Multicotyle purvisi and Lobatostoma manteri
are discussed as examples (Rohde, 1972, 1973, further references therein).
Pre-adult juveniles differ from adults in that only the latter produce
sperm and egg cells. An oral sucker is lacking; the mouth opening is
located anteriorly to the pharynx which opens into the single caecum.
Two testes are found in the middle of the body and anteriorly to them
an ovary. The vitellarium extends along both sides of the body and is
confluent posteriorly. The uterus occupies much of the anterior half
of the body. It opens, together with the sperm duct, close to the anterior
end of the body. A septate oviduct is characteristic of all aspidogastreans
which have been examined in this respect (Fig.1).
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Figure 1. The female reproductive organs near the ovary
of Multicotyle purvisi.
Note the septate oviduct, indicated by the narrow lumen surrounded
by a thick wall in the cross-section (redrawn from Rohde, 1972).
In both M. purvisi and L. manteri,
a Laurer's canal extends from the female reproductive ducts to the dorsal
surface where it opens to the outside (Fig.1 and Fig. 2).
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Figure 2. Adult Multicotyle
purvisi (redrawn from Rohde, 1972).
L. manteri has a powerfully developed eversible copulatory
organ, a cirrus. In M. purvisi, the ventral disc extends
along most of the body (Fig.2). In its anterior half, it is separated
from the main body by a septum of connective tissue (Fig.3).
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Figure 3. Sagittal (longitudinal section along the midline)
of adult Multicotyle purvisi.
Note the septum of connective tissue between the anterior ventral
and dorsal parts of the body (redrawn from Rohde, 1972).
It is subdivided into four longitudinal rows of alveoli (suckerlets).
So-called marginal bodies are located between the marginal alveoli.
Electron-microscopic studies of the marginal bodies of M. purvisi
and L. manteri have shown that they are ampullae (Fig.4),
which serve to store secretion produced by the marginal gland cells
in the dorsal parts of the marginal alveoli. The secretion is transported
in the ducts to the ampullae.
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Figure 4. Marginal organ of Lobatostoma
manteri (redrawn from Rohde, 1973).
The ampullae open on the ventral surface through short ducts (Figs.4
and 5).
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Figure 5. Marginal organs of Multicotyle
purvisi showing marginal glands, ampullae
and connecting ducts (redrawn from Rohde, 1972).
Nervous system
The nervous system both of the larva and the adult is of extraordinary
complexity (Figs. 6 and 7).
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Figure 6. Nervous system in anterior part of body. Note
that the brain is the dorsal part of a ring commissure, and that there
is a second more external ring commissure at the same level (redrawn
from Rohde, 1972).
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Figure 7. Main nerves in middle part of Multicotyle
purvisi (redrawn from Rohde, 1972).
Whereas most flatworms have a ladder system of nerves consisting of
a number of longitudinal nerve cords (connectives) below the surface
epidermis or tegument connected by ring commissures, both the larva
and the adult of Multicotyle purvisi have two such
systems in the anterior part of the body, one below the surface and
one surrounding the mouth cavity. Furthermore, the number of connectives
is very large, i.e., 14, and the brain is the dorsal part of a ring
commissure surrounding the mouth cavity (Fig.6). The latter observation
may have important phylogenetic implications, it may indicate that the
brain in the Platyhelminthes and perhaps in all invertebrates has evolved
as a processing centre in the interior of the body and not, as usually
assumed, as an aggregation of nerve cells around anterior sensory receptors.
- Among the posterior nerves, the ventral connectives are best developed
(Fig.7). Some branches of these connectives enter the ventral disc of
the adult or posterior sucker of the larva, and other branches enter
the pharynx of the adult and larva. Intestine and septum dorsal to the
ventral disc are also well innervated. Anteriorly and posteriorly, the
longitudinal nerves are connected by well developed commissures. Of
particular importance, parts of posterior nerves were found to be surrounded
by a multilamellate sheath, one of the few instances of glial elements
in the nervous system of flatworms. Clusters of glandular, apparently
neurosecretory cells were shown to occur at the junctions of connectives
and commissures dorsal and lateral to the ventral disc.
Sensory receptors
Electron microscopic studies of Lobatostoma manteri
and Multicotyle purvisi have shown an amazing variety
of sensory receptors, which differ in the absence or presence of cilia,
in the shape and length of the cilia, in the absence or presence of
ciliary rootlets and their shape, in the number of axonemal microtubules,
and whether they are part of a complex organ or not. In pre-adults of
the first species, eight (and possibly a further six) types, in adults
of the second species seven (and possibly a further two) types were
found (Fig.8) (Rohde, 1989a, 1990; Rohde and Watson, 1989a).
Figure 8. Sensory receptors of pre-adult Lobatostoma
manteri. Note differences in presence
or absence of a cilium or a ciliary rootlet, length of cilium, shape
of ciliary rootlet, presence or absence of tubules, location in the
tegument (redrawn from Rohde, 1989a).
It has been estimated that pre-adult L. manteri has
at least 8500 surface receptors and many sub-surface receptors as well.
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