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BIOL2009 LT3 Deuterostomes £3.49
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Lecture notes

BIOL2009 LT3 Deuterostomes
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The phylogeny of the deuterostomes, distinct characteristics and introduction to the major clades. Extra reading with sources cited.

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  • April 6, 2016
  • 7
  • 2014/2015
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Deuterostomes = Chordates + Hemichordates + Echinoderms
Protostomes Deuterostomes
Spiral cleavage (8 cell stage) Radial cleavage
Determinate Indeterminate
Schizocoelous: solid masses of Enterocoelus: folds of archenteron
mesoderm split to form coelom form coelom (fluid filled cavities)
Trimery: body has 3 distinct regions
Mouth forms from blastopore Anus forms from blastopore
Evolution within Deuterostomes
- Members of protostomes and deuterostomes always subject to debate –
different analyses give different results
- Deuterostomes maintaine primitive radial
cleavage: evolved enterocoelus,
archenteric mesoderm, tripartite coelom
(from enterocoely)
- First deuterostome was probably a
vermiform burrower – main trunk coelom
(metacoel) for peristaltic burrowing
- Protocoel and mesocoel evolvable, available
for modification during deuterostome
radiation
- Evolution of hemichordate-chordate line
involved origin of pharyngeal slits, dorsally
concentrated CNS, iodine binding epithelial
tissues
- Many anatomical similarities – though several recent DNA analyses propose
Echinodermata Hemichordata sister-group (Ambulacraria) rather than
Echinodermata-Chaetognath
- Chordates – Urochordates arose first followed by cephalochordate and
vertebrate lineages

Phylum Chaetognatha
- Arrow worms, benthic, planktonic
- Highy motile, pelagic, predatory lifestyle
- Bilateral symmetry, trimeric form: head, trunk and tail, grasping spines for
food capture
- Deuterostome characteristics: entercoely (coelom forms from pouches
“pinched off” the digestive tract during gastrulation), mesoderm derived
from archenteron
- But they are now allied to Protostomes – MAD5 mitochondrial DNA analysis
finds genes only in protostomes

,Phylum Echinodermata
- Sea cucumbers, sea urchins, sea cucumbers, brittlestars, crinoids
- Embryogeny is deuterostomous – radial cleavage, enterocoely, through gut,
nervous system diffuse, no excretory organs
- Calcareous endoskeleton arising from mesodermal tissue, composed of
separate places or ossicles – each plate originates as a single calcite crystal
develops as an open meshwork structure (steroeom), interstices filled with
living tissue (stroma)
- Adults pentamerally symmetrical, larvae bilaterally symmetrical
- Unique coelomic water vascular system – evident as muscularpodia
- Originated from Pre-Cambrian invasion of epibenthic habitats by ancestral
burrowing deuterostome
- Calcichordates (extinct fossils) related to echninoderms

Phylum Hemichordata
- Enteropnuests (acorn worms), pterobranchs
- ~75 or 85 living species belong to Enteropneusta – all hemichordates are
bethic marine animals (except for Planctosphaera)
- In addition to deuterostome qualities – possess pharyngeal gill slits, most
have dorsal (sometimes hollow) nerve cord similar to those seen in Chordates
- Once thought to possess notochord – but is actually evagination of anterior
gut (buccal diverticum – stomochord), not homologous with notochord
- Unique excretory – glomerulus, open circulatory system, through gut
- Lophophore – convergent evolution
- Pterobranchs are largely colonial – the two classes represent highly
divergent clades within the phylum, each adapted to different ways of living
- Both represent exploitations of basic tripartite detuerostome architecture –
evolutionary plasticity of this fundamental body plan
- Hemichordate larva almost identical to echinoderm larvae
- Stylophora, cothurnocysts possess gill slits
- Bayesian Phylogenetic Analysisof 18S ribosomal DNA – supports monophyly
of Ambulacraria

Phylum Chordata
- Urochordates and vertebrates is a sistergroup to cephalochordates
- Notochord (dorsal stiffening rod - synapomorphy), dorsal hollow nerve cord,
segmented muscular postanal tail, endostyle (ciliated glandular groove on the
floor of the pharynx that secretes mucus for trapping food particles during
filter feeding)
- Vertebrates – skeletal vertebral column housing notochord, others are

, invertebrates
- Notochord = dorsal, elastic rod derived from middorsal strip of embryonic
mesoderm, provides structural and locomotory support in body of
larval/adult urochordates



Phylum Urochordates (Tunicates)
- Present day tunicates – marine animals that filter particles of food from
water with basketlike perforated pharynx (~3000 iving species, 0 or so
sedentary as adults)
- Adult tunicates bear little apparent similarity to cephalochordates and
vertebrates – most tunicate species have brief free-swimming larval period
lasting a few minutes to a few daysm, after which the larvae metamorphose
into sedentary adylts attached to the substrate, although some remain
motile
- Tunicate larvae – notochord, dorsal hollow nerve cord, muscular postanal tail
- At some point, stopped being mobile, sessile lifestyle adopted
- Almost all marine suspension feeders – ascidians, salps
- Adult body from varies, usually lacking obvious trimeric organisation (sea
squirt)
- Body covered by thick/thin cellulose like polysacchacaride (no bony tissue)
- Notochord restricted to tail – usually found only in larval stage, coelom not
developed
- Tadpole larva has chordate characteristics, most lost in adults
- Previously believed that the earliest chordates would have been sessile as
adults, and that cephalochordates and vertebrates evolved from an
ancestor that resembled a tunicate larva – however it seems more likely
that a sessile adult stage was a derived character
- Ancestral chordate was proobaby free swimming wormlike creature using
gill slits for filter feeding

Phylum Cephalochordata
- ~27 species – small, fishlike marine animals usually less than 5 cm long
(lancelet – Branchiostoma lanceolatum/amphioxus)
- Demonstrates many qualities intermediate between invertebrates and
vertebrates
- Lancets/amphioxus – shallow marine environments lie burrowed in clean
sands with only head protruding above sediment
- Notable characteristic is myomeres – blocks of striated muscle fibres
arranged along both sides of the body and seprated by sheets of connective
tissue, biticgird acts ab ab ubcinpressible elastic rod, extending the full

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