AcoelematesSlides - The Major Lineages of Evolution in...

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9/14/11 1 Next up: Acoelomate Bilateral Animals TWO PHYLA: Phylum Platyhelminthes (“flatworms”) Phylum Nemertea aka Rhynchocoela or Nemertina (in lab manual) The Major Lineages of Evolution in Animals Fundamental evolutionary divergence among the Eumetazoa based on tissues vs. organs and also symmetry . if you don ’t move, radial symmetry is useful (e.g., cnidarian polyps) (why….???) however, if you do move, it’s advantageous to put sense organs on the end that encounters the new environment first (i.e., anterior ); this leads to cephalization (the anterio r or “head” placement of sense organs/nerve center) this sets up an anterior-posterior axis which leads to bilateral symmetry With directed (“forward”) movement also comes the need for musculature specializations to coordinate this movement there is a need for a better nervous system Becoming bilateral Fundamental evolutionary divergence among the Bilateria , but based on what? The Major Lineages of Evolution in Animals
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9/14/11 2 “protostome” phyla (see inside cover of text) “deuterostome” phyla Phyla Echinodermata, Hemichordata, Chordata (incl. humans) - cells divide (cleave) such that daughter cells do not lie on top of each other, but obliquely in the furrows -cell fate determined early = determinate or mosaic development -cells divide (cleave) such that daughter cells lie on top of each other - cells remain totipotent longer = indeterminate or regulative development Embryogeny - the developmental pattern giving rise to animal body plans among various animal groups (see text, pp. 58-63***) (1) Patterns of cleavage : (for blastula) See Fig. 3.4 spiral cleavage radial cleavage Embryogeny (cont’d) (2) also see different patterns of mesoderm & coelom (body cavity) formation among the various Bilateria phyla -blastopore becomes mouth Most protostomes: -blastopore becomes anus Deuterostomes: -schizocoelous coelom formation… -enterocoelous coelom formation… See Fig. 3.6 Embryogeny (cont’d) (3) also see different patterns of gastrulation & formation of a body cavity ( coelom ) among the various Bilateria phyla meso. arises as solid massses of cells at lip of the blastopore that SPLIT to form the coelom (body cavity). Most protostomes have schizocoelous coelom formation: meso. arises as outpockets of the archenteron that enclose the coelom (cavity) vs. same end result Most deuterostomes have enterocoelous coelom formation: See Fig. 3.6 See 3 different coelomic architectures in bilateral animals: Fig. 3.5 devel.may end here with 2 germ layers (a diploblastic embryo with a “2-way”, blind” or “incomplete” gut: only 1 opening); e.g.,Cnidarians a tubular, “1-way” or “complete” gut with 2 openings forms, then a 3 rd germ layer in a triploblastic embryo (one exception today).
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