Studies on the pigmented and unpigmented photoreceptors of the cercaria ofCryptocotyle lingua(Creplin) fromLittorina littorea(L.)

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Abstract
The cercaria of Cryptocotyle lingua possesses a pair of pigmented rhabdomeric photoreceptors situated antero-dorsally, and a median unpigmented rhabdomeric photoreceptor lying slightly ventral to the middle region of the body immediately anterior to the transverse commissure. Each pigmented photoreceptor consists of two asymmetrical pigment cells containing two symmetrical back-to-back pigment cups facing respectively antero-laterally and postero-medianly, with one retinular cell associated with each pigment cup. The microvilli of the rhabdomere in each cup are radially arranged around a triradiate dendritic process; in the centre of each microvillus is a strand presumably the visual pigment. The retinular cell and dendritic process contain a large number of mitochondria. The rhabdomere of the median unpigmented photoreceptor is enclosed in a thin-walled unpigmented cup opening posteriorly and giving off processes, some of which contain mitochondria, into the surrounding parenchyma. This rhabdomere resembles the equivalent of two rhabdomeres of the paired photoreceptors. Axonal processes from the retinular cells of the paired and unpaired photoreceptors enter the neuropile of the cerebral ganglia and transverse commissure respectively. The cercaria is markedly photopositive and responds also to shadow by active bursts of swimming. It is suggested that the paired dorsal photoreceptors may be sensitive to light of higher intensity and the median more ventral photoreceptor to light of lower intensity and to shadow.