Extra Ocular Photoreceptors in the Land Leech, Haemadipsa zeylanica agilis (Moore) from Nainital, Almora (India)

Abstract

THE special function of the segmental papillæ of the medicinal leech, Hirudo medicinalis, has long been in doubt. As far back as 1885, Whitman, in a study¹ describing the external morphology of the leech Hirudo, suggested serial homology of the segmental papillæ with the eyes. He supported this by the evidence that in sections of the segmental papillæ are found all the elements of the eye, except the pigment. Owing to the absence of a pigment cup holding the glassy cells, however, as is found in true visual organs, Whitman found it difficult definitely to regard the segmental papillæ as visual organs. The possibility that the segmental papillæ may be concerned with vision has also been suggested by me in my memoir on the Indian leech Hirudinaria ².