Earthworm populations in grassland and plantations in the area of Bulolo, Papua New Guinea were compared with populations in nearby undisturbed rain forest. The grasslands had been maintained by burning for many years. The Pinus plantation had been developed on a burned grassland site and the Araucaria plantation on a site which had been cleared of secondary forest.

The grasslands and the Pinus plantation supported moderate populations of exotic earthworms including Pontoscolex corethrurus, but no indigenous species. The Araucaria site supported a native species, Amynthas zebrus only, which was also found together with two other native species at very low density in rain forest.

Exotic earthworm species widespread throughout the tropics, were present in disturbed soils and formed moderately high density populations in burned grasslands. There was no evidence that they displaced native Megascolecidae in rain forest.

The evolution of a character is better appreciated if examples of convergent emergence of the same character are available for comparison. Three instances are known among invertebrates of the evolution of axonal sheaths possessing the functional properties and many of the structural properties of vertebrate myelin. Comparison of these invertebrate myelins raises the question of what structural features must a sheath possess in order to produce the two principal functional characteristics of impulse speed enhancement and energy savings. This essay reviews the features recognized by early workers as pertaining to myelin in vertebrate and invertebrate alike: osmiophilia, negative birefringence and saltatory conduction. It then examines common features revealed by the advent of electron microscopy: multiplicity of lipid membranes, condensation of those membranes, specialized marginal seals, and nodes. Next it examines the robustness of these features as essential components of a speed-enhancing sheath. Features that are not entirely essential for speed enhancement include membrane compaction, spiral wrapping of layers, glial cell involvement, non-active axonal membrane, and even nodes and perinodal sealing. This permissiveness is discussed in relation to the possible evolutionary origin of myelin.

Cette étude porte sur les communautés d'Oligochètes aquatiques vivant dans une petite rivière atlantique, le Tambre, et ses milieux associés (lagune, lac de barrage, ria). Elle concerne la répartition spatiale des différentes populations et l'analyse des principaux facteurs qui la déterminent.Des données de l'évolution saisonnière des principales populations et des périodes de reproduction complètent l'inventaire faunistique des Oligochètes récoltés dans ces milieux.