Ultrastructure and significance of the transitory nephridia in Erpobdella octoculata (Hirudinea, Annelida) |
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Authors: | B Quast Thomas Bartolomaeus |
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Institution: | Zoomorphologie und Systematik, Fakult?t für Biologie, Universit?t Bielefeld, Postfach 100131, 33501 Bielefeld, Germany e-mail: bjoern.quast@biologie.uni-bielefeld.de Tel.: +49-521-1062800, Fax: +49-521-1066426, DE
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Abstract: | In early developmental stages of Erpobdella octoculata two pairs of transitory nephridia occur which degenerate during the formation of the body segments. Because in the ground
pattern of Annelida the first nephridia formed during ontogenesis are protonephridia, it can be assumed that the transitory
nephridia of E. octoculata are homologous to the larval protonephridia (head kidneys) of Polychaeta. To test this hypothesis two cryptolarvae of E. octoculata were investigated ultrastructurally. Both pairs of transitory nephridia are serially arranged to either side of the midgut
vestigium. Each organ consists of a coiled duct that opens separately to the exterior by an intraepidermal nephridiopore cell.
The duct is percellular and formed by seventeen cells. Adluminal adherens and septate junctions connect all duct cells; the
most proximal duct cell completely encloses the terminal end of the duct lumen. A filtration structure characteristic for
protonephridia is lacking. Additionally, the entire organ lacks an inner ciliation. Morphologically and ultrastructurally
the transitory nephridia of E. octoculata show far reaching congruencies with the segmental metanephridia in different species of the Hirudinea. These congruencies
support the assumption that formation of transitory nephridia and definitive metanephridia in Hirudinea depends on the same
genetic information. The same inherited information is assumed to cause the development of larval head kidneys and subsequently
formed nephridia in different species of the Polychaeta. Thus, the presumed identical fate of a segmentally repeated nephridial
anlage supports the hypothesis of a homology between the transitory nephridia in Hirudinea species and the protonephridial
head kidneys in the ground pattern of the Polychaeta. We, therefore, assume that functional constraints lead to a modification
of the protonephridial head kidneys in Hirudinea and explain ultrastructural differences between the transitory nephridia
in Hirudinea and the protonephridia in Polychaeta.
Accepted: 11 December 2000 |
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