A reevaluation of specimens of Mesocoelium monas (Platyhelminthes: Digenea: Mesocoeliidae) from the Natural History Museum, UK and the United States National Parasite Collection, USA Author Calhoun, Dana M. Author Dronen, Norman O. text Zootaxa 2012 3589 1 29 journal article 10.5281/zenodo.209761 2b78cedc-fd40-4bff-98c5-12aacd319fc4 1175-5326 209761 Mesocoelium cf americanum— NHMUK 1979.9. 12 17-19 ( Figure 9 ; Table 2) Host: Duttaphrynus melanostictus (Schneider) (syn. Bufo melanostictus Schneider ), Asian toad ( Anura : Bufonidae ). Locality: Malaysia , Penang. (Native) Description: Based on one specimen: Body 2,236 by 646; forebody, 630 long, representing 28% of body length. Mouth subterminal; oral sucker, subspherical, 250 by 240; prepharynx absent; pharynx subspherical, 105 by 130; ratio of width of pharynx to width of oral sucker 1:1.8; esophagus 112 long; ceca surpassing ovary posteriorly, occupying 46% of postovarian space. Ventral sucker subspherical, situated in posterior portion of anterior 1/3 of body, 200 by 190; ratio of ventral sucker width to oral sucker width 1:1.3. Right testis 117 by 104 (left testis could not be measured). Genital pore, submedian, located immediately posterior to midlevel of esophagus. Cirrus sac 210 long, representing 9% of body length. Ovary oval, 151 by 187; postovarian space 1,300 long, representing 58% of body length. Uterus extensive, filling hindbody. Vitelline fields distributed in lateral fields from level of esophagus to near level of cecal ends posteriorly. Eggs 43 (39–47) by 21 (18–23). Excretory system not visible. Remarks: This specimen appears to be twisted at the level of the pharynx and oral sucker, tearing the esophagus. This specimen has a reduced postovarian space with relatively few fully developed eggs present, and is likely a young adult. The poor quality of the specimen precluded observation of some structures (e.g. left testis, excretory system). Although this specimen has ceca that surpass the ovary posteriorly and a prebifurcal genital pore, and is assigned to the monas body type , it cannot be assigned specifically to M . monas . This specimen differs from M . monas by having vitelline fields that terminate near to the cecal ends compared with terminating well short of the cecal ends, a shorter forebody (630 compared with 970), a smaller ratio of the pharynx width to the oral sucker width of (1:1.8 compared with 1:2.6), a narrower ventral sucker (200 compared with 344), a narrower oral sucker (240 compared with 382), and a larger percentage of the ceca that surpass the ovary into the postovarian space (46% compared with 31%). This specimen is consistent with M . americanum by having the posterior extent of the vitelline fields terminating near to the cecal ends, gonads that overlap the area of the ventral sucker, an oval shaped body where that is widest from the level of the ventral sucker to the midlevel of the body, a prebifurcal, submedian genital pore that is located posterior to the midlevel of the esophagus, a similar ratio of the ventral sucker width to the oral sucker width (1:1.3 compared with 1:1.5–1:1.7), a similar body length (2,236 compared with1,325–4,038), a similar length of the postovarian space (1,300 compared with 740–2,626), and similar sized eggs (43 [39–47] by 21[18–23]) compared with 41 [39–45] by (24 [21–26]). This specimen differs from M . americanum by a having a smaller ratio of width of pharynx to oral sucker width (1:1.8 compared with 1:2.5 [1:2.2–1:2.7]), a larger percentage that the ceca surpass the ovary into postovarian space (46% compared with 26% [25–27%], and M . americanum was described from Texas, USA not Malaysia . Based on those differences this specimen may represent an undescribed species of Mesocoelium ; however, without additional, museum-quality specimens it cannot be adequately distinguished or described