Abstract
The acotylean polyclads are characterized by the absence of a ventral sucker located posteriorly to the female gonopore. These flatworms are mostly marine, benthic or pelagic, except by two species, one freshwater Limnostylochus borneensis and other semi-terrestrial Myoramyxa pardalota. Oaxaca is a state located in the southern Mexican Pacific, where only three acotyleans species have been formally recorded until now. After sampling littoral to sublittoral zones, a total of 34 specimens were collected and fixed in frozen formalin. Some specimens were stained with Mayer´s carmalum and hematoxylin and cleared in methyl salicylate for whole mounts; the portion containing the reproductive structures of other specimens were dissected and histologically processed in sagittal and frontal sections. Four species were identified as belonging to four genera and three families. The new genus Bisacculosuteri gen. nov. is established, close to the genus Itannia, but lacking double ventral suckers placed on both sides of the female gonopore. Two species are new to science: Bisacculosuteri marcelae sp. nov., characterized by a highly branched central pharynx, prominent seminal vesicle, a penis papilla with stylet, paired strongly muscularized uterine sac and uterine vesicles; and Paraplanocera oligoglenoides sp. nov., with translucent body, light brown pigmentation and black dots distributed in the pharyngeal and reproductive regions, and an oval cirrus sac with intramuscular surrounding hollow spaces and armored with spines and conspicuous teeth. The third species is Euplanoida cf. pacificola that could not be determined as the nominal species, due to ambiguity in its description. The fourth species, Bivesiculoplana lamothei, was previously described from the region and now is recorded in new localities and hosts from Oaxaca.
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