wow...you should publish this...
usually people refer to sponges or the phylum of placozoa as the most primitive metazoans.
and for reproduction in sponges:
asexual reproduction occurs, but is not that common (although they have a remarkable capacity for regeneration!)
As for sexual reproduction both hermaproditic and dioecious sponges exist, and mostly sperm and eggs are produced at different times (which would proclude self-fertilization as occurs in c.elegans)
After that come the bilateral animals. There are 2 major groups of them, the protostomia and the deuterostomia. And of course nematodes fall under protostomia and we vertebrates fall under deuterostomia. Nematodes are therefore not a primitive ancestor of ours. They went there own way with a whole group of other animals a long time ago. I think you may have to adjust your view on the evolutionary tree slightly.
The most primitive bilateral animal would be something as the flatworm. They are largely hermaphroditic, but also some are dioecious.
and why are they hermaphroditic?
From 'Invertebrate zoology', Ruppert anbd Barnes. 6th edition. p209.
'hermaphroditism is not an adaptation of self-fertilization. In fact most hermaphrodites have structural and physiological barriers that prevent self-fertilization. In stead, hermaphroditism generally prevails under conditions in which potential mates meet infrequently. Such conditions may occur when population densities are low and contacts are infrequent. The security of hermaphroditism is that contact between any two individuals can result in a succesful mating'
self-fertilizing hermaphrodites are therefore a special adaptation.
Maybe you should check things first before you claim something with authority.
edit: On nematodes - 'MOST nematodes are diocecious!!!!, but hermaphrodites, such as the well-studied C elegans, are not uncommon.' p297 invertebrate zoology
