Animals that live as endoparasites are bathed in the nutritious tissues of their host or in the digested food that fills their host’s digestive tract. Thus they may not need to exert much energy to obtain food, but to survive they must overcome the host’s defenses. Furthermore, either they or their offspring must disperse to new hosts while their host is still living, because they die when their host dies.

The fertilized eggs of some parasites are voided with the host’s feces and later ingested directly by other host individuals. Most parasite species, however, have complex life cycles involving one or more intermediate hosts and several larval stages (Figure 30.12). Some intermediate hosts transport individual parasites directly between other host species. Others house and support the parasite until another host ingests them. Complex life cycles may thus facilitate the transfer of individual parasites among hosts.