Since the discovery of sporocysts and rediae by Steenstrup in 1842, the study of the life histories of different Trematodes has received much attention by investigators, for instance Wagener in 1857, Leuckart in 1882, Thomas in 1883, Heckert in 1889, Creutzberg in 1890 and Looss in recent times. It was from the observations of these workers that the life cycle of Trematodes was put on a firm basis. But no real importance was attached to the study of miracidia, sporocysts, rediaeand corcariae until after the classical work of Thomas in 1883, which resulted in-a thorough explanation of the life history of the liver fluke and its transmission from sheep to sheep. Since'1883 numerous investigations have been carried out from the economical and medical point of view, and the bulk of the contributions*to the knowledge of cercariae consists of the results of observations and experiments on the cercariae of the Trematodes infecting man and the domestic animals Particular attention was paid to the different intermediate hosts and the conditions under which they are capable of causing the infection of the definitive hosts. A serious study of the organisation of cercariae was long neglected and in spite of the numerous cercariae described by many investigators, it is impossible to identify many of the species correctly. It was only after the study of the cercariae of the Schistosomidae infecting man that the study of the morphology and histology of other cercariae was adequately undertaken, and authors began to attach importance to features of the internal organisation, which may throw light on the structure of the adult. The systems which have been carefully studied are the alimentary tract, the excretory and genital systems, and specific differences were observed even among cercariae closely resembling one another. Certain glands in the body have also been observed, and have proved in many cases to play an important part in the process of infection of the definitive host.