Internal
The evolutionary history of the Cestoda has been studied using ribosomal RNA, mitochondrial and other DNA, and morphological analysis and continues to be revised. "Tetraphyllidea" is seen to be paraphyletic; "Pseudophyllidea" has been broken up into two orders, Bothriocephalidea and Diphyllobothriidea. Hosts, whose phylogeny often mirrors that of the parasites (Fahrenholzs rule), are indicated in italics and parentheses, the life-cycle sequence (where known) shown by arrows as (intermediate host1 [→ intermediate host2 ] → definitive host). Alternatives, generally for different species within an order, are shown in square brackets. The Taeniidae, including species such as the pork tapeworm and the beef tapeworm that often infect humans, may be the most basal of the 12 orders of the Cyclophyllidea. Interactions with humans
Infection and treatment
Like other species of mammal, humans can become infected with tapeworms. There may be few or no symptoms, and the first indication of the infection may be the presence of one or more proglottids in the stools. The proglottids appear as flat, rectangular, whitish objects about the size of a grain of rice, which may change size or move about. Bodily symptoms which are sometimes present include abdominal pain, nausea, diarrhea, increased appetite and weight loss.There are several classes of anthelminthic drugs, some effective against many kinds of parasite, others more specific; these can be used both preventatively and to treat infections. For example, praziquantel is an effective treatment for tapeworm infection, and is preferred over the older niclosamide.
All but amphilinids and gyrocotylids (which burrow through the gut or body wall to reach the coelom) are intestinal, though some life-cycle stages rest in muscle or other tissues. The definitive host is always a vertebrate but in nearly all cases, one or more intermediate hosts are involved in the lifecycle, typically arthropods or other vertebrates. Infections can be long-lasting; in humans, tapeworm infection may last as much as 30 years. No asexual phases occur in the lifecycle, as they do in other flatworms, but the lifecycle pattern has been a crucial criterion for assessing evolution among Platyhelminthes.Cestodes produce large numbers of eggs, but each one has a low probability of finding a host. To increase their chances, different species have adopted various strategies of egg release. In the Pseudophyllidea, many eggs are released in the brief period when their aquatic intermediate hosts are abundant (semelparity). In contrast, in the terrestrial Cyclophyllidea, proglottids are released steadily over a period of years, or as long as their host lives (iteroparity). Another strategy is to have very long-lived larvae; for example, in Echinococcus, the hydatid larvae can survive for ten years or more in humans and other vertebrate hosts, giving the tapeworm an exceptionally long time window in which to find another host.Many tapeworms have a two-phase lifecycle with two types of host. The adult Taenia saginata lives in the gut of a primate such as a human, its definitive host.