Abstract
Sperm structure among species in each Class of Aplacophora is highly conserved but between the two Classes is radically different. This together with numerous morphological differences between the two groups, suggests a long separation and a likely paraphyletic relationship, which has been suggested in the past but is not supported by molecular analyses. All Caudofoveata examined have unique externally-fertilizing ect-aquasperm found nowhere else in the Animal Kingdom. Solenogastres fertilize internally with introsperm like those described for Epimenia australis but differ in details of length and number of specific components, providing insights to relationships among them. Furthermore, the solenogaster introsperm shares at least nine characters with introsperm of the bilaterian lineage Nemertodermatida, but shares none of these characters with the sperm of Caudofoveata, Polyplacophora (chitons), or Xenoturbellida and few with Acoela. However, a recent re-analysis of molecular data points to the re-organization of Xenacoelomorpha due to Long Branch Attraction and its separation into a basal Nemertodermatida plus a “Xenacoela” clade related to Ambulacraria. If the shared sperm characters of Solenogastres and Nemertodermatida are plesiomorphies not homoplasies, this would provide support for phylogenies that place Solenogastres basal to Mollusca. If true, then basal Bilateria would have a direct link with Protostomia. Recent discoveries of minute worm-like bilaterian body and trace fossils in Ediacaran deposits suggest that earliest bilaterians were meiofaunal and preceded the great Cambrian explosion by millions of years.