Morphology and phylogenetic implications of oesophageal modifications in the Limnodriloidinae (Oligochaeta, Tubificidae)

Two features have been considered apomorphic for the subfamily Limnodriloid inae (Tubificidae): the lobed prostate glands, which are broadly attached t o prostatic pads inside the atrial ampullae, and the 'modified oesophagus' in segment IX. The homology of the modified oesophagus may be questioned, a s it is not of the same kind in all taxa. In Limnodriloides, Smithsonidrilu s, and Tectidrilus, there is a pair of diverticula, whereas in Thalassodril ides, Parakaketio, and Doliodrilus, the oesophagus is dilated to form a bar rel-shaped portion. In a few species of Limnodriloides lacking diverticula, a short part of the corresponding oesophagus is swollen. A scrutiny of the se features shows that they are morphologically different in several aspect s: the thickness and the structure of the epithelium, the granulation, the shape of the cells and the length of the modifications. Moreover, a regular , semi-embedded blood plexus always occurs around the barrel-shaped portion but rarely around the diverticula (if present, plexus irregular), and neve r around the swollen part of the oesophagus. These different types of modif ications fail the similarity test and thus should be regarded as independen tly evolved structures and coded as different characters in a parsimony ana lysis. Species from other subfamilies within the Tubificidae were also morp hologically investigated, and modifications of the alimentary canal were fo und in a few of them: Ainudrilus lutulentus, Heronidrilus bihamis, H. fasti gatus (all Rhyacodrilinae) and Clitellio arenarius (Tubificinae). The modif ication in segment VIII of Heronidrilus spp. resembles the barrel-shaped po rtion in, for example, Thalassodrilides, and should primarily be coded as t he same character in a congruence test of homology.