HISTOLOGICAL AND PHYSIOLOGICAL ASPECTS OF THE MEDULLA EXTERNA X ORGAN, A NEUROSECRETORY CELL GROUP IN THE EYESTALK OFPALAEMON SERRATUSPENNANT (CRUSTACEA, DECAPODA, NATANTIA)

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Abstract
The medulla externa X organ (MEX) is composed of two cell types containing two different categories of neurosecretory elementary granules also found in the sinus gland. Changes in the volume and activity of the organ were observed in relation to the intermolt cycle and season. After extirpation of the medulla externa X organ, the volumes of both the sinus gland and the organ of Bellonci increased in post- and intermolt stages. In the sinus gland, lysis and clumping were observed in axon endings containing one (large) granule type. Also as a consequence of the extirpation, large droplets were secreted in vacuoles of the organ of Bellonci. Neurosecretory cells in the medulla terminalis ganglionaris X organs (MTGX) showed increased activity and hypertrophy. The medulla externa X organ also had an impact on the distal retinal and chromatophoric red pigments.