Riserius pugetensis gen. n., sp. n. (Nemertina: Anopla), a new mesopsammic species, and comments on phylogenetics of some anoplan characters

Riserius pugetensis gen. n., sp. n. is described from the northwest coast of Washington, U.S.A. It is probably a heteronemertine and, as such, would be the first known mesopsammic member of that order; it lives in the interstices of medium to coarse, moderately clean sands. Its morphology presents some attributes considered characteristic of mesopsammic fauna. The effect of some of these attributes is a generalized appearance of anatomical ‘simplicity’ and, as with other mesopsammic metazoans, this presents difficulties in distinguishing between primitiveness and reduction. In the new species such problematic features include the lack of subepidermal glandular cells and connective tissue, reduced proboscideal musculature, simple blood-vascular system, and the presence of a ‘secondary’, outer circular musculature in the body wall. The general appearance of this new species is very similar to the so-called palaeonemertine Hubrechtella and characteristics of both suggest relationship with the baseodiscid heteronemertines. These relationships are explored in light of a modified interpretation of proboscideal musculature, a traditional cornerstone of heteronemertinean taxonomy.