The habitat of trematodes frequently varies over the course of their lives. Members of the subclass Digenea are endopar-asites with indirect life cycles, which means that they infect different hosts during the various stages of their life cycles. Digenetic trematodes typically have two larval stages and at least two hosts. Several, including Halipegus occidualis, have four hosts, ending up at the base of the tongue of a green frog (Rana clamitans), which is the definitive host of H. occidualis. The life cycles of members of the subclass Aspidogastrea, which sometimes take a shorter path with only one host and no asexually reproducing larvae, are described as direct.

A typical digenetic individual begins its life as an egg in its so-called definitive host, and then passes with the host's feces either into water or onto land. After hatching, the larva takes up residence in a first intermediate host, which is often a mollusk. It then exits and moves to a second intermediate host, which is frequently either another mollusk, a fish, or an amphibian. The characteristic digenetic life cycle continues when the definitive host eats the secondary host, at which point the trematode infects the definitive host. Definitive hosts often include predatory mammals or birds.

Among species of Aspidogastrea, the life cycle is a bit simpler with usually only one host. After birth, the ectoparasitic species generally latch immediately onto the outside of a host organism, usually the skin or gills of bivalves or fishes. En-doparasitic species of Aspidogastrea infect such taxa as mol-lusks and fishes, but also the shark-like holocephalans and elasmobranchs.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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