Parasites obtain nutrients from another organism—a host—by living on or within the host. Some animal parasites consume parts of the host itself (such as ticks that suck body fluids); others highjack nutrients the host would otherwise consume (such as tapeworms that may live in the intestines of mammals). Most animal parasites are much smaller than their hosts, and many parasites can consume parts of their host without killing it. To set up residence within a host, a parasite must first overcome the host’s defenses. Parasites often have complex life cycles that rely on multiple hosts, as you will see in Key Concept 30.4.

Parasites that live inside their hosts are called endoparasites, and these are often morphologically very simple. Endoparasites often function without a digestive system, absorbing their food directly from the host’s gut or body tissues. Many flatworms are endoparasites of humans and other mammals, as we will describe in Chapter 31.

Parasites that live outside their hosts are called ectoparasites; they are generally more complex morphologically than endoparasites. Ectoparasites have digestive tracts and mouthparts that enable them to pierce the host’s tissues or suck on the host’s body fluids. Fleas and ticks are ectoparasitic arthropods that feed on many vertebrates, including humans.