On the phylogenetic significance of sagittocysts and copulatory organs in acoel turbellarians

Viewed by SEM and TEM, sagittocysts of Convoluta bifoveolata Mamkaev, 1971, and ‘needles’ of C. sagittifera Ivanov, 1952, have the same structure. Both are capsule-form extrusomes containing a protrusible needle. Only seven similar species of convolutimorph acoels symbiotic with green algae and C. sagittifera, without algae, possess extrusomes of this peculiar and complicated type. The sagittocyst is a clear synapomorphy of all these species. A sacciform ciliated antrum lacking a seminal vesicle is also characteristic of these species and also of three Japanese species of green (algae-symbiotic) convolutimorph acoels lacking sagittocysts. We suggest schemes of the possible evolution of male and female copulatory organs to provide a basis for better using such organs as phylogenetic characters. We regard the formation of a ciliated sacciform antrum as an independent evolutionary trend. This conclusion forms the basis for establishing the separate family Sagittiferidae. Species of this family seem to have originated in the West Pacific.