Fine Structure of the Eyespot in the Miracidium of Philophthalmus megalurus (Cort, 1914)

Miracidia from worms reared in the eyes of chicks were prepared as sections for light and electron microscopy. The eyespot consists of two closely apposed pigment-cup cells, each occupied by two sensory cells with one anterior and the other posterior to a vertical septum formed by the cup cell. This orientation extends also to the rhabdomeres of the two sensory cells in each cup and facilitates interpretation of the phototactic behavior of the miracidium. The fine structure of photoreceptors has been described for a variety of animals, but evidently the use of electron microscopy to investigate such receptors in larval trematodes has been limited to two species of miracidia. Kiimmel (1960) described the eyespot in that larva of Fasciola hepatica, and studies dealing with the miracidium of Philophthalmus megalurus were reported in a brief abstract by the author (Isseroff, 1963). This paper presents more fully the observations included in that part of the abstract concerned with fine structure of the eyespot. In reporting the life history of this species in Indiana, West (1961) identified it as Philophthalmus gralli, which had previously been found in Indochina and Formosa. In Hawaii, Alicata (1962) also identified as P. gralli, a trematode whose life history differed in several respects from that reported by West. However, Cable and Hayes (1963) determined that West and Alicata were dealing with different species and designated the North American one as P. megalurus (Cort, 1914) because the specific name of its larva, Cercaria megalura, has priority over other names that may have been given to the same species.