Phylum Echinodermata

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Phylum Echinodermata
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Introduction

• Echinodermata are all marine, triploblastic
  unsegmented coelomates
• Phylum has 3 unique features
• pentagonal symmetry (bilateral in larvae)
• calcite spicules embedded in the skin, often
  partly fused
• Tube feet (podia)

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Affinities

• The only connected phylum is our own, the
  chordates - based on embryological evidence.

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An unhurried phylum..

• No echinoderm moves fast, apart from a very few
  deep sea holothurids which swim actively
• Crinoids are sessile, the others crawl at a rate
  of mm / minute
• During one Antarctic marine survey a starfish was
  tagged. A year later the same animal was in the
  same exact spot, having apparently done nothing
  at all!

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Anatomical basics

• There is no cephalization
• There is a meaningful gradient in all echinoderm
  bodies one surface has the mouth and tube feet
  (ORAL or AMBULACRAL), while one does not (ABORAL)
• The anus is often, but not always, aboral.

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Originally

• The ancestral echinoderm was a sessile
  filter-feeder, extending its oral surface upwards
  to capture food
• This sedentary design has evolved into motile
  forms where the feeding surface faces downwards

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Functional groups 1 nerves

• Echinoderms have a diffuse nervous system with no
  brain
• There is a 5-radial circum-oral nerve ring, and a
  superficial net running close to ectoderm

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Hydraulics

• These are far more complex than the nervous
  system!
• Main hydraulic systems are derived from the
  coelom, although separate sections of the coelom
  also surround viscera
• The podia are operated by a hydraulic system
  called the water-vascular system

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5-radial layout

• Many organ systems in the echinoderms follow the
  same basic structure as the water-vascular and
  nervous systems a 5-radial circum-oral ring
• These rings give rise to 5 radial branches
  (canals in the case of the WVS)
• A few asteroids have 7, 10, 11 arms - in which
  case 7,10, 11 radial branches

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Hydraulics, contd.

• Each radial canal of the WVS supplies water to
  tube feet, each with its ampulla
• There is one asymmetric element a single tube
  (the stone canal) running from the oral WVS
  ring to the outside via the madreporite

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Surface features

• Echinoderm skin has several distinctive sets of
  organs protruding from their skin
• Tube feet (podia)
• Spines
• Pedicillaria

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Tube feet..

• Podia are not scattered haphazardly over the body
  surface
• They lie in 10 rows (5 pairs), the ambulacral
  grooves
• Each tube foot its ampulla is isolated from the
  WVS by a valve
• Tube feet vary - starfish have muscular suction
  cups, other forms have sticky tips.
• Crinoids are different - primitive

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Tube feet..

• Originally began as outgrowths of the WVS. In
  crinoids and ophiuroids these remain essentially
  as tentacles.
• In other radiations, notably asteroids, these
  have evolved a highly specialised suction cup
  used for locomotion and prey capture.

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Tube feet..

• Have retractor muscles and can bend, but no
  extensors
• To extend, muscles around the ampulla contract
• Each podium has a nervous arc to its branch of
  the hyponeural system

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Role of WVS

• Hydraulics
• Respiration - O2 is exchanged between ampulla and
  perivisceral coelomic fluid
• Probably (?) this was the ancestral function of
  the WVS, with tubes podia lining arms to
  exploit ciliary current already used in food
  collection

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Pedicillaria

• Are defensive organs, assumed to protect against
  encrusting organisms
• Are active, independent local effector units able
  to inject toxins on contact

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Madreporite

• Allows pressure equalization and top up water
  supply to the WVS
• Is absent in crinoids

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Gonads

• Lie as 10 (2N) paired structures at the base of
  ambulacral grooves.
• Sexes are separate, and discharge gametes into
  the sea water
• Gonads can be large - echinoid gonads almost fill
  the test, and can be eaten as a delicacy.

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Sadly...

• Of the 13 classes of echinoderms known, 7 are
  extinct.
• Echinoderms were dominant forms in Carboniferous
  seas, but have suffered a long-term decline in
  phyletic richness

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Crinoidea

• Feather stars Sea lilies
• Abyssal filter feeders
• 5000 fossil spp, 620 living

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Crinoidea

• Body made of ossicles
• 10 arms have podia (no ampullae) feeding
  particles to the mouth.
• Arms can move
• Mouth and anus are both on oral side (!)

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Asteroidea

• Starfish
• Active predators
• feed on bivalves
• use suction cups to pull open the shells with
  forces of up to 5kg
• The stomach is eversible, and can be partially
  inserted inside preys shell (enzymes but no
  toxins)

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Echinoidea

• Recipe take a starfish and roll its 5 arms
  together into a ball, then fuse and calcify with
  an external armor
• The armor is called the test
• Very small aboral surface

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Echinoidea

• Herbivores, preferring macro-algae
• They can be highly effective grazers, creating
  urchin barrens devoid of algae
• The mouthparts are unique, known as Aristotles
  Lantern.
• 5 continually growing chisel teeth
• Each tooth with 8 supporting skeletal pieces

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Irregulars

• All are sand burrowing
• Heart urchin Echinocardium has no lantern
• Sand-dollars (Clypeaster) are more flattened with
  a lantern

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Noli tangere

• Many echinoids have wickedly sharp spines, which
  break off in your skin.
• Only a few fish, trigger fish attack long-spined
  species
• Spines are under muscular control, and can be
  used to move

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Noli tangere

• Very few echinoids are lethal to touch - their
  pedicillaria inject a neurotoxin
• Toxopneustes is feared by pearl divers

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Ophiuridae - brittle stars

• Have arms sharply demarcated from the body disc.
• The internal structure of the arms involves
  interlocking internal ossicles, confusingly
  called vertebrae
• Are primarily detrital or filter feeders, raising
  their arms in a current to capture particulates

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Holothuridae- Sea Cucumbers

• They have no calcitic skeleton, except for
  spicules embedded in a leathery skin
• Most are immobile, and lie on the sea bed rolling
  back and forth with the swell. Some have limited
  mobility using their tube feet.
• Despite retaining 5-radiate anatomy, they have
  re-evolved bilateral symmetry along their long
  axis (the oral-aboral)

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Holothuridae

• They mainly feed on detritus
• Oxygen exchange is performed using gills inside
  their anus
• They have 2 odd defensive strategies
• Squirting a sticky goo
• Voiding their entire intestines