The Larvae of Some Monogenetic Trematode Parasites of Plymouth Fishes

The Order Monogenea of the Class Trematoda contains (Sproston, 1946) upwards of 679 species, but in only twenty-four of these species has a larval form been described. Of these larvae, fourteen belong to adults that parasitize fresh-water fishes, amphibians or reptiles, and ten to adults that parasitize marine fishes. Udonella caligorum is known (Sproston, 1946) hot to have a larval stage in its life history. In the present study, accounts will be given of eleven hitherto undescribed larvae which belong to adults that parasitize marine fishes at Plymouth, and which represent seven of the eighteen families (Sproston, 1946) of the Monogenea. For the most part the literature on larval monogeneans consists of isolated studies of individual species, and these have been listed by Frankland (1955), but the descriptions of four new larvae by Euzet (1955) appeared too late for inclusion in Frankland's list. More general observations on monogenean life cycles have been made by Stunkard (1937) and Baer (1951), and Alvey (1936) has speculated upon the phylogenetic significance of monogenean larvae. Previous accounts of monogenean larvae have included but scant reference to culture techniques, presumably because the methods adopted have been simple and successful; in my hands, however, many attempts to rear larvae have been unsuccessful, and so some details of those procedures which have yielded successful cultures are included. At the same time it must be pointed out that the main object of the present study was to obtain specimens of monogenean larvae for morphological investigation, and properly designed experiments to determine factors influencing embryonic development were not attempted. In addition to the studies made upon the eleven previously undescribed larvae, observations were made upon the larvae of Diclidophora luscae and Polystoma integerrimum in order to facilitate comparisons and to provide practice in techniques. In past accounts of monogenean larvae it has been customary to describe the egg capsules, but the variations in the form of the capsules of the species studied here appear likely to be related more to variations in habits of oviposition than to larval characters, and so it is intended later to study the egg