Mesoglossus, Deland & Cameron & Rao & Ritter & Bullock, 2010

Deland, C., Cameron, C. B., Rao, K. P., Ritter, W. E. & Bullock, T. H., 2010, A taxonomic revision of the family Harrimaniidae (Hemichordata: Enteropneusta) with descriptions of seven species from the Eastern Pacific, Zootaxa 2408 (1), pp. 1-30 : 6

Genus Mesoglossus n. gen.

Type species. Saccoglossus bournei Menon, 1904

Diagnosis. Proboscis about twice as long as wide, with no conspicuous dorsal groove; its longitudinal musculature arranged diffusely, not in concentric rings or radial bundles. Proboscis pore is present usually on left side. Perihaemal cavities usually present. Collar canals present. A ventral mesentery present in proboscis, with both dorsal and ventral mesenteries present in collar. Dorsal gonads absent; only lateral gonads present. Peribuccal spaces present or absent.

Remarks. It is evident from the above characters that the closest relations of Mesoglossus bournei n. comb. are three other species previously assigned to the genus Saccoglossus but here transferred to Mesoglossus — M. caraibicus (van der Horst, 1924) n. comb., M. gurneyi ( Robinson, 1927) n. comb. and M. pygmaeus (Hinrichs & Jacobi, 1938) n. comb.. Although these four species and M. intermedius resemble species of Saccoglossus a great deal, they are distinguished from the latter by the fact that the longitudinal muscle fibers of the proboscis are uniformly distributed without being arranged into concentric rings as in Saccoglossus (or into radial groups as in Harrimania and Stereobalanus ). Mesoglossus is too distinct to be confused with Stereobalanus . Hence all of the forms that have previously been put under Saccoglossus but which exhibit no regular arrangement of the proboscis longitudinal muscle fibers into concentric rings are here transferred to the genus Mesoglossus , with M. bournei as the type species.

The diagnosis given above is primarily based on Ritter’s account in his manuscript (written ca. 1900) plus a re-examination of his histological slides and of material collected by others. At the beginning of the twentieth century, no Saccoglossus species had been described with a diffuse distribution of longitudinal fibers in the proboscis, except for Benham's description of S. otagoensis (Benham, 1899) . But it is now known that in S. otagoensis the longitudinal muscle fibers in the proboscis are actually arranged in three or four concentric rings (van der Horst, 1935). Thus S. otagoensis cannot be considered as belonging to Mesoglossus , which means that M. bournei is the earliest described species under the genus and should be considered as its type species.