little is actually known
about the function of the nervous system in
flatworms. The worms themselves are small
and the individual cells are quite small and
tightly packed, making rare any flatworm
preparations where one can study the nervous
system directly. Further, in the case of parasitic
worms, the observation of in situ behavior is
difficult at best, and the effects of isolation of
the parasites from their hosts is largely
undefined. Consequently, there are very few
data elucidating how flatworm nervous systems
function, or the role of the nervous system
in various physiological processes or
behaviors. Largely unknown are relationships
between the central and peripheral nervous
systems, and their relative roles in controlling
various aspects of behavior. Free-living flatworms
are capable of coordinated locomotion
and feeding responses in the absence of CNS
input, demonstrating that much of the neural
control of basic flatworm behavior occurs in
the peripheral plexuses. Similarly, in a number
of parasitic flatworms, removal or damage to
the central nervous system has little observable
effect on the worms. Most of what is
known about the roles of putative neurotransmitters
in flatworms is focused on the somatic
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS