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Since the life history stages responsible for asexual and sexual phases are so distinctive in these groups, and occur in a regularly alternating pattern, this general reproductive strategy is often called alternation of generations. Gametogenesis During sexual reproduction, each parent animal must form specialized cells known as gametes, which are genetically recombined and in which the chromosome number is reduced by half from a diploid double set to a haploid single set. Both processes occur during meiosis, which is the first stage of ga16 mete formation, or gametogenesis.

Hexactinellida sponges colonize soft surfaces; they prefer to live in deep water. Demosponges can live on such different substrates as rock, unstable shell, sand, and mud; in some cases they burrow into calcareous material. They are found in a variety of underwater habitats ranging from upper intertidal to hadal depths (below 20,000 ft or 6,100 m). The ecological dominance of the Demospongiae reflects their diversity in form, structure, reproductive capabilities and physiological adaptation. Cnidarians and ctenophores are mostly marine; however, a few groups have successfully made their way into freshwater habitats.

Most are broadcast spawners, but some are fertilized internally and may even brood their larvae. Embryogenesis results in a cydippid larva that swims freely during its metamorphosis into an adult. Asexual reproduction is not known to occur in this phylum. Sexual reproduction is the rule among the roundworms; most species are gonochoristic with some sexual dimorphism. Some hermaphroditic species do exist. Reproductive systems are tubular; in copulation, the male introduces amoeboid sperm into the vagina of the female.

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