Structure and ultrastructure of the central nervous system of the polychaete Nephtys, with special reference to photoreceptor elements

Abstract

Organs of presumed photoreceptor significance in Nephtys exhibit great simplicity. The pigmented eyes of Nephtys are embedded in the posterior region of the brain. Each consists of a pigment cup and two closely-associated sensory cells. The cells exhibit the characteristic features of rhabdomeric-type photoreceptor structures. The anterior or accessory eyes are identical to the pigmented eyes except that each may consist of only a single cell and that pigment cups are absent. A pair of “sensor sacs” is also present, embedded in the brain. Each consists of a single cell which has the appearance of possessing a large intracellular vesicle (the phaosome) into which project irregularly arranged microvilli. The limiting membrane is, in fact, continuous with the cell membrane. Evidence is presented of the secretion of material of unknown character into the vesicle.