Except for members of the order Taeniidae, the first intermediate host is an arthropod, and except for in the case of Archigetes spp. (which can attain sexual maturity in freshwater oligochaeta), the second host is usually a fish, but can be another invertebrate or vertebrate. After the scolex has differentiated and matured in the larval stage, growth will stop until a vertebrate eats the intermediate host, and then the strobila develops. Adult tapeworms often have a high final host specificity, with some species only found in one host vertebrate. Common infective species
Medical importance
Taeniasis
Taeniasis is an infection within the intestines by adult tapeworms belonging to the genus Taenia. It is due to eating contaminated undercooked beef or pork. There are generally no or only mild symptoms. Symptoms may occasionally include weight loss or abdominal pain. Segments of tapeworm may be seen in the stool. Cysticercosis
Cysticercosis is a tissue infection caused by the young form of the pork tapeworm. Infection occurs through swallowing or antiperistaltic contractions during regurgitation carrying eggs or gravid proglottids to the stomach. At this point, larvae hatch when exposed to enzymes and penetrate the intestinal wall, travelling through the body through blood vessels to tissues like the brain, the eye, muscles, and the nervous system (called neurocysticercosis).At these sites, the parasites lodge and form cysts, a condition called cysticercosis, producing inflammatory reactions and clinical issues when they die, sometimes causing serious or fatal damage.
Brain infection can cause granulomas, hematomas, and abscesses. Subdivisions
The evolutionary history of the Eucestoda 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. References

Bale, James F. "Cysticercosis." Current Treatment Options in Neurology. 2000. pp. 355–360. Dunn, J., and Philip E. S. Palmer. "Sparganosis." Seminars in Roentgenology. 1998. pp. 86–88. Esteban, J. G., Munoz-Antoli, C., and R. Toledo. "Human Infection by a "Fish Tapeworm," Diphyllobothrium latum, in a Non-Endemic Country." Infection. 2014. pp. 191–194. Kim, Bong Jin, et al. "Heavy Hymenoleptis nana Infection Possibly Through Organic Food: Report of a Case." The Korean Journal of Parasitology. 2014. pp. 85–87. Mehlhorn, Heinz. "Eucestoda Classification." Encyclopedia of Parasitology. 2008. pp. 495–497. Rohde, Klaus. "Eucestoda." AccessScience. McGraw-Hill Ryerson. Usharani, A., et al. "Case Reports of Hydatid Disease." Journal of Epidemiology and Global Health. 2013. p. 63–66.