Invertebrates Classification – The Difference between Protostomes and Deuterostomes. What characteristics distinguish protostomes from deuterostomes ? to differing the mode of coelom formation and the embriological origin of the mouth, they also typically differ in the number of coelomic cavities formed as they develop. Among protostomes, the number of coelomic cavities is highly variable, for example, an annelid worm can have as many coelomic cavities as it has segments, hundred in some species. Among deuterostomes, however the original coelomic cavity generally subdivides to form 3 pairs of coelomic pouches. Protostomes and deuterostomes may also differ with respect to the orientation of the spindle axes of the cells during cleavage, the point in development at which cell fates become irrevocably fixed and how the mesoderm originates.
Cleavage is often referred to as being either radial or spiral, depending on the orientation of the mitotic spindles relative to the egg axis. Generally, yolk is asymmetrically distributed within eggs and the nucleus occurs in, or moves to, the region of lower yolk density. This is the animal pole and it is here that the polar bodies are given of during meiosis. The opposite end of the egg is termed the vegetal pole.
Invertebrates Classification – The Radial Cleveage
Invertebrates Classification – The Radial Cleveage. In radial cleavage (deuterostome), the spindles of a given cell and thus the cleavage planes are oriented either parallel or perpendicular to the animal vegetal axis. Thus, daughter cells derived from a division in which the cleavage plane is parallel to the animal vegetal axis end up lying in the same plane as the original mother cell. The two daughter cells resulting from a division perpendicular to the animal vegetal axis come to lie directly one atop the other, with the center of the upper cell lying directly over the center of the underlying cell.
Invertebrates Classification – The Spiral Cleveage
Invertebrates Classification – The Spiral Cleveage. In contrast with radial cleveage, the spindle axes of cells undergoing spiral cleavage are oriented (after the first two cleavages) at 450 angles to the animal vegetal axis. Moreover, the division line does not cecessarily pass through the center of the diiding cell. As a result, by the eight cell stage we often see a group of smaller cells (micromeres) lying in the space between the underlying larger cells (macromeres). Cell division continues in this fashion, with the cleavage plane always oblique to the polar axis of the embryo.
Cleaving embryos of protostomes and deuterostomes also typically differ with respect to when their cells become fully committed to a particular fate. Among deuterostomes, one can separate the cells of a two cell or four cell embryo and each cell will typically develop into a small but complete and fully functional animal. Thus, deuterostomes are said to show indeterminate (or regulative) cleavage. Each cells retains, sometimes as late as the eigh cell stage, the capacity to differentiate the entire organism if that cell loses contact with its associates. Among most protostomes, in contrast, the developmental potential of each cell is irrevocably determined at the first cleavage, separate the blastomeres of two called protostome embryo and each cell will, in most species, give rise only to a short lived, malformed monster. Protostome cleavage is therefore said to be determinate or mosaic. Species with determinate development can never produce identical twins, which in deuterostomes arise from the natural separation of blastomeres during early cleavage. Interestingly, both protostomes and deuterostomes can regenerate body parts as adults, regardless of whether their development is determinate or indeterminate.
Further difference between the two groups of coelomates concerns the source of mesoderm. Among protostomes, much of the mesodermal tissue derives from a single cell of the 64 cell embryo, located at the edge of the blastopore. This is not true of deuterostomes, which produce mesoderm from the walls of the archenteron. During their first 1 or 2 cleavages, the embryos of some protostomes form polar lobes. A polar lobe is a conspicuous bulge of cytoplasm that forms prior to cell division and that contains no nuclear material. After cell division is complete, the bulge is resorbed into the single daughter cell to which it is still attached. Although the functional significance of this phenomenon for the embryo is still not fully understood, polar lobe formation has provide developmental biologists with an intriguing system through which to study the role of cytoplasmic factor in determining cell fate. In the basic experiment, the fully formed polar lobe is detached from an embryo and the development of the lobeless embryo is subsequently monitored. Polar lobe formation is characteristic of only some protostome species (some annelids and some molluscs), but is never encountered among deuterostomes.
Finally, the ciliary bands involeved in feeding and locomotion among deuterostome larvae (and adults) are typically monociliated, while those found among larval protostomates are typically composed of multiciliated cell.
The biologist find a simpler way to construct logical classification system than to neatly distribude animals. As faith in the phylogenetic significance of body cavities has diminished, the definition of “protostome” has been broadened to include both acoelomate and pseudocoleomate animals (example : flatworms and nematodes). Few protostome species exhibit all of the other listed protostome characteristics. Not all protostomes exhibit spiral determine cleavage. Similarly, although the blastopore generally becomes the moth during protostome development, in some protostome species it becomes instead the anus, as in deuterostomes. And some deuterostome species (example : the sea squirts) show the fully determinate cleavage pattern typically associated with protostomes, while at least one protostome species (a targidrate) show the indeterminate cleavage pattern typically associated with deuterostomes.
Finally, some species exhibit a combination of protostome and deuterostome characteristics. Because all animal groups have had ancestral forms in common at some time during their evolution, because evolution is an ongoing process and because embryos as well as adults are subject to the modifying force of natural selection, some species are likely to have developmental characteristic that fall outside the mainstream. In any event, those species exhibiting entirely or primarily protostome characteristics are most likely to be more closely related to each other than to those species exhibiting purely deuterostome characteristics.
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