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Aspidogastrea: Structure of the Juvenile and Adult

Klaus Rohde

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).

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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.

References

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About This Page

Klaus Rohde
University of New England, Armidale, New South Wales, Australia

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