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 The only connected phylum is

our own, the chordates - based on embryological evidence.

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: 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.

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

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

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

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.

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

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

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 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 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

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

Pedicillaria

…Are defensive organs, assumed to protect against encrusting organisms

Are active, independent local effector units able to inject toxins on contact

Madreporite

Allows pressure equalization and top up water supply to the WVS

Is absent in crinoids

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.

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

Crinoidea

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

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 (!)

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 prey’s shell (enzymes but no toxins)

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

Echinoidea

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

Irregulars

All are sand burrowing Heart urchin Echinocardium has no

lantern; Sand-dollars (Clypeaster) are more

flattened with a lantern

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 tangere

Very few echinoids are lethal to touch - their pedicillaria inject a neurotoxin

Toxopneustes is feared by pearl divers

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 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

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