phylum echinodermata. introduction n echinodermata are all marine, triploblastic unsegmented...
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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 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