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Frontal glandular and sensory structures inNemertoderma (Nemertodermatida) andParatomella (Acoela): ultrastructure and phylogenetic implications for the monophyly of the Euplathelminthes (Plathelminthes)

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Summary

The ultrastructural organization of the frontal glandular and sensory structures in aNemertoderma species and in the acoelParatomella rubra is described. In both species, a frontal gland complex with an accumulation of different body wall glands and of ciliated sensory receptors exists at the anterior tip of a specimen. Such an organization can be hypothesized as a ground pattern characteristic of the Acoelomorpha. A “frontal organ” with a common apical pore of many necks of mucoid glands is known for most species of the Acoela; however, such an organ is not a basic feature either of this taxon or of the Acoelomorpha. This substantiates the hypotheses of a homology of the frontal glandular structures in the Acoelomorpha and the Rhabditophora and of the monophyly of the taxon Euplathelminthes. Within the Euplathelminthes, positional, morphological and histochemical differences of the frontal glands and the secretions exist between distinct taxa. These different organizations contribute to the phylogenetic systematization of the Plathelminthes, as shown in a diagram of the phylogenetic relationships among the basic closed descendent communities of this taxon.

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Abbreviations

e,e 1,e 2 :

ellipsoid granules, types 1 and 2

ep :

epidermis cell

f :

mucoid granules

gf :

mucoid gland neck

is :

intensively staining secretions

mit :

mitochondria

r :

rod-shaped rhabdoids

sr 1 :

monociliated sensory receptors with vertical rootlet

sr 2 :

monociliated sensory receptor with rhizomelike structure

sr 3 :

monociliated sensory receptor with tubularlike structure

t :

target granules

tw :

terminal web

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  1. II. Zoologisches Institut und Museum der Universität Göttingen, Berliner Strasse 28, W-3400, Göttingen, Federal Republic of Germany

    Ulrich Ehlers

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Ehlers, U. Frontal glandular and sensory structures inNemertoderma (Nemertodermatida) andParatomella (Acoela): ultrastructure and phylogenetic implications for the monophyly of the Euplathelminthes (Plathelminthes). Zoomorphology 112, 227–236 (1992). https://doi.org/10.1007/BF01632820

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