Yolk cells travel in a duct system to the oviduct, where, in a modified region, the ovum is enclosed in a shell with yolk cells. After the gonads and their ducts have finished maturing, the female reproductive organs begin to mature. The oviduct develops a vagina and enlarges into the uterus, where fertilization and embryonic development occur.Egg formation is a result of copulation. A proglottid can copulate with itself, with other proglottids in the same worm, or with proglottids in other worms, and hypodermic fertilization sometimes occurs. When a gravid proglottid that is distended with an embryo reaches the end of the strobila, it detaches and passes out of the host intact with feces, with or without some tissue degeneration. In the order Pseudophyllidea, the uterus has a pore and the proglottid sheds the shelled embryo, only becoming detached when exhausted.Some members of the Eucestoda (such as Echinococcus, Sparganum, Taenia multiceps sp., and Mesocestoides sp.) can reproduce asexually through budding, which initiates a metagenesis of alternating sexually and asexually reproducing generations. Life stages
A tapeworm can live from a few days to over 20 years. Eucestoda ontogenesis continues through metamorphosing in different larval stages inside different hosts. The initial six-hooked embryo, known as an 
oncosphere or hexacanth, forms through cleavage. In the order Pseudophyllidea, it remains enclosed in a ciliated embryophore. The embryo continues to develop in other host species, with two intermediate hosts generally needed. It gains entry to its first intermediate host by being eaten.
Members of the Eucestoda have no mouth or digestive tract, and instead absorb nutrients through a layer of microtriches over the tegument at the shared body wall surface. In addition to the body wall, several other systems are common to the whole length of the tapeworm, including excretory canals, nerve fibers, and longitudinal muscles. The excretory system is responsible for osmoregulation and consists of blind-ending flame bulbs communicating through a duct system. The nervous system, often referred to as a "ladder system," is a system of longitudinal connectives and transverse ring commissures. Reproduction
The reproductive systems develop progressively along the differentiated proglottids of the strobila region, with each proglottid developing one or two sets of sexual organs that differentiate at different times in a species-specific pattern, usually male-first. Thus, moving in the posterior direction of the continuously maturing proglottid chain, there are proglottids with mature male reproductive organs, then proglottids with mature female reproductive organs, and then proglottids with fertilized eggs in the uterus, a condition commonly referred to as "gravid." Proglottids
An atrium on the lateral margin of each proglottid contains the openings to both the male and female genital ducts. Follicular testes produce sperm, which are carried by a system of ducts to the cirrus, an eversible copulatory organ that usually has a hypodermic system of spines and a holdfast system of hooks. The main specialized female reproductive organs are an ovary that produces eggs and a vitellarium that produces yolk cells.