3, 4) the interior of the dorsal valve is variously furrowed to receive the lophophore folded in two or more lobes.
Its two lips fusing together at the corners of the mouth are prolonged into the socalled arms. These arms, which together form the lophophore, TV.
The great arm-sinus of each side of the lophophore lies beneath the fold or lip which together with the tentacles forms the ciliated groove in which the mouth opens.
The small arm-sinus runs along the arms of the lophophore at the base of the tentacles, and gives off a blind diverticulum into each of these.
- Diagrammatic section through an arm of the lophophore of Crania.