Viviparous Animals

Viviparous animals, a term for animals that carry fertilized eggs within the female reproductive system until the embryo is fully formed: offspring leave the maternal organism in the form of young or larvae at various stages of development. Viviparity occurs in various animal groups, starting with coelenterates (sea anemones, etc.): in some worms (trichina, rotifers), some mollusks, occasionally in echinoderms (some ophiuroids), arachnids (scorpions) and insects. Among vertebrates, there are viviparous fish (some sharks, Cyprinodonta), reptiles (some lizards and snakes) and almost all mammals (except monotremes). In general, viviparity is more common among terrestrial than aquatic animals. Sometimes a connection between viviparity and climate is noticeable. Thus, among amphibians and reptiles, northern and high-altitude forms are more often viviparous than their southern or plain relatives (compare the mountain black salamander and the plain spotted salamander, the southern Russian "Lacerta viridis" and the northern Lacerta vivipara). Different parts of the reproductive system can serve for carrying embryos: the vagina (in many flies), the ovary (in aphids, some fish, etc.), and most often a section of the oviduct transformed into a uterus (scorpions, mammals, etc.). The number of offspring in viviparous animals is usually significantly less than in their egg-laying relatives. Various adaptations for nourishing the embryo during its life in the maternal reproductive system are often associated with viviparity. These adaptations are of two kinds: in some cases, liquid nutrients are simply secreted by the walls of the reproductive ducts and consumed by the embryo (some flies, etc.); in other cases, there is a local fusion of the embryo or its membranes with the uterine wall and the formation of a placenta, through which nutrients from the mother's blood diffuse into the embryo's blood (some sharks, mammals, etc.).