The majority of cestodes are hermaphroditic (only the members of the tetraphyllidean family Dioecotaeniidae and the cyclophyllidean family Dioecocestidae are dioecious). As a rule, each proglottis contains one set of male reproductive organs and one set of female reproductive organs. Often the maturation of the male organs and female organs do not coincide in time. In the majority of the families, the male organs mature first and proglottides initially act as male. This type of strobilar development is known as protandry. In some families (e.g., in the Progynotaeniidae), female reproductive organs develop first. In the latter case, the sperm production in testes coincides with the development of the eggs in the uterus. This type of development is known as proterogyny. Both types of strobilar development should be considered adaptations allowing more complete utilization of resources— though hermaphroditic, each proglottis is initially functionally male and after that functionally female (or visa versa), thus being able to produce larger number of gametes. It also prevents self-fertilization.
The life cycle of each cestode species includes at least two hosts, final and intermediate. The final or definitive host is that harboring adult (reproducing sexually) worms. The intermediate host is that where larvae (known also as metaces-todes) develop. The two hosts are in close ecological associations, facilitating the transmission of the parasite. The intermediate host lives in habitats where the final host feeds and defecates. The intermediate host is also a common component of the diet of the final host. The transmission of the cestode from the intermediate host to the final host is along the food chains only. Often the transmission is facilitated by a parasite-induced modification of the intermediate host's behavior, color, or health condition, in order to make it easier prey for the final host.
The general scheme of the life cycle is as follows. The ces-tode eggs pass with host's feces into the environment. Each egg encompasses an embryo named oncosphere. The latter possesses six embryonic hooks and several glandular cells and is surrounded by several envelopes. The egg is eaten by the intermediate host. In the gut of this host, the oncosphere hatches and, using its hooks and glands, penetrates through the intestinal wall and locates in the body cavity or in any internal organ. There, it metamorphoses into an infective larval stage (metacestode). In the most common case, the metacestode has a fully developed scolex identical to that of the adult worm. The final host is infested by eating infected intermediate hosts. The scolex of the metacestode attaches to the intestinal wall of the final host. The neck of the worms starts the production of proglottides and thus the strobila is formed. With the further development of the proglottides, the worm starts producing eggs, which are released with feces into the environment.
There are numerous cestode species exhibiting details in the life cycle differing from that described in the general scheme. Some species have two intermediate hosts or mobile embryos able to swim (see the account for Diphyllobothrium latum). The embryos of Amphilinidea and Gyrocotylidea are not oncospheres but lycophoras (see the account for Amphilina foliacea). The range of the intermediate hosts used is also impressive.
Metacestodes of various orders and families exhibit an immense morphological variability. Procercoid is the metaces-tode in the first intermediate host that has an elongate body and cercomer (e.g., in Pseudophyllidea). Entering into the second intermediate host, it develops into the plerocercoid. The latter possesses a differentiated scolex and is able to infect the final host. Embryos of other cestodes directly develop into plerocercoids (e.g., Proteocephalidea and Nippotaeni-idea).
The most widespread type of metacestodes in the order Cyclophyllidea is the cysticercoid. It is a solid-bodied organism with fully developed scolex retracted into the body. Among the Taeniidae, the most widespread metacestode is the cysticercus. Its scolex is introverted.
As in 2001, life cycles of about 200 cestode species are known (out of 5,100-5,200 species described). Obviously, the discovery of their enormous diversity, in terms of the range of hosts utilized and the morphological specialization of metacestodes, is a matter for the future.
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