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The Deuterostomes Deuterostomes Why Are Echinoderms Considered

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The Deuterostomes Deuterostomes Why Are Echinoderms Considered Protostome The Deuterostomes Deuterostomes Echinoderms and Chordates The SECOND opening of Ch 35 the digestive system (in the gastrula stage) becomes the mouth Deuterostomes Characteristics: • Bilaterally symetrical • Coelomate • Blastopore fate (anus) – Second opening in the gastrula becomes the mouth – This developmental similarity between echinoderms and chordates is supported by DNA sequence data Phylum Echinodermata Why are Echinoderms considered (Gk: echin = spiny; derma = skin) to be bilaterally symmetrical? • 5 Classes: Sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers • A thin skin covers an endoskeleton of hard calcareous plates • 7,500 species – all marine 1 Adults - Pentaradial Echinoderm larvae are bilateral Starfish larvae Sea Urchin larva Sea Cucumber Some species are commercially harvested for human food Echinoderm Features • Unique “water vascular system” • A network of hydraulic canals used in: • locomotion • feeding • gas exchange • excretion 2 Sea Star anatomy Tube feet of a sea cucumber • Muscle and nervous tissue, but no cephalization Sea Star feeding on a bivalve in adults • Endoskeleton of hard calcareous plates • Complete (but short) digestive system • No specialized respiratory, circulatory, or excretory systems • Separate male and female individuals • External fertilization Everts stomach into the bivalve to digest it INSIDE ITS OWN SHELL!! Sea stars = Keystone predators Crown-of-thorns sea star • Keystone species - plays a unique role in the • Feed on the living cnidarians that produce coral reefs way ecosystems function – greater than • Population explosions of the crown-of-thorns sea star what their biomass would suggest. can severely damage coral reefs • Sea stars eat shell fish (urchins, mussels, • Causes of outbreaks appear to be removal of natural etc) that most others do not eat predators and increased nutrient levels in the water • If sea urchins and/or mussels (herbivores) (both due to human activities) take over what will happen? • Control efforts include injection of ! individual starfish with chemical toxins 3 Phylum Chordata Phylum Chordata • Divided into 3 subphyla, which include 11 This phylum includes the animals that major groups of chordates are most familiar to us - and includes humans. • Some of the 11 groups are defined as “Classes” by biologists • Others of the 11 groups are currently considered to be “clades”, i.e. formal taxonomic level has not been agreed upon Chordates! Fig 35.12 • Chordate does NOT = Vertebrate All chordates have four characteristics 1. Pharyngeal slits (at some developmental stage) • Openings to the outside of the body at the pharynx (region just behind the mouth) 1. Pharyngeal gill slits – Allows water that enters the mouth to exit without passing though the gut in invertebrate chordates 2. Dorsal hollow nerve cord – Modified for gas exchange or other functions in vertebrates 3. Notochord 4. Post-anal tail Fig. 34.3 Pharyngeal slits 4 2. Dorsal hollow nerve cord 2. Dorsal hollow nerve cord • Unique to chordates • Nerve cord develops into the central nervous – Other animals have a solid nerve cord that is usually system ventrally located – Brain and spinal cord Dorsal hollow nerve cord Ventral solid 3. Notochord 4. Muscular post-anal tail • A flexible rod located between nerve cord and gut • Skeletal elements (notochord) and muscles – Provides skeletal support for muscle attachment • Lost during embryonic development in many – Only remnants of embryonic notochord found in most species adult vertebrates – Non-chordates have a digestive tract that extends the length of the body Notochord Post-anal tail Phylum Chordata Subphylum Cephalochordata Three Subphyla: • Called “lancelets” because of their blade-like shape 1. Cephalochordata • Chordate characteristics persist into adulthood • Globally rare, but some places have high 2. Urochordata population density (e.g., Tampa Bay, Florida) 3. Vertebrates 5 Lancelet Anatomy Suspension feeders that use their pharyngeal slits to filter out small food particles Typically present buried tail-first in sand Subphylum Urochordata Adult tunicates don’t look much like a “chordate” • Tunicates (“sea squirts”) • No notochord, nerve cord or tail • Filter feeders with incurrent and excurrent siphons Adult tunicates don’t look much Some tunicate species are like a “chordate” colonial • But these features are present in the tunicate larval form… 6 The Vertebrates! Subphylum Vertebrates • Chordates with a skull (either bone or cartilage) • Includes all of the animals we call “vertebrates” Finally - something we’ve heard of!! All vertebrates have the same • Includes some large and rather obvious characteristics as chordates animals that are quite familiar to us (at some developmental stage), plus • But remember that vertebrates represent • Pronounced cephalization less than 5% of the known animal species – With a skull (cranium) on Earth • Vertebral column that encloses the nerve cord – Replaces the notochord function • Endoskeletons that grows with the animal • Closed circulatory system • Lampreys 7 Hagfish (Clade Myxini) Jawless Lost vertebrae through evolution (ancestors had vertebrae) 8 .
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