The structure of the ctenidium of the ark-shell Scapharca kagoshimensis (Arcoida: Arcidae) histomorphologically ascertained as follows: 1). The dorsal bends of ctenidia do not fuse to the mantle (MT) and the visceral mass (VM). 2). The ridges of the ventral bent of ctenidium (VB) do not formed the food groove. 3). The demibranchs are filibranchiate, flat and homorhabdic. 4). The interlamellar connecting membranes are observed in all filaments between the inner and outer lamellae near VB. 5). The interlamellar extensions of filament are formed at abfrontal side of all filaments in the outer lamella of inner ctenidium and in the inner lamella of outer ctenidium, and are spreading out between the vessel of ordinary filament and the vessel-like interlamellar connecting vessels. 6). The fine frontal cilia and the coarse frontal cilia are arranged at the center and the both sides of the frontal surface of filament, respectively. 7). The tips of the inner palp of labial palp (IPP) and the outer palp of labial palp (OPP) are fixed with the suspensory membrane of labial palp on VM and MT, respectively. 8). The lateral oral grooves extend between IPP and OPP. 9). The proximal oral grooves extend between the inner lip and the outer lip (OL) on either side of the oral aperture. 10). OL is equipped with the membranous flap of lip.
Ark-shell
Dorsal bent of ctenidium
Interlamellar extension of filament
Membranous flap of lip
Ordinary filament
Ventral bend of the ctenidium