FIG. 5.

Generalized phylogeny of the major groups of leeches (after reference 29). Losses of blood feeding are depicted here with white branches leading to taxa. The date of divergence of Placobdella spp. is estimated to be 250 Mya, at the point of separation of North and South America. The origin of the glossiphoniids is estimated to be ∼350 Mya, and the ancestral morphology of bacteria-associated organs is hypothesized to be a cluster of small cells encircling the esophagus, as is seen in present-day Marsupiobdella and the Placobdelloides spp.