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(C) Formation of the second micromere quartet by laeotropic division the first-quartet cells in this species also divide laeotropically, but with a slight delay (a much longer delay occurs in Ilyanassa).
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(B) Eight-cell interphase the first-quartet micromeres (1a–1d) are nestled between neighboring macromeres (1A–1D). (A) Mitotic spindles in the quadrant founder cells are skewed clockwise, preparatory to dexiotropic division. (A) through (E) show the third through fifth rounds of cleavage in the basal gastropod Trochus, as seen from the animal pole (top row) and from one side (bottom row). Original figure caption: "Spiral cleavage: elaborating fourfold rotational symmetry about the primary axis of polarity. Image is from Goulding (2009) in PLoS ONE (Creative Commons Attribution license). In particular, observe B (top and bottom), the eight-cell stage. Illustration of spiral cleavage in the snail Trochus. Additional details about individual phyla are covered in later chapters. The major groupings, or clades, that comprise animal phyla are very briefly introduced below.
#IN THE PROTOSTOME LINE THE BLASTOPORE BECOMES THE PLUS#
(animals with backbones, plus a few others). Chordata: you, your dog, your cat, your goldfish, your lizard, your parakeet, the squirrel outside, etc.Hemichordates: a somewhat obscure group that includes the paleontologically-signficant graptolites.Echinodermata: starfish, sea urchins, crinoids, and others.Cnidaria: jellyfish, corals, sea anemones, Hydra, and others.It is recommended that you have familiarized yourself with phylogenetic terminology and how cladograms work before exploring this chapter ( Systematics chapter here).īefore considering the major clades of animal life, let's introduce some examples of the paleontologically-significant phyla that form the "tips" of this phylogenetic tree: This page, therefore, is meant to serve as a highly simplified introduction to the major features of animal evolution as suggested by recent molecular phylogenetic research, as well as the relationships amongst those groups with significant fossil records. The following chapters of this textbook cover only paleontologically-significant animal phyla and these chapters are arranged phylogenetically. ( Telford et al., 2015 provides an especially helpful recent overview of animal phylogeny, its history of discovery, and outstanding questions). See the reference list below for research papers that were especially significant during this period, as well as several useful broad reviews of animal phylogeny. Some long-held hypotheses of relationships were supported by genetic data, while others were found to have little support at all. The advent of molecular phylogenetic systematics-especially from the late 1980's through the late 1990's-began to provide some clarification regarding the major features of Metazoan phylogeny.