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Echinodermata
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Evolution and Classification, Echinoderm Phylogeny and Anatomy, Classes: Asteroidea, Crinoidea, Echinodea, Ophiuroidea, Holothuroidea,
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InvertebratesWeek 11
Dr Glyn Barrett
Echinodermata
Characteristics;
- Spiny skin, internal skeleton [endoskeleton]
- Pentaradial symmetry [radial, branching out from central point – cnidarians]
- Water vascular system with tube feet
- 7000 species, 5 classes
Evolution and Classification;
- Cambrian 500mya, evolved from bilaterally symmetrical ancestor
- Ancestral larvae similar to modern sea star larva
- Evolved from sessile to free living [are re-evolved back into sessile for some]
- Choanoflagellates; free living, unicellular, colonial flagellate eukaryotes.
- Sponges lack true tissues, no body symmetry, sessile poriferans multicellular, not
organized into tissues
- Porifera + cnidaria are diploblastic with 2 main cell layers; jellylike mesoglea
separates
- Cell differentiation;
o Protostome and deuterostomes – embryological development of mouth and
anus
o Proto [mouth then anus] Deuto [mouth second, anus first]
o Blastopore; dent in cell wall goes on to create anus/mouth
o Archenteron; embryonic gut
Key differences which set apart Hemichordates and
Chordates;
1. Share common ancestry with them. Evidence in
deuterostome development
2. Bilaterally symmetrical at larval stages.
3. Extinct echinoderms not radial symmetry.
- Hemal system; primitive, poorly developed
blood vascular system.
, Invertebrates
Week 11
Dr Glyn Barrett
5 extant classes;
Phylum; Echinodermata;
- All marine, triploblastic, unsegmented coelomates with unique features.
1. Adults exhibit pentamerous symmetry [2nd radial symmetry / larvae bilaterally
symmetrical undergo metamorphosis = radial symmetry in adults]
2. Calcite spicules embedded in skin [partly fused]
3. Tube feet [podia]
- Round body with parts radiating from center, lack cephalization, no posterior/anterior
end, mouth on underside and anus on aboral surface.
- Echinoderm body wall;
o Poorly ganglionated, possess few sensory structures [ganglia related to PNS]
o Body wall contains endoskeleton of calcareous places – ossicles
a. Mutable connective tissue [MCT] ability to convert from stiff to soft
[instantaneous] MCT; capacity to change mechanical properties within
<1sec.
o Ossicles not linked together, muscle and connective tissues which allows
movement
o Spines [specialized ossicles]] protective purpose and movement
o Size, quality, function varies between echinoderms;
b. Urchins; ossicles attached to hard outer “test” = rigid
c. Holothurians; ossicles highly reduced
- Nervous System;
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