phylum echinodermata l.
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Phylum Echinodermata 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) Affinities

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  • 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)
  • The only connected phylum is our own, the chordates - based on embryological evidence.
an unhurried phylum
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!
anatomical basics
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.
  • 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
functional groups 1 nerves
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
  • 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
5 radial layout
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
hydraulics contd
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
surface features
Surface features
  • Echinoderm skin has several distinctive sets of organs protruding from their skin:
    • Tube feet (podia)
    • Spines
    • Pedicillaria
tube feet
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
tube feet13
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.
tube feet14
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
role of wvs
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
  • …Are defensive organs, assumed to protect against encrusting organisms
  • Are active, independent local effector units able to inject toxins on contact
  • Allows pressure equalization and top up water supply to the WVS
  • Is absent in crinoids
  • 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.
  • 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
  • Feather stars & Sea lilies
  • Abyssal filter feeders
  • 5000 fossil spp, 620 living
  • 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 (!)
  • “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 prey’s shell (enzymes but no toxins)
  • 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
  • Herbivores, preferring macro-algae
  • They can be highly effective grazers, creating “urchin barrens” devoid of algae
  • The mouthparts are unique, known as Aristotle’s Lantern.
    • 5 continually growing chisel teeth
    • Each tooth with 8 supporting skeletal pieces
  • All are sand burrowing
  • Heart urchin Echinocardium has no lantern;
  • Sand-dollars (Clypeaster) are more flattened with a lantern
noli tangere
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
noli tangere28
Noli tangere
  • Very few echinoids are lethal to touch - their pedicillaria inject a neurotoxin
  • Toxopneustes is feared by pearl divers
ophiuridae brittle stars
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
holothuridae sea cucumbers
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)
  • 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