Metacercariae of Apatemon gracilis (Rudolphi 1819) (Trematoda: Strigeidae) were found to encyst primarily in the posterior half of their leech hosts (Helobdella stagnalis, Placobdella parasitica, Erpobdella punctata) in numbers of from 1 to 13 larvae per cyst. Presumably the posterior localization of these larvae in their leech hosts is determined by the host's behavior in response to cercarial penetration. It is possible also that leeches may become infected by ingesting snail tissues containing mature cercariae.
Palmieri, James R. and James, Hugo A.
"The effects of leech behavior on penetration and localization of Apatemon gracilis (Trematoda: Strigeidae) cercariae and metacercariae,"
Great Basin Naturalist: Vol. 36
, Article 7.
Available at: https://scholarsarchive.byu.edu/gbn/vol36/iss1/7