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Lecture 16

BIO153 Lecture 16.pdf

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University of Toronto Mississauga
Christoph Richter

2009 BIO153: Lecture 16 Invertebrates (II) th Mar 18 , 2009 The organisms we will look at today are in the monophyletic group Ecdysozoa (the other Protostome lineage). Characteristics of Ecdysozoa: ▯ grow by molting of exoskeleton ▯ exoskeleton: non-living external covering (flexible cuticle or hard, chitinous exoskeleton) ▯ exoskeleton permits new methods of locomotion & respiration and allows for the colonization of new environments There are 4 major phyla in Ecdysozoa; we will focus on two of them: Nematoda and Arthropoda. new feature! 1. Nematoda monophyletic group ▯ unsegmented worms ▯ have a tough outer cuticle composed of collagen ▯ pseudocoelomate ▯ most free-living, some parasitic (on plants; animals; fungi) ▯ Caenorhabditis elegans: model organism for geneticists & developmental biologists (genome completely mapped; fates of all cell divisions have been mapped) ▯ Parasitic nematodes have a significant impact on human health 2. Arthropoda ▯ arthro = joint; pod = foot ▯ most successful group of eukaryotes! 1 ▯ ¾ of all described organisms are arthropods Arthropod characteristics: ▯ fusion of body segments into 2 or 3 regions ▯ jointed appendages ▯ compound eyes ▯ antennae ▯ external skeleton composed of chitin ▯ exhibit metamorphosis Metamorphosis: ▯ 2 main types: holometabolous and hemimetabolous metamorphosis Holometabolous: ▯ also called complex or complete metamorphosis ▯ egg ▯ larva ▯ pupa or chrysalis (inactive phase) ▯ adult ▯ inside the pupa, digestive juices are excreted to destroy much of the body; new parts are regenerated from the stored nutrients released from the dead cells Hemimetabolous: ▯ also called simple or incomplete metamorphosis ▯ egg ▯ several instar or nymph stages (juvenile stages) ▯ adult ▯ instars undergo a series of molts (ecdysis) ▯ nymphs or instars are wingless ▯ ecdysis is under hormonal control (many insecticides interrupt this hormonal control) The arthropods are such a successful group primarily because of their jointed appendages. These appendages appear in aquatic organisms, but are a preadaptation to terrestrial life (a preadaptation is a characteristic that evolves under one set of 2 circumstances but allows an organism to be successful in a different set of conditions). ▯ appendages may be modified for many functions ▯ swimming, crawling, leaping, breathing (e.g. sand bubbler crab)… Having jointed appendages set the stage for another great terrestrial invasion (the first of two waves of terrestrial invasion by the animals): Oldest terrestrial arthropod fossils: ~ 400 mya (in the Rhynie chert!) ▯ wingless insects ▯ centipedes ▯ spiders ▯ scorpions Why did they move on to land? Probably to avoid predation. Moving onto land was possible due to: ▯ rising O2(terrestrial plants are getting going!) ▯ rising O3(ozone) – blocks UV As we have seen in other groups of organisms, there are advantages associated with living in the terrestrial environment, but there are challenges to be overcome. Problem A: water loss Solutions: 1. Cuticle ▯ aquatic arthropods: waxy cuticle on exoskeleton ▯ protects from water loss, UV, fungal attack 2. Internal respiratory surfaces ▯ internal gills (land crabs, isopods) ▯ book lungs (spiders etc.) ▯ tra
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