Miracidium

The Miracidium is the second stage in the life cycle of trematodes. When trematode eggs are laid and come into contact with fresh water, they hatch and release miracidium. In this phase, miracidia are ciliated and free-swimming. This stage is completed upon coming in contact with, and entering into, a suitable intermediate host for the purposes of asexual reproduction. Many different species of Trematoda exist, expressing some variation in the physiology and appearance of the miracidia. The various trematode species implement similar strategies to increase their chances of locating and colonizing a new host. The trematode Hirundinella ventricosa releases eggs in strings. Each egg contains a single miracidium, while the string contains living spermatozoa. Miracidia have cilia that are only present in the upper portion of the body near an apical gland with 12 hook-like spines in the opening. Miracidia usually need to enter a Mollusca host before they can start growing and begin reproduction, however certain species can use other animals as intermediate or main hosts. Echinostoma paraensei miracidia have 18 plates along the outside of their body. Even when about to hatch, their eggs show no signs of specialization such as projection or spine-like structure. They have elongated bodies with one intraepidermal ridge in the anterior row. They display a single "excretory vesicle". The miracidia are oval-shaped and their body is almost entirely covered in cilia except for the most anterior portions, taken up by "apical papilla". The miracidia have four papillae on each side, which contain sensory hairs. They each have an apical gland that leads to the apical papilla. They have four rows of epidermal plates, with row two made up of eight plates, while the other three rows each have six. Their eyespots are dark brown and shaped like an inverted capital letter L, located between the first and second row of plates. A single "large cephalic ganglion" along with several smaller nuclei, make up the nervous system.