Echinodermata

ECHINODERMATA

(Greek.echin = hedgehog + derma = skin)
Echinodermata includes the starfish, sea urchins, brittle stars, sea cucumbers and feather stars. Just like the name says they have spines or spicules on their skins to a varying degree in the different groups. The usually have a radial symmetry with no anterior or posterior, but radiating out from a central point. Radial symmetry is present only in the adult form (larvae bilateral). They reproduce sexually and most of the species release eggs and sperm into the water where the fertilization then occurs. The huge number of gametes produced compensates for the wastefulness of this type of fertilization. The larvae are attractive, planktonic creatures that are transparent and has long slender arms.

Characteristics:

1. Symmetry is usually radial in adults, bilateral in larvae. Triploblastic. Most of the organs are ciliated. No segmentation.

2. Body surface of five symmetrical radiating areas, or ambularca, from which the tube feet project.

3. Body covered by a delicate epidermis over a firm mesodermal endoskeleton of moveable or fixed calcareous plates, usually in a definite pattern. Often with spines (skin leathery and plates usually microscopic in Holothuroidea).

4. No head. The body is arranged on a oral, aboral axis.

5. The Coelom is enterocoelous, large and lineated with ciliated peritoneum and subdivided during development to give rise to the unique water-vascular system. They poses tubular feet for motion, food handling and respiration. A complex hemal system present.

6. Respiration by minute dermal branchiae (skin gills) or papulae protruding from the coelom by tube feet and in Holothuroidea by coral respiratory trees.

7. The nervous system is diffuse and composed typically of three rings centered on the mouth region with radiating branches.

8. The sexes are seperate, with rare exceptions, and all are alike externally. The gonads are large with simple ducts. Eggs are abundant and are usually fertilized in the sea. The larvae are bilateral, microscopic, ciliated, transparent and usually free swimming, with conspicuous metamorphosis.

9. No excretory system is present.

ASTEROIDEA

Starfish

The starfish are probably the most familiar echinoderms. They are identified by their flattened bodies which merge into five gradually into five (somtimes more) thick and fleshy, tapering arms.

Asteroidea viewed form above while in different stages of disection.

If one of these arms were to break of the animal would quite simply grow a new one and the lost arm will simply grow itself a new animal. The mouth is located on the central part of the under surface of the body and the anus in turn is located on top.

Oral surface of disc.

Each one of the arms contain their own respiratory, digestive and reproductive organs. Some starfish are dotted with tiny pedicellaria, resemling grappling irons. These structures are three-jawed, usually equiped with venom glands and serve to protect the animal against predators.

A small area on the dorsal surface enlarged.

A groove, that runs from the mouth along the underside of each arm, protects rows of tiny hydraulically-opperated tubefeet with suckered tips. These tubefeet are responsiple for the creeping movements of the starfish.

Interior view of a part of an ambulical ridge.

Some species of Astroidea feed on detritus or microalgae, but many are predators. Among those that are predators some species prevent the mussels from a monopoly of the shores and some also attack corals. Some species are scavengers.

CRINOIDEA

Feather stars

These are elegant echinoderms with small, soft bodies that are surrounded by 10 or more elongate and upwardly raised arms, that each comprise of a central axis and then numerous side branches(pinnules).

A stalked feather star.

The oral surface of a feather star.

Food particals are then captured by the arms and are passed along ciliated grooves back to the mouth (situated on the upper surface). Most of the body cavity is filled by a plain U-shaped gut that ends in an cone in close vicinity to the mouth. The gonads are situated in the pinnules closest to the body and they swell greatly in size during the breeding season, and ultimately tearing and releasing the sperm or eggs into the water. The Crinoids are essentially sedentary. They poses a ring of claw-like segmented limbs (the cirri) beneath the body with which they grip to the substratum, but they have the abillity to crawl or even swim with the use of their arms. Attached stalked crinoids (sea lilies) are abundant in the fossil record but very few species survive today.

OPHIUROIDEA

Brittlestars

Brittle stars consist of a flat, circular body (disc) with five or more long thin arms that perform snakelike movements. These arms break of extremely easyly, hence the name "Brittlestars". The segments of the arms are each covered by 1 - 3 plates (their number and shape help with the identification). The sides of the arms are often spiny. They move by the snake-like undulation of their legs. The mouth is situated at the lower surface and is surrounded by five toothed jaws. The texture of the disc allows division of the brittlestars into 3 groups:
1. Those with grannules
2.Those with short spines
3.Those with a leathery or scaley texture
Just above the beginning of each arm there are usually two enlarged scales (radial shields).

Oral view of the disk of a brittle star.

Diagramatic cross section through the arm of a brittle star.

A brittle star on the surface of a sponge.

Most of the brittle stars have minute planktonic larvae, but a few brood their young in their bodies and give birth to extremely small copies of themselves. Most of the species are detritus feeders.

ECHINOIDEA

Sea urchins

Most of these creatures have a globular body that is encased in a hard calcium carbonate shell (the test). The test is formed by the fusion of the spines in the skin. The mouth occurs centrally on the underside, with the anus ussually on the upper side. Through tiny pores in the test, tube feet protrude (five double rows of tube feet run from the apex down the sides of the test) and serve for movement for the sea urchin. Long protective spines project from the test.

Internal structure of the sea urchin Arbacia (lateral view).

Oral view of the Common Atlantic sea urchin (Arbachia Punctilata).

Aboral view of the Common Atlantic sea urchin (Arbachia Punctilata).

Diagramatic cross section through the body wall of a sea urchin.

They are mounted on a ball-and-socket and can be swiveled to face any threat directly. In most of the cases the spines are harmless to humans, but if stung by one it can be irritating. A few species have long lance-like spines with backwards pointing serrations. They can penetrate very deeply and then break of.Some of these longspined urchins discharge poisons through the tips of their broken arrows. Some sea urchins are dotted with tiny pedicellaria, resemling grappling irons. These structures are three-jawed, usually equiped with venom glands and serve to protect the animal against predators. Most Urchins are grazers (the flattened sand-dwelling species feed on detritus), and the more mobile species control the growth of the seaweed population. In the topics where there are more predatory fish, most of the urchins shelter in crevices and feast upon drift-weed. Some are even able to burrow into the rock, creating holes in which they shelter.

The Hawaiian slate pencil urchin (hetrocentrotus mammilatus).

The purple sea urchin (strongylocentrotus purpurates).

HOLOTHUROIDEA

Sea cucumbers

The cucumbers have traded in their starshaped, radial symmetry and skeletal structure, common to other echinoderms, for a elongate sausage-shaped body with a leathery skin that lies on its side. Up to five rows of tube feet that run along the side is the only reminder of their ancesteral radial symmetry. They have also become a lot more flexible with their spines being reduced to microscopic spicules. A mouth that is surrounded by 10-20 retractable feeding tentacles is situated on one end, with the anus on the other. Their sticky tentacles are used to gather detritus or to catch plankton that floats overhead.

A North-Atlantic sea cucumber (cucmaria frondosa).

The sea cucumber, disected.

When some species are desturbed they employ special tactics in order for them to survive. Some species eject long sticky threads from the anus, others digorge part or all of the digestive canal, which they subsequently regenerate while the intruder is left with the feast.