Sponges-The pore bearer
The phylum Porifera consists of Sponges meaning “Pore bearer”,are mostly marine animals.These animals are among simplest of animals lacking gastrulated embryos (Gastrulation is a phase early in the development of animal embryos,during which the morphology of the embryo is dramatically restructured by cell migration.Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers) and sponges are also devoid of proper digestive system.Diffusion is responsible for circulation and excretion in these organisms.They also lack nerves,muscles and tissue organization.
The choanocytes(feeding cells) appear to be a homologous to choanoflagellates, a group of unicellular and colonial protists that are believed to be the immediate precursors of animals.But recent phylogenomics from Cassey Dunn et al indicated Ctenophores(comb jellies) as most primitive organism group ,replacing sponges which were always believed to a result of possible jump from unicellular to multicellular organisms.
Modern sponges are predominantly marine, with some 150 species adapted to freshwater environments. Their habitats range from the inter-tidal zone to depths of 6,000 metres.There are three main classe of porifera :Calcarea,Hexactinellida and Demospongiae(some believe fourth Sclerospongiae)
Reproduction occurs by both asexual and sexual ways.Sponges are hermaphrodites ,but not self fertile as they are sequential hermaphrodites.Sponges in-spite of being so simple when compared to primitive group of animals(comb jellies) but still phylogenomic analysis took away their basal spot suggesting that these group of animals have over simplified(could be because of loss of genes or thier functions) in course of evolution.This also indicates that evolution is not always gaining things but losing also an important part of it.
Related information about sponges on Tree of life
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