Dr. Morris Maduro, UC Riverside Biology 168 – 10F – Lecture 14, page 1Lecture 14: Chordates and early XenopusDevelopment Text: 2nd Ed.: 29-32; 67-73. 3rd Ed.:4-5; 89-96; 113-114. Some figures in these notes are redrawn from Wolpert et al. (2nd edition). ChordatesMembers of Phylum Chordata have the following features, as shown on the following diagram showing longitudinal and transverse cross sections at the phylotypic stage (the pharyngula). On the left figure, anterior is to the left and dorsal is up. On the right figure, dorsal is up. 1. Notochord. The notochord is a flexible rod of cells that forms the supporting axis of the body in lower chordates (e.g. lamprey) and in the embryos of higher vertebrates (i.e., replaced by vertebrae). 2. Dorsal hollow nerve cord.3. Gill slits. Gill slits (or pharyngeal arches) are used to capture food or remove oxygen from water (may be present transiently). 4. Post-anal tail.Where do chordates fit evolutionarily? If we consider the evolutionary relationships among all animals, excluding sponges and the radiata(animals with radial symmetry, such as comb jellies and jellyfish): Recent work studying 18S rDNA sequences has revealed that the two model systems we studied at the beginning of the course are fairly closely related. The unifying feature is that both Drosophilaand C. elegansundergo molting, and as such this group of animals is often called ecdysozoa. Annelids and molluscs form a sister group, the lophotrochozoa, that have a ciliated larva and extensive metamorphosis to adult. These four together form a classical group called the protostomes, in which the blastopore becomes the mouth (or both the mouth and anus). The latter group are called deuterostomes, the defining feature of which is that the blastopore will become the anus. The deuterostome clade includes the chordates, which can be subdivided into three subphyla: the urochordatessuch as the ascidian, the cephalochordatessuch as Amphioxus, and vertebrates. The last common ancestor to the chordates lived about 530 million years ago. Within the vertebrates, there are six classes.
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