The structure of the ctenidium in the chest-shell Scapharca broughtonii (Arcoida: Arcidae) was histomorphologically examined. The histological structures are as follows: 1). The fused border of outer lamella of outer ctenidium and the fused border of inner lamella of inner ctenidium do not fuse to the mantle and the visceral mass (VM), respectively. 2). The dorsal bent of ctenidium (DB) is filaments (OF) bending to the branchial cavity and the food groove is not formed on the ridge of DB. 3). The interlamellar connecting membrane is observed about 10-20% length of OFs which are ascending from the ventral bend of the ctenidium (VB). 4). The interlamellar extension of filament is formed at abfrontal side of all OFs in the outer lamella of inner ctenidium and in the inner lamella of outer ctenidium is spreading out between the vessel of ordinary filament and the vessel-like interlamellar connecting vessel. 5). Neighboring OFs are connected each other with the cilia at the interfilamentar junction, VB and DB. And the coarse frontal cilia and the fine frontal cilia are arranged at the center and the both sides of the frontal surface of OF, respectively. 6). In the labial palp, the proximal oral grooves extend between the inner lip and the outer lip on either side of the oral aperture and OL is equipped with the membranous flap of lip. 7). The lateral oral grooves extend between the inner palp of labial palp (IPP) and the outer palp of labial palp, and the tip of IPP is fixed on VM with the suspensory membrane of labial palp.
Chest-shell
Dorsal bent of ctenidium
Interlamellar extension of filament
Membranous flap of lip
Ordinary filament
Ventral bend of the ctenidium