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The bivalve Limopsis cristata pursues a semi-endobenthic life in abyssal soft sediments. It attaches to particles by up to three byssal threads and filter feeds by inhaling water from posterior and anterior directions. Because of partial burial, however, only the latter is functionally significant. Complex ciliary currents in the mantle cavity concerned with the rejection of unwanted particles keep most material out of the simplified intestine. It is generally considered that the ligament is the constraining force in arcoid evolution. This may be true in part, but the lack of pallial fusions and the retention of anterior and posterior inhalant flows are more powerful limits to radiation in the Arcoida. In the deep sea, the Limopsidae has radiated into many micro-niches through micro-morphological adaptations. Loss of the arcoid ‘heel’ has resulted in the union of the separate rejectory currents of the visceral mass and foot, creating a single discharge point in more advanced bivalves. This greatly simplifies the rejectory roles of the visceral mass and foot and is thus of functional and selective advantage.

Summary. 1 The internal structure and the ciliary sorting mechanisms of the stomach are described for representatives of twelve families of Filibranchia and Pseudolamellibranchia. 2 The variable structural features of the stomach are examined individually, and it is shown that only one of these is suitable for use as a diagnostic in assigning the stomachs of these lamellibranchs to a limited number of types. This feature is the form of the major typhlosole. 3 The major typhlosole may sweep smoothly from the orifice of the mid-gut to that of the left pouch, accompanied on its course by the ciliated rejection tract of the intestinal groove. Alternatively a long slender tongue may pass forwards from the major typhlosole and penetrate deeply into the food-sorting caecum, this tongue being accompanied by a similar extension of the intestinal groove. No intermediate condition was found, and on this basis the stomachs in eight families were assigned to Type III, in which the major typhlosole possesses such a tongue. These families are the Arcidae, Glycimeridae, Limopsidae, Pinnidae, Vulsellidae, Pteriidae, Mytilidae and Ostreidae. 4 It is considered that the evolution of a tongue of the major typhlosole, extending into the food-sorting caecum, could not have occurred independently on two or more occasions, nor is it likely that such a tongue, once having been formed, could have been lost independently on two or more occasions. It is concluded, therefore, that the above mentioned eight families constitute the end products of one phylogeny of lamellibranchs. 5 In the remaining four families, the Trigoniidae, Limidae, Pectinidae and Anomiidae, the major typhlosole lacks a tongue. The stomachs in these four families are assigned to Type IV, and attention is drawn to the fact that in many Eulamellibranch families the internal structure of the stomach is similar. 6 It is suggested that evolution of the structure and function of the interior of the stomach has played an important part in the adaptive radiation of the Lamellibranchia. 7 It is reiterated that the Protobranchia constitute a phylogeny which is distinct from all other lamellibranchia and that it is undesirable to link the Protobranchia with the Arcacea to form the order Taxodonta.