EXTERNAL MORPHOLOGY AND PHYLOGENETIC SIGNIFICANCE OF THE DORSAL NECK ORGAN IN THE CONCHOSTRACA AND THE HEAD PORES OF THE CLADOCERAN FAMILY CHYDORIDAE (CRUSTACEA, BRANCHIOPODA)/

On basis of a SEM study the homology between the neck/dorsal organ of the Conchostraca and the head pores of the cladoceran family Chydorida e is established. Species of Lynceus (Conchostraca) and Eurycercus (Ch ydoridae) show a characteristical similar arrangement of four elevated areas within a circular/oval organ. Presence of two lateral pores may be an apomorphy for the Chydoridae lost in the Chydorinae and in some other genera. Some species of the Chydoridae (Rhynchotalona falcta an d Tretocephala ambigua) display what can be interpreted as intermediat e stages between the circular/oval organs in Eurycerus and more aberra nt neck organ structures in the remaining Chydoridae. A characteristic neck organ morphology - two widely separated median pores with two sm aller pores in between and without lateral pores - is considered as a synapomorphy for the Chydorinae. In contrast, no component of the neck organ morphology could be given synapomorphic status for all the spec ies of the Aloninae. A number of potential apomorphies, related to the neck organ, seem to place subgroups of the Aloninae closer to the Chy dorinae than to the rest of the subfamily. These apomorphies include, among others, 'elongation of the neck organ' after Eurycercus has been branched off and subdivision of the neck organ into discrete pores af ter Rhynchotalona and Tretocephala has been branched off. If this inte rpretation is correct it will leave the Aloninae paraphyletic with res pect to the Chydorinae.