Animals that harbour adult digenetic Trematodes thus become infected by ingesting encysted cercariæ, which either occur (1) in certain animals (second intermediate hosts) on which they feed, or (2) in water, or (3) on plants, or finally (4) in the first intermediate host; whereas animals harbouring encysted cercariæ have been directly infected by the corresponding tailed stage, and animals harbouring germinal tubes (sporocysts or rediæ) have been infected by the miracidia. [Illustration: FIG. 131.--Development of _Fasciola hepatica_, L. _a_, the miracidium in optical section showing cephalic lobe, X-shaped eye-spot resting on the cerebral ganglion, two germ balls; below each of these a flame cell, and still lower germ cells lying in a cavity (primitive body cavity). _b_, young sporocyst with two eye-spots, and germ balls; the cells lining the cavity are not shown. _c_, older sporocyst with a young redia. Magnified. (After Leuckart.)] Thus certain species of ducks and geese become infected with _Echinostoma echinatum_ by devouring certain water-snails (_Limnæus_, _Paludina_) in which the encysted cercariæ occur. Oxen become infected with _Paramphistomum cervi_ (= _Amphistomum conicum_) by swallowing with water, cysts of this species which occur at the bottom of puddles and pits. Sheep are infected with _Fasciola hepatica_ by eating grass to which the encysted cercariæ of the liver-fluke are attached; our song-birds infect themselves or their young with _Urogonimus macrostomus_ by tearing off pieces containing the corresponding sporocysts which are full of encysted cercariæ from snails (_Succinea amphibia_), which act as the first intermediate hosts, and eating, or offering their young these pieces. (1) The MIRACIDIA of the digenetic Trematodes are comparatively highly organized, and the mode of their formation from the segmentation cells of the ovum is only imperfectly known. They have a cuticular epithelium (fig. 129) entirely or partly covered with cilia, beneath this a dermo-muscular tube composed of circular and longitudinal muscles; also, a simple gut sac with an œsophagus, occasionally also with pharynx, salivary glands and boring spine, also a cerebral ganglion on which, in some species, there are eyes (fig. 131, _a_). As to the excretory organs, they are represented by two symmetrically placed terminal flame cells, with excretory vessels opening separately; there is a more or less ample (primary) body cavity between the parietes of the body and the gut; from the cellular parietal lining of this cavity single cells (germ cells) become free (fig. 131, _a_, _b_), and become rediæ or cercariæ. [The germ cells of the miracidium and the germ balls of the sporocyst arise, according to some observers, by further division of undifferentiated blastomeres; according to others from the cells of the lining wall of its body cavity. It is from these free germ balls that the redia stage is developed.