The structure and function of the British Pholadidae (Rock‐Boring Lamellibranchia)

S ummary.1 A detailed investigation has been made into the structure of the principal organ systems and the ciliary mechanisms of the mantle cavity and of the interior of the stomach, in the five British species of Pholadidae (Lamellibranchia). 2 Tabulations have been made of the features characteristic of the majority of British Pholadidae. 3 In the Pholadidae two anatomical features were noted, which may ultimately be of systematic value: –a.  the presence of an accessory visceral ganglion. b.  the presence of an appendix on the side of the stomach.4 It was shown that the appendix of the Pholadidae was homologous with the postero‐dorsal caecum of the Tellinacea, and probably also with the wood‐storing caecum of the Teredinidae and of Xylophaga . 5 In Xylophaga dorsalis , the absence of a shell apophysis is primitive, and in a number of the Pholadidae a ligament is present. It was concluded that the characteristic (but not diagnostic) features of the Adesmacea may have evolved progressively in response to the assumption of a rock‐boring habit, and that these characteristics may therefore be lacking in certain congeners which diverged at an early stage from the main stock of the Adesmacea. 6 It is therefore suggested that careful anatomical comparisons should be made between the Pholadidae and other rock‐boring genera such as Gastrochaena , to investigate the possibility that such genera might be early offshoots from the parent stock of the Adesmacea. 7 Among the burrowing Lamellibranchia various genera show similarities with the Pholadidae, which could be regarded as pre‐adaptive in nature towards the rock‐boring mode of life. 8 It was found that the cilia in the proximal oral groove of the Pholadidae were typically inactive under experimental conditions. Various reasons for this ciliary inactivity have been offered. 9 It was concluded that the ciliary mechanisms in the stomachs of all five species here considered are primarily concerned with preventing particles from entering the ducts of the digestive diverticula. 10 It was suggested that, most particles having been eliminated from the stomach, the stomach contents are then injected or sucked into the digestive diverticula by volumetric changes brought about primarily by movements of the shell valves and foot, and perhaps by muscles in the stomach wall. 11 The progressive treatment of the stomach contents as outlined above would be assisted by intermittent refusal of food in the proximal oral groove.