Phylum echinodermata
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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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Introduction l.jpg

  • 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 l.jpg

  • 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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  • 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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  • 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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  • …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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  • Allows pressure equalization and top up water supply to the WVS

  • Is absent in crinoids

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  • 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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  • 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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  • Feather stars & Sea lilies

  • Abyssal filter feeders

  • 5000 fossil spp, 620 living

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  • 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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  • “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)

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  • 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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  • 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

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  • 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)

Holothuridae l.jpg

  • 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