Flatworms

Tapeworms are the dominant member of the class Cestoidea. They are ribbon-like, segmented creatures living in the intestines of their vertebrate hosts. There are a dozen orders in this class, most living in fish but two that use humans as hosts. Tapeworms cling to the intestinal wall of their hosts with suckers, hooks, or other adhesive devices. Having no mouth or gut, they receive their nourishment through their skin. Further, they have no type of sensory organs. White or yellowish in color, species in this class vary from 0.04 in (1 mm) long to over 99 ft (30 m).

The broad fish tapeworm (Diphyllobothrium latum), a large tapeworm present in humans, can illustrate the typical life of a tapeworm. As an adult, it attaches itself to the intestinal wall of the human host. Its body, composed of roughly 3,500 sections, probably measures 33-66 ft (10-20 m) long. At this point, it lays about one million eggs each day. Each egg, encased in a thick capsule so that it will not be digested by the host, leaves the host through its feces. When the egg capsule reaches water, an embryo develops and hatches. The larva swims until it is eaten by its first host, a minute crustacean called a copepod. The larva feeds on the copepod and grows. When a fish eats the copepod, the young tapeworm attaches itself to the fish's gut. The tapeworm continues to grow and develop until the fish is eaten by a suitable mammal, such as a human. Only at this point can the tapeworm mature and reproduce.