Structure of the Photoreceptors in the Compound Eyespots of Branchiomma Vesiculosum

Each photoreceptor unit within the eye is located immediately beneath the surface cuticle and is composed of 2 cells; a superficially placed lens cell and a large, deeply situated receptor cell which is composed of two segments. The deep segment of the receptor cell is filled by a large invaginated cavity containing a stack of some 450 disc-shaped membranous sacs; these are the expanded and flattened membranes of cilia, the basal bodies of which line the cytoplasmic wall of one side of the cavity. The basal bodies have a 9 + 0 arrangement of fibrils and lack striated rootlets and orthogonal centrioles. On the side of the cell opposite this field ofbasal bodies the cavity is continuous with a tunnel which passes through the superficial segment of the cell at one side and opens at its top. The tunnel is crescent-shaped in cross-section, and its inner wall is covered by an array of microvilli. The cytoplasm of the superficial segment of the receptor cell is mostly filled by some 3 000 long, rod-shaped, hexagonally packed mitochondria, whose long axes lie normal to a plane dividing the basal body and tunnel sides of the cell. The arrays of mitochondria and microvilli correspond in number and spacing; and fibrils arising in the microvilli project into the cytoplasm running in the space between adjacent mitochondria.