Septibranchia

(invertebrate zoology) A small order of bivalve mollusks in which the anterior and posterior abductor muscles are about equal in size, the foot is long and slender, and the gills have been transformed into a muscular septum.

A subclass of bivalve mollusks (class Bivalvia) that are unique in their possession of a muscular septum instead of a filamentous gill. The Septibranchia equate in great part to the superfamily Poromyacea, which includes the septibranch families Cuspidariidae and Poromyidae, and the Verticordiidae. The Verticordiidae have gills that are greatly reduced in size. Although there are a few cuspidariid species in shallow seas, the great majority of septibranchs are found at lower slope and abyssal depths which are deficient in food for filter-feeding bivalves. They live close to the surface in soft sediments. Most species are less than 20 mm maximum length. See also Bivalvia; Mollusca.

The term septibranch remains extremely useful because it describes mollusks having a septum and other morphological specializations that relate to the septibranch's unique carnivorous habits. The septum is a muscular, horizontal partition dividing the mantle cavity. It is derived from the enormous gill found in the mollusks of the subclass Lamellibranchia: The filaments are reduced in size and modified. Other modifications include a muscular stomach for crushing the prey and high proteolytic activity of the gastric juice.