A comparison of lip structure and function correlated with other aspects of the functional morphology of Lima lima, Limaria (Platilimaria) fragilis, and Limaria (Platilimaria) hongkongensis sp.nov. (Bivalvia: Limacea)

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Abstract
A study of three bivalves of the genera Lima and Limaria has demonstrated that two evolutionary processes are at work within the Limidae; both are of major significance. The first process concerns a reduction in the importance of the shell as a means of protection and the assumption of such responsibility by the middle mantle fold possessing autotomising tentacles. The tentacles also aid swimming by undertaking a rowing action and the thin shell aids swimming by reducing the sinking rate. A second major evolutionary trend which facilitates swimming concerns the gradual changeover from a ciliary to a muscular based means of expelling pseudofaeces both posteriorly (as in the typical bivalve) and via a subsidiary anteriorly directed aperture; in less specialised limids this rejection tract is located ventrally. The anterior exhalant current has also profoundly affected the structure of the lip apparatus. To prevent food material from being flushed out of the oral grooves by this stream the lips have become fused over the mouth. Successive stages in a process of lip fusion have been recognised and correlated with the evolution of the anteriorly directed rejection tract, the full importance of which, both in terms of mobility and morphological specialisation, is now better understood. A new limid, Limaria (Platilimaria) hongkongensis, from Hong Kong is described.

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