Key to species of Mesacanthion with simple spicules shorter than 2 abd and subventral precloacal setae present

This key can be taken as an addition of the last entrance (22) in the key to species of Mesacanthion in Jeong et al. (2019) to include two new species.

1. Precloacal supplement organ less than 2 spicule lengths distant to cloaca ...... M. pali Wieser, 1959

– Precloacal supplement organ more than 2 spicule lengths distant to cloaca .................................... 2

2. Gubernaculum without dorsal apophysis ........................................ M. longigubernaculum sp.nov.

– Gubernaculum with dorsal apophysis ............................................................................................... 3

3. De Man´s b index greater than 5 .............................................. M. longissimesetosum Wieser, 1953

– De Man´s b index smaller than 5 ............................................................. M. sanantoniensis sp. nov.