DOI: 10.1186/2041-9139-5-48
Background: The evolutionary emergence of the Mollusca is unclear. Some have proposed that molluscs stem from unsegmented organisms, while others say they stem from a segmented annelid-like ancestor.
In this study, the authors investigated the development of the nervous system in two species of solenogasters to describe the larval nervous system and also to test the hypotheses on segmented or unsegmented ancestry of molluscs.
Observations: During the embryonic development, first neurons appear at the apical and abapical pole; the flask-shaped cells of the apical organ and the large cells associated with the suprarectal commissure are lost.
The neuropile beneath the apical organ develops into the cerebral commissure. The cellular posterior connection of the lateral neurite bundles becomes the suprarectal commissure.
Interestingly early nervous system development in the polychaetes shows strong similarity to the mode of neural development described here for solenogasters. They both develop apical organ with flask-shaped cells, and a single pair of longitudinal neurite bundles. Similarity in the pattern of serotonin-like immunoreactivity, and formation of the CNS from anterior and posterior ends.
Conclusions: This study supports a nonsegmented ancestry of molluscs, but there is similarities between solenogasters and polychaetes during early nervous system development, such as the formation of the nervous system from an apical and abapical neurogenic domain.
The authors suggest that they share neural features descent from the last common ancestor, which had no segmentation. Segmentation may have evolved only along the line leading to the annelids.
The authors suggest that they share neural features descent from the last common ancestor, which had no segmentation. Segmentation may have evolved only along the line leading to the annelids.
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