Cercaria douglasi Cort, 1917 and Its Relation to the Cercaria of Cotylurus flabelliformis (Faust, 1917)

Abstract

Cercaria douglasi was described briefly by Cort (1917) from a species of Physa from the Douglas Lake Region, Michigan. Later Cort and Brooks (1928) identified as this species cercariae from Lymnaea stagnalis appressa Say, L. stagnalis perampla Walker, Stagnicola emarginata angulata (Sowerby), and Physa parkeri Currier. Their description, however, was from the cercariae that emerged from three specimens of the varieties of L. stagnalis. Subsequent investigations have revealed that cercariae in the Douglas Lake Region, previously identified as C. douglasi, actually represent two distinct but closely related species. One of these, which is found only in members of the PHYSIDAE, we identify as C. douglasi Cort, 1917; the other, which develops only in members of the LYMNAEIDAE, was mistakenly identified by Cort and Brooks as C. douglasi. This second species has been shown by van Haitsma (1930) to be the cercaria of Cotylurus flabelliformis (Faust, 1917). Since Cort's original description of C. douglasi was very brief and was made from cercariae obtained by autopsy from a single snail, a redescription of this species is presented. Also, C. douglasi is compared with the cercaria of C. flabelliformis with which it has been confused.