In Fasciola miracidium develo into the next stage in
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The Miracidium
The miracidium is a ciliated, nonfeeding larva (Fig. 9.18). Under favorable conditions, it escapes from the eggshell, usually through the operculum, into the environment. The miracidium is elongated and covered with flattened, ciliated epidermal plates. At the junctures of adjacent epidermal plates are cytoplasmic ridges, which play a prominent role during the miracidium’s metamorphosis into the next larval stage. Beneath the epidermal plates are well-developed circular and longitudinal muscles.
At the anterior tip of the miracidium is a flexible apical papilla with sensory organs and three secretary glands, the apical gland and two lateral glands, which secrete materials at the tip of the papilla during host penetration (Fig. 9.23). During penetration, the papilla becomes partially invaginated and secretions from the glands are captured in the depression. The papilla thus acts as a suction cup, holding the miracidium to the site of penetration and allowing the secretions to exert both adhesive and lytic actions. In addition to the sensory structures in the papilla, there may be two to three anterior eyespots, as well as lateral papillae, on each side of the body. The “brain” of the miracidium lies in the parenchyma behind the apical region from which nerve fibers innervate various tissues and organs of the body. The miracidium also has a simple, protonephridial excretory system. Waste-containing body fluids are collected by two or three pairs of flame cells and excreted through two lateral excretory pores. During differentiation of the miracidium, germ cells grow and divide to form germ balls. Each germ ball eventually develops into a distinct, membrane-enclosed entity that is the next larval generation.