2IntroductionEchinodermata are all marine, triploblastic unsegmented coelomatesPhylum has 3 unique features:pentagonal symmetry (bilateral in larvae)calcite spicules embedded in the skin, often partly fusedTube feet (podia)
3AffinitiesThe only connected phylum is our own, the chordates - based on embryological evidence.
4An unhurried phylum..No echinoderm moves fast, apart from a very few deep sea holothurids which swim activelyCrinoids are sessile, the others crawl at a rate of mm / minuteDuring 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!
5Anatomical 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.
6Originally…The ancestral echinoderm was a sessile filter-feeder, extending its oral surface upwards to capture foodThis sedentary design has evolved into motile forms where the feeding surface faces downwards
7Functional 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
8Hydraulics 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 visceraThe podia are operated by a hydraulic system called the water-vascular system
95-radial layoutMany organ systems in the echinoderms follow the same basic structure as the water-vascular and nervous systems: a 5-radial circum-oral ringThese 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
10Hydraulics, contd.Each radial canal of the WVS supplies water to tube feet, each with its ampullaThere is one asymmetric element: a single tube (the “stone canal”) running from the oral WVS ring to the outside via the madreporite
11Surface featuresEchinoderm skin has several distinctive sets of organs protruding from their skin:Tube feet (podia)SpinesPedicillaria
12Tube feet.. Podia are not scattered haphazardly over the body surface They lie in 10 rows (5 pairs), the ambulacral groovesEach tube foot + its ampulla is isolated from the WVS by a valveTube feet vary - starfish have muscular suction cups, other forms have sticky tips.Crinoids are different - primitive
13Tube 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.
14Tube feet.. Have retractor muscles and can bend, but no extensors To extend, muscles around the ampulla contractEach podium has a nervous arc to its branch of the hyponeural system
15Role of WVSHydraulicsRespiration - O2 is exchanged between ampulla and perivisceral coelomic fluidProbably (?) this was the ancestral function of the WVS, with tubes + podia lining arms to exploit ciliary current already used in food collection
16Pedicillaria…Are defensive organs, assumed to protect against encrusting organismsAre active, independent local effector units able to inject toxins on contact
17MadreporiteAllows pressure equalization and top up water supply to the WVSIs absent in crinoids
18GonadsLie as 10 (2N) paired structures at the base of ambulacral grooves.Sexes are separate, and discharge gametes into the sea waterGonads can be large - echinoid gonads almost fill the test, and can be eaten as a delicacy.
22Crinoidea Body made of ossicles 10 arms have podia (no ampullae) feeding particles to the mouth.Arms can moveMouth and anus are both on oral side (!)
23Asteroidea “Starfish” Active predators feed on bivalvesuse suction cups to pull open the shells with forces of up to 5kgThe stomach is eversible, and can be partially inserted inside prey’s shell (enzymes but no toxins)
24EchinoideaRecipe: take a starfish and roll its 5 arms together into a ball, then fuse and calcify with an external armorThe armor is called the testVery small aboral surface
25Echinoidea Herbivores, preferring macro-algae They can be highly effective grazers, creating “urchin barrens” devoid of algaeThe mouthparts are unique, known as Aristotle’s Lantern.5 continually growing chisel teethEach tooth with 8 supporting skeletal pieces
26Irregulars All are sand burrowing Heart urchin Echinocardium has no lantern;Sand-dollars (Clypeaster) are more flattened with a lantern
27Noli tangereMany echinoids have wickedly sharp spines, which break off in your skin.Only a few fish, trigger fish attack long-spined speciesSpines are under muscular control, and can be used to move
28Noli tangereVery few echinoids are lethal to touch - their pedicillaria inject a neurotoxinToxopneustes is feared by pearl divers
29Ophiuridae - brittle stars Have arms sharply demarcated from the body disc.The internal structure of the arms involves interlocking internal ossicles, confusingly called vertebraeAre primarily detrital or filter feeders, raising their arms in a current to capture particulates
30Holothuridae- Sea Cucumbers They have no calcitic skeleton, except for spicules embedded in a leathery skinMost 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)
31Holothuridae They mainly feed on detritus Oxygen exchange is performed using gills inside their anusThey have 2 odd defensive strategies:Squirting a sticky gooVoiding their entire intestines