1. Phylum Echinodermata

2. 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)

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

4. 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!

5. 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.

6. 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

7. 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

8. 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

9. 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

10. 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

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

12. 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

13. 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.

14. 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

15. 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

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

17. Madreporite
Allows pressure equalization and top up water supply to the WVS
Is absent in crinoids

18. 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.

20. 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

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

22. 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 (!)

23. 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)

24. 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

25. 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

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

27. 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

28. Noli tangere
Very few echinoids are lethal to touch - their pedicillaria inject a neurotoxin
Toxopneustes is feared by pearl divers

29. 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

30. 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)

31. 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