Trigonostomum vanmecheleni sp nov., a new species of Trigonostomidae (Rhabdocoela: Dalytyphloplanida) from the channels of Venice (Italy), with a discussion on the T-lilliei species group

Trigonostomum vanmecheleni nov. sp. (Rhabdocoela: Dalytyphloplanida) was collected from algae taken from the channels in Venice (Italy). It can easily be distinguished from its congeners by the detailed morphology of the stylet and of the bursal appendage, which is a coiled tube. Only T. lilliei has a bursal appendage with a comparable morphology. All available material of that species was also studied, leading to the conclusion that T. prytherchi is not a synonym of T. lilliei, as was proposed in earlier literature. T. prytherchi can be distinguished by the fact that the stylet proper is proximally bent over 180 degrees, whereas in the other two species it is bent over 90 degrees, and by the fact that the mantle of the stylet consists of one plate only, where it comprises two plates in the other two species. T. vanmecheleni nov. sp. can be distinguished from T. lilliei by the fact that the two plates of the mantle are different in shape and by the fact that the bursal appendage splits distally into six finer tubes. In T. lilliei the two plates are identical in shape and the bursal appendage splits distally into three to four finer tubes.