Fine structural studies on the eyes of the harvestman Mitopus morio revealed the presence of microvilli in the rhabdom. The microvilli vary in length between 1 µ and 2 µ, are about 800 Å wide, and curved or straight. They derive from the plasma membranes of the four retinula cells which surround the rhabdom. Approximately cylindrical in shape, the rhabdoms are about 40 µ long by about 4-6 µ in cross-diameter. Each rhabdom is situated at the centre of a retinula, and these retinulae are packed in a hexagonal array to form the retina. Distally, rhabdom fusion occurs to form a rhabdom network. The retina lies beneath the dioptric apparatus which consists of a single lens, surmounting a glassy body composed of lentigen cells.

The cytoplasmic organelles of the retinula cells include mitochondria, lysosomes, sparse elements of endoplasmic reticulum, vesicular components, prominent Golgi complexes and pigment granules which possess a laminated structure. An important feature of the retinula cell is the presence of many small vesicles, about 0.1 µ in diameter, clustered beneath the rhabdom. Incubation of glutaraldehyde-fixed eyes in a Gomori medium with acetylthiocholine as substrate, coupled with inhibition of controls by 62C47, indicates the presence of a presumed acetylcholinesterase in these vesicles. Similar vesicles also occur in the proximal cytoplasm of the retinula cells. Other larger vesicles, often with a core of whorled membranes, as well as dense bodies, also show acetylthiocholine-splitting activity. This latter activity is not inhibited by 62C47 and is probably the effect of lysosomal non-specific esterase. These bodies also exhibit acid phosphatase activity when incubated in a Gomori medium with β-glycerophosphate as substrate.

The presence of acetylcholinesterase activity, as distinct from non-specific esterase, in vesicles closely associated with the rhabdom and in more proximally situated vesicles is significant. It would point to the presence of an acetylcholine/acetylcholinesterase system involved in the generation and/or propagation of the sensory impulse arising from photo-stimulation of the rhabdom.

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