The structure and ciliation of the lip-apparatus of two species of Lima—L. hians (Gmelin) and L. excavata (Fab.)—and one species of Pecten—P. maximus (L.)—is described.

It is suggested that the lips of these three species of bivalves have been hypertrophied (and in one case partially fused) in order to retain particulate material being carried from the ctenidia to the mouth while at the same time allowing the escape of water currents, generated by the ciliary tracts of the ctenidia and palps, from the proximal oral grooves. Such water currents must emerge from between the lips of all bivalves but do not usually interfere with the passage of particulate material. In the Limidae and Pectinidae, these water currents have been intensified by the reduction of the anterior regions of the mantle cavity. Since this change in the proportions of the body can be correlated with the evolution of the monomyarian condition in the Anisomyaria, it is postulated that the lips of all monomyarian Anisomyaria will be similarly hypertrophied.