Correlated with the presence of the genital pleurae there is a pair of vascular folds of the basement membrane proceeding from the dorsal wall of the gut in the postbranchial portion of the branchio-genital region, and from the dorsal angles made by the pleural folds with the body-wall in the pharyngeal region; they pass, in their most fully developed condition, to the free border of the genital pleurae.
In Carinella, Cephalothrix and Polia, as well as in all Metanemertines, the basement membrane of the skin already alluded to is particularly strong and immediately applied upon the muscular layers.
In the Heteronemertines there is a layer in which the cutaneous elements are largely represented below the thin basement membrane (fig.
The connective tissue of the integument and basement membrane imperceptibly merges into that which surrounds the muscular bundles as they are united into denser and definite layers, and this is especially marked in those forms (Akrostomum) where the density of the muscular body-wall has considerably diminished, and the connective tissue has thus become much more prominent.
In most Heteronemertines these elements are separated by a thin homogeneous basement membrane (fig.