Download Grzimek's Encyclopedia - Protostomes by Michael Hutchins PDF

By Michael Hutchins

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Extra resources for Grzimek's Encyclopedia - Protostomes

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Photo by A. Flowers & L. Newman. ) Phylum Onychophora The exclusively tropical and terrestrial velvet worms are gonochoristic, with well-developed gonads and a copulatory behavior that involves the deposition of spermatophores by the male externally, onto the body of the female. Fertilization is internal, following movement of the sperm directly through the body wall into the female. Cleavage is holoblastic in most species, and superficial in others, but always resembles that of various arthropods.

The relevant characteristics among the protostomes are described in the following sections. Gametogenesis As higher animals, all sexually reproducing protostomes form gametes, generally in the form of sperm (in male systems) and oocytes (in female systems). ”) Most protostomes are gonochoristic, meaning that they have separate male and female individuals. However, hermaphroditism, or the formation of male and female gametes in the same individual, is very common in many protostome phyla, reaching high levels in groups such as the leeches and earthworms (phylum Annelida), and the 16 pulmonate snails (phylum Mollusca).

Conversely, the fate of the blastopore in deuterostomes is to form the anus. Shortly or immediately after gastrulation is complete, protostomes form their body cavity, the coelom. By definition, a true coelom is always a body cavity within mesodermal tissue. The mechanism by which the coelom is formed is the third primary distinction between protostomes and deuterostomes. In most deuterostomes, the coelom forms by outpocketing from the original archenteron, a process known as enterocoely, since the coelomic cavities are thus derived directly from embryonic enteric cavities.

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