Parazoans represent the first body plan that moves beyond that of a single cell to a multicellular animal. But unlike all other animals, Parazoa are characterized by their absence of tissues and we refer to the group as being organized at the cellular grade rather than the tissue grade, that we associate with the true metazoans. Even though the cells are arranged in layers it is the absence of a basement membrane (basal lamina), or any type of tight junctions between the adjacent cells, that confirms that tissues do not exist.
The Parazoans that you are probably most familiar is are the sponges, Porifera. These unusual animals owe their evolutionary longevity to two important things; choanocytes that pump water through the body and totipotent cells that allow sponges to adapt and change their shapes to meet the changing demands of the aquatic environment in which they live. Even though tissues are missing there are still distinct cell layers each with its own unique cells. The outer covering of the body is maintained by a pinacoderm and the inner surface of the spongocoel is lined with choanocytes that form the choanoderm of the spongocoel, radial canals or choanocyte chambers.
The choanoderm's location reflects the three different types of sponge architecture: from the simple asconoid, to syconoid and the most complex of all, the leuconoid forms. Each allows a much more efficient flow of water through the sponge for trapping the particulate food suspended in it..
Between the choanoderm and pinacoderm is an acellular matrix, the mesohyl containing a variety of cells including amebocytes, archaeocytes, and sclerocytes that secrete the spicules that support the sponge and help defend against predation. Spicules, composed of silica and calcium along with collagen like spongin help to support the sponge and their composition and appearance are used by poriferan taxonomists to differentiate between the classes. Sponges exhibit both asexual and sexual reproduction and, in the case of the latter, the convolutions that a sponge goes through to bring the sperm and zygote together and form the larval stage are unique and in some cases rather unusual.
Another parazoan that is of interest to us is the placazoan Tricoplax adhaerens found wandering across the surface of marine aquaria in the late 1800s. In 1971 it was realised that this wasn't an unusual larval cnidarian, as it was originally thought, but its own one species phylum, the Placazoa. T. adhaerens has a distinct dorsal and ventral surface and when it feeds it forms a hollow ball of cells with an uncanny resemblance with some of the early embryological stages of the blastula and gastrula. T. adhearens offers some tantalizing clues, and raises just as many interesting questions on the origins of the true metazoa.
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BIO3334 Amphiblastula larva, Apopyle, Coeloblastula larva, Collagen, Inversion, Macromere, Megascleres, Micromere, Microscleres, Parazoan grade, Parenchymula larvae, Placozoa, Prosopyle, Prosopyle, Protandry, Reduction bodies, Sclerocytes, Spongin, Tricoplax adherens, Cellularia, Choanosyncytium, Hexaxon, Spongocytes, Symplasma, Syncytium, Tetraxon, Triaxon, Tropocollagen.
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Parazoans (Porifera and Placozoa)
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