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