THE STOMACH IN THE EULAMELLIBRANCHIA; STOMACH TYPES IV AND V

Abstract

The internal structure and function of the stomach is described for representatives of twenty‐three families of Eulamellibranchia. The stomachs of two of these are assigned to Stomach Type IV and the remainder to Stomach Type V. A detailed analysis is made of structural variation in the stomach in the twenty‐one families with stomachs of Type V and a definition of Stomach Type V is given. The appendix is limited in its occurrence to the Tellinacea and the Adesmacea, which suggests that these two sub‐orders are closely related to each other. It is concluded that Stomach Type IV is an ancestral character which has persisted to the present time in a large section of the Lamellibranchia; it is suggested that there arose from the ancestral stock a separate phylogeny which is characterized by the possession of Stomach Type V. It is concluded that within the phylogeny that is chara***cterized by Stomach Type V it is possible for the stomach to be simplified and to resume secondarily the ancestral form of Stomach Type IV. This is considered to have occurred in the Sphaeriidae, Lucinidae, Thyasiridae, Donacidae and in Chama multisquamosa. 1960 The Zoological Society of London