Reproductive biology

Digenetic flukes engage in both asexual and sexual reproduction. When the first-stage larvae reach their destination within the first intermediate host, a polyembryonic asexual reproductive phase begins. After the larvae develop into sporocysts, the sporocysts of some species produce a second generation of sporocysts, while others produce rediae. Tri-chobilharzia physellae and Diplostomum flexicaudum take the two-generations-of sporocysts route. Metorchis conjunctus and Proterometra dickermani develop from miracidium to sporo-cyst to redia. A few species, including Stichorchis subtriquetres, go directly from miracidium to redia, bypassing the sporocyst stage altogether. The rediae are also capable of asexual reproduction to produce a second generation of rediae. Via sporocysts and rediae, the number of invading trematodes can multiply very quickly within the first intermediate host.

Both sporocysts and rediae produce the next stage, the cer-cariae and metacercariae. Cercariae are free-living and swim to the second intermediate host, which is typically a prey for the definitive host. Once on or in the second intermediate host, the cercariae transform themselves into metacercariae. Neither the cercariae nor metacercariae reproduce. The metacercariae finally enter the definitive host and become adults, which participate in sexual reproduction either by mating with a second fluke or by self-fertilization. Almost all trematodes are hermaphrodites. Some of these species, including members of the family Hemiuridae, even have fused male and female reproductive ducts. Members of the family Schistosomatidae are the exception. The schistosomatids have separate sexes with the adult females usually found coupled with adult males.

The eggs of both subclasses of trematodes are typically light-colored oval structures. The eggs of some species are embryonated (Heterophyes heterophyes) while others are not (Paragonimus westermani). Several species, including Metago-nimus yokogawai and Opisthorchis felineus, have eggs that contain mature miracidia in addition to embryos. In digenetic trematodes, the eggs develop in a large loop-shaped uterus. The number of eggs varies by species, but production of several hundred per day is not unusual. The common digenetic marine trematode Cryptocotyle lingua can produce thousands of eggs over a 24-hour period. Trematode eggs exit the host through the feces. In some species, miracidia form in the egg before it is shed; more typically, however, the miracidia develop two or three weeks after oviposition. In a few digenetic species, the miracidium forms only after the egg is taken into the first intermediate host.

The life cycle of the subclass Aspidogastrea is simpler; it has no asexual phase of reproduction. The entire life cycle typically occurs in one host, usually a mollusk. Predators of the host species have been known to eat infected mollusks and temporarily harbor the flatworms. Flatworms can survive in the predator's digestive tract for a short period, but cannot reproduce or develop there. There are other exceptions. Muli-calyx cristata is thought to use marine crustaceans as intermediate hosts. After the crustaceans are eaten by sharks or rays, M. cristata matures in the gall bladder of the shark or ray.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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