The complex life cycles of animals - is an alternation of generations of diploid organisms. In contrast to plants and fungi, in which the haploid ph

Life cycle of animals [Complex]

The complex life cycles of animals — is an alternation of generations of diploid organisms. In contrast to plants and fungi, in which the haploid phase is usually represented by a separate generation of organisms, in animals is represented by a stage of gametes. In this case the sex cells are formed by meiosis, which is called the gamete. Therefore, complex life cycles of animals may be associated with the alternation of generations of diploid organisms that differ from each other only by the breeding process. Most of the species has a simple life cycle — from zygote to sexual reproduction. Only primitive and most parasitic representatives of Animal kingdom have preserved complex life cycles.

The alternation of sexual and asexual reproduction is inherent to coelenterates (diagram 99). In this case time the polyp stage — is the asexual phase of the life cycle. Polyps reproduction usually occurs by budding. Medusoid stage — is sexual. The complex life cycles with a change of asexual and sexual reproduction are also peculiar to specific groups of tapeworms, particular to Echinococcus that during the larval stage reproduces by fragmentation of the fin (its larvae stage name) and as an adult in the final host — sexually. Taken from http://worldofschool.org

Diagram 99. The complex life cycles of animals on the example of scyphoid coelenterates: 1 — egg; 2 — larva; 3 — mature polyp; 4 — asexual reproduction; 5 — sexual reproduction; 6 — immature jellyfish; 7 — mature jellyfish

Another kind of complex life cycle that that is inherent only for the animals should be considered as an alternation of amphimictic and parthenogenetic forms of sexual reproduction. This type of life cycle is required for all flukes. Their amphimictic reproduction takes place in the final host, which is the vertebrate, whereas larvae reproduce by parthenogenesis in the body of gastropods, which are bound to be their intermediate hosts.

Questions:
  • What are the features of complex life cycle of the animals?