Illutsrated by:

Introduction

The echinoderms are exclusively marine

enterocoelous coelomata, triploblastic animals having

pentamerous radial symmetry derived from a original bilateral

symmetry without definite head or brain with an endoskeleton

of calcareous plates or spicules embedded in the skin with a

peculiar water vascular system of coelomic origin, pushing

out of the body surface, numerous small delicate projections

the podia or tube feet, with an ectodermal nervous system

without nephridia and with gonads opening directly to the

interior by special ducts.

The name “Echinodermata” literally means “spines or

prickly skinned” and refers to the conspicuous spines

possessed by their skin. This name was at first used by

“Jacob Klein” for echinods.

The Greeks applied the name echinas to the

hedgehog as well as the sea urchin; both having a prickly

appearance. It was “Leukart” who first established

Echinodermata as a distinct group of animal kingdom.

General Characteristics

- The echinoderms are exclusively marine and are

among the most common and widely distributed of marine animals.

- They occur in all seas from the intertidal zone to the great depths.

- Radially symmetrical and nearly always pentamerous.

- Body is triploblastic, coelomate with distinct oral and aboral surfaces and without definite head and segmentation.

- They are moderate to considerable size but none are microscopic.

- Body shape rounded to cylindrical or star-like with simple arms radiating from a central disc or branched

- Surface of the body is rarely smooth, typically it is

covered by five symmetrically space radiating grooves

called ambulacra with five alternating inter-radii or

inter ambulacra.

- Body wall consists of an outer epidermis, a middle

dermis and an inner lining of peritoneum.

- Endoskeleton consists of closely fitted plates

forming a shell usually called theca or test or may be

composed of separate small ossicles.

- Presence of water vascular or ambulacral system is

the most characteristic feature. It consists of tubes

filled with a watery fluid.

- Alimentary tract is usually coiled tube extending

from the mouth located on the oral surface to the

annus on the oral or aboral surface.

- Circulatory or haemal or blood lacunar system is

typically present.

- Excretory system is wanting.

- Nervous system is primitive, consisting of networks

concentrated into the radial ganglionated nerve cords.

- Sense organs are poorly developed.

- Sexes are usually separated (dioecious) with few

exceptions. Gonads are simple with or without simple

ducts.

- Reproduction is usually sexual; few reproduce

asexually or by regeneration.

- Fertilization is external, while few echinoderms

are viviparous.

- Development is indeterminate including

characteristic larvae which undergo

metamorphosis into the radially symmetrical

adults.

- Radially symmetrical and nearly always pentamerous.

- Body is triploblastic, coelomate with distinct oral and

aboral surfaces and without definite head and

segmentation.

- Surface of the body is rarely smooth, typically it is

covered by five symmetrically space radiating grooves

called ambulacra with five alternating inter-radii or inter

ambulacra.

- Endoskeleton consists of closely fitted plates forming a

shell usually called theca or test or may be composed of

separate small ossicles.

- Presence of water vascular or ambulacral system is the

most characteristic feature. It consists of tubes filled with a

watery fluid.

Identifying Characters of Echinodermata

- Alimentary tract is usually coiled tube

extending from the mouth located on the oral

surface to the annus on the oral or aboral surface.

- Excretory system is wanting.

- Nervous system is primitive, consisting of

networks concentrated into the radial

ganglionated nerve cords.

- Sense organs are poorly developed.

- Sexes are usually separated (dioecious) with

few exceptions. Gonads are simple with or without

simple ducts.

Classsification: Classification of Echinodermata is not universally

taken yet. But at present, two format of classification can be seen.

One of them is given by G.Ulbaughs (1967). This format is

based on the basis of external features of living and extinct species.

Other one is given by A.B. Smith (1984). In this format,

Phylum Echinodermata is divided into two sub-phylum. Now we’ll

discuss about this classification________

Sub-phylum: Eleutherozoa

Class:Asteroidea

Characteristics:

- Starfishes or sea stars.

- Arms five or more and not sharply marked off from the

central disc.

- Tube feet in orally placed ambulacral grooves; with

suckers.

- Annus and madreporite aboral.

- Pedicellariae present.

- Free-living, slow creeping, predaceous and

scavengerous.

Characteristics:

Fossil Pelazoic Sea stars.

Platasterias latiradiata is

the only living species.

Class: Asteroidea

Characteristics:

They are living sea

stars.

Class: Asteroidea

Characteristics:

- Brittle stars and allies.

- Body star like with arms sharply marked off from the

central disc.

- Pedicellariae absent.

- Stomach sac like; no annus.

- Ambulacral grooves absent or covered by ossicles;

tube feet without suckers.

- Madreporite oral.

Example: Ophiura, Gorgonocephalus, Asteronyx.

Class: Ophiuroidea

Class: Echinoidea

Sub-Class: Bothriocidaroida

Sub-Class: Regularia

Sub-Class: Irregularia

Characteristics:

- A single row of plates in

each inter ambulacral

area.

- Without typical lantern.

- Madreporite radial.

Example: Single extinct

Ordovician genus

Bothriocidaris.

Class: Echinoidea

Sub-Class: Bothriocidaroida

Sub-Class: Regularia

Sub-Class: Irregularia

Characteristics:

- Body globular pentamerous,

with two rows of inter-

ambulacral plates in existing

members.

- Mouth central.

- Aristotle’s lantern well

developed.

- Annus central on aboral

surface with well developed

apical plates.

- Madreporite oral.

Example:

Palaeodiscus,Melonechinus,

Diadema,Echinus.

Class: Echinoidea

Sub-Class: Regularia

Sub-Class: Bothriocidaroida

Sub-Class: Irregularia

Characteristics:

- Body oval or circular, flattened

oral-aborally.

- Mouth oral or displaced

anteriorly on oral surface.

- Annus marginal, outside the

apical system of plates.

- Tube feet generally not

locomotor.

Example:

Echinoneus,Cassidulus,Echinoc

yamus, Spatangus.

Class: Echinoidea

Sub-Class: Irregularia

Sub-Class: Bothriocidaroida

Sub-Class: Regularia

Characteristics:

- Sea cucumbers.

- No arms and no spines.

- Body elongate on oral-aboral axis, body wall

leathery.

- Mouth anterior surrounded by tentacles.

- Ambulacral grooves concealed; tube feet with

suckers.

- Usually with respiratory tree for respiration.

Example: Cucumaria, Holothuria, Molpadia, Synapta.

Class: Holothuroidea

Sub-Phylum: Pelmatozoa

Greek word “pelmatos”

means stalk and Greek word

“zoois” means animal. That’s

mean Pelmatozoa stands for

“stalked animal”. Their

characteristics are stalked

and sedentary. Class:CrinoideaCharacteristics:

- Sea lilies.

- Body attached during part or whole of life by an aboralstalk.

- Mouth and annus on oral surface.

- Arms with pinnules.

- Tube feet without suckers; no madreporite, spines and pedicellariae.

- Ciliated ambulacral grooves on oral surface.

Example: Antedon, Neometra.

General Morphology of Star Fish or Sea Star

There are too many varieties of species consist

in phylum Echinodermata. Among them Class

Anthozoa is rich of species. Metridium

marginatum, known as sea-star or star fish under

this class, here’s some description about this.

Classification:Phylum: Echinodermata

Sub-phylum: Eleutherozoa

Class: Asteroidea

Order: Euasteroidea

Family: Forcipulata

Genus: Asterius

Species: Asterius

rubens

Shape :

The echinoderms present at a great diversity of organization.

The original biradial symmetry is often obscured by a well developed

radial symmetry. The most striking characteristic of the group is their

pentamerous radial symmetry, i.e. body can be divided into five parts

arranged around a central axis. The radially arranged parts are

called antimeres, with the mouth at centre.

Star fishes have a flattened body which is pentagonal or

produced into five, six or even more tapering arms. Bristle stars have

a small rounded central disc with five distinct, slender, jointed and

flexible arms. Sea urchins are hemispherical while sand dollars are

disc like with a cup like central disc bearing five branched arms.

Size:

Some fossil and dollars measure only 6mm in diameter, whereas a

fossil crinoid had a stem about 22 meters long. However, the living

echinoderms are of moderate size. The largest star fish, Pycnopodia

helianthoides is about 30 cm. across. The shell of the largest urchins

Echinosome hoplacantha has a diameter of 30 cm. One sea cucumber,

Synapta maculata, measures about 2 meters in length and 5cm in diameter.

Coloration:Echinoderms adapted various types of color, such

as yellow, orange, brown & purple. The body has two

surfaces. The upper convex & darker side is called the

aboral & abactinal surface. The lower surface is flat, less

pigmented & is called the oral actinal surface.

Habit & Habitat:Asterias are exclusively marine, bottom dwelling or

benthic animal, inhabiting various types of bottom mainly in the

littoral zone where they crawl about or may remain quiescent at

times. Asterias forbesi is found equally on hard, rocky, sandy or

soft bottom, while other species of Asterias are generally

solitary but under certain ecological conditions. Such as to

avoid direct sunlight or excessive drying, many individuals may

gather at some place for the purpose of protection. Most of

them are nocturnal, remain quiet in day time and become

active during night. They move by crawling on the bottom

mostly at a rather slow rate. All sea stars are carnivorous and

feed voraciously and almost any available slow moving or

sessile animals, chiefly on polychaetes, crustaceans, mollusks,

and other echinoderms. Many species of Asterias exhibits

various types of biological relationships such as parasitism and

commensalism etc. Sea stars have remarkable power of

autonomy and regeneration.

Food & Feeding

Mechanism:

Sea star is a

voracious carnivore. It

prefers to prey on

sedentary marine

animals like clams,

oysters, mussels,

snails, crabs,

barnacles, worms, and

even sea urchins and

other smaller star fish.

At times it also feeds

on small fishes and

injured and dead

animals.

Sea star ingests and digests food in a

fascinating manner. Prey is captured and held in

position by the arms and tube feet. It may be

interesting to note how a sea star devours clams and

other bivalves. It conveniently creeps over the clam,

arches its body over it and firmly attaches its tube feet

to two shell valves in such a way that the ventral

margin of the clam comes to lie in front of its mouth. It

then tries to pull apart the two valves held tightly

together by the powerful adductor muscles. A few

tube feet at the tips of arms are also attached to the

substratum to aid in the process. Pull is steadily

maintained till the adductor muscles of clam are

exhausted and give way. Adductor muscles cannot, as

a rule, remain in a continuous state of contraction for

long time. When the valves finally gape, cardiac

stomach of sea star is everted into the mantle cavity

of the clam to devour it.

Body Structure:

Oral Surface:The side of the body, which in natural condition remains towards

the substratum and contain the mouth or oral opening is called oral

or actinal surface. The oral surface bears the following structures.

(i) Mouth:

On the oral surface, in the centre of the

pentagonal central disc is an aperture, the actinosome or

mouth. It is a pentagonal aperture with five angles, each

directed towards an arm. The mouth is surrounded by a soft

and delicate membrane. The peristomial membrane or

peristome and is guarded by five groups of oral spines or

mouth papillae.

(ii) Ambulacral Grooves:

From each angle of the mouth radiates a

narrow groove called the ambulacral groove which runs all

along the middle of oral surface of each arm.

(iii) Tube Feet or Podia:

Each ambulacral groove contains four rows of

locomotory organs, food capturing, respiratory and sensory

organs called tube feet or podia. The tube feet are soft.

Thin walled, tubular, retractile structures provided with

terminal discs or suckers.

(iv) Ambulacral Spine:

Each ambulacral grooves is bordered and

guarded laterally by two or three rows of movable

calcareous ambulacral spines which are capable of

closing over the groove.

(v) Sense Organs:

Sense organs include five unpaired

terminal tentacles and five unpaired eye spots. The lip

of each arm bears a small median, non retractile and

hollow projection. The terminal tentacles act as a

tactile and olfactory organ. At the base of each

tentacle occurs a bright red photo-sensitive eye spot

made up of several ocelli.

Aboral Surface:The side of the body, which remains directed upward or

lowers the upper surface is convex & of light orange to purplish

color is called aboral or abactinal surface. The aboral surface

bears following structures________

(i) Annus:

A minute circular aperture, called the annus. It

is situated close to the centre of the central disc of aboral

surface.

(ii) Madreporite:

At the aboral surface of the central disc occurs

a flat sub-circular, asymmetrical & grooved plate called

madreporiteplate. Madreporite between the basis of two of

the five arms. The surface of the madreporite is marked by

a number of radiating, narrow, straight or slightly wavy

grooves with pores in them.

(iii) Spines:

The entire aboaral surface is covered with

numerous short, blunt, calcareous spines or tubercles. The

spines are variable in size and arranged in irregular rows

running paralleled to the long axes of the arms.

(iv) Papulae or Gills:

Between the ossicles of integument are

present a large number ofminute dermal pores.

Through each dermal pore projects out a very small,

delicate, tubular conical finger like, thin walled,

membranous & retractile projection called the dermal

branchia or gill or papillae.

(v) Pedicellariae:

Besides the spines & gills, entire aboral

surface is covered by manywhitish modified spines

like tiny pincers or jaws called pedicellariae. The oral

surface also bears pedicellariae. Each pedicellariae

consists of a long or short, flexible stalk having no

internal calcareous support.

Water Vascular SystemThe water vascular

system is a modified part of

coelom & it consists of a

system of sea water filled

canals having certain

corpuscles. It plays most vital

role in the locomotion of the

animal.

It comprises________

(i) Madreporite

(ii) Stone Canal

(iii) Ring Canal

(iv) Tiedemann’s Bodies

(v) Polian vesicles

(vi) Radial Canal

(vii) Tube feet

Here’s some description____________

(a) Madreporite:

The madreporite is rounded calcareous plates

occurring on the aboral surface of the central disc in inter radial

position. Its surface bears a number of radiating, narrow,

straight or wavy minute pores at its bottom.

(b) Stone Canal:

The ampulla opens into an s-shaped stone canal.

The stone canal extends downwards & opens into a ring canal,

around the more the walls of stone canal is supported by a

series of calcareous ring.

(c) Ring Canal:

The ring canal or water ring is located to the inner

of the peristomial ring of ossicles and directly above to the

hyponeural ring sinus. It is wide & pentagonal or five sided.

(d) Tiedemann’s Bodies:

The ring canal gives out inter-radially nine, small,

yellowish, irregular or rounded glandular bodies called

racemose or tiedemann’s bodies from its inner margins. The

tiedemann’s bodies rest upon the peristomial ring of ossicles.

(e) Polian vesicles:

The ring canal gives off on its inner side in the inter radial

position one, two or four little, pear-shaped, thin walled,

contractile bladders or reservoirs with long necks called polian

vesicles.

(f) Radial Canal:

Along each radius, the ring canal gives out a

radial canal which extends up to the tip of the corresponding

arm. Radial canal ics below the ambulacral ossicles and

terminates as lumen of the terminal tentacle.

(g) Lateral Canal:

In each arm, the radial canal gives out two

series of short, narrow, transverse branches called

lateral or podial canals. Each lateral canal is attached

to the base of tube feet & is provided with a valve to

prevent backward flow of fluid into the radian canal.

(h) Tube feet:

There are four rows of tube feet in each

ambulacral groove. A tube feet or podium is a hollow,

elastic, thin walled, closed cylinder or sac-like

structure having an upper sac-like ampulla, a middle

tubular podium & a lower disc like sucker.

Locomotion of Echinoderms:

Locomotion is performed with the help of water-vascular

system which sets up a hydraulic pressure. By the action of cilia

lining the ambulacral canals, sea water enters through madreporite

and fills up all canals of the system including the tube feet. Body is

moved by the stepping action of tube feet which are alternately

adhered to and released from the substratum. One or two arms, in

the desired direction of movement, are raised from the substratum.

Simultaneously the ampullae of tube feet to these arms contract by

the action of their circular muscles. This increases the hydraulic

pressure within the tube feet, which consequently elongate, extend

forward and adhere firmly to the substratum by vacuum action of

their suckers. Adhesion is further strengthening by mucus secreted

by the tips of the tube feet. Then, by muscular activity, tube feet

assume a vertical posture, dragging the body forward. Tube feet

then shorten by contracting their longitudinal muscles and forcing

some water back into their ampullae. Consequently, the suckers

release their hold on the substratum.

During locomotion, one or two arms serve as leading

arms, and all the tube feet extend in the same

direction in a coordinated manner. However, the tube

feet may not work in unison. As a result, the star

moves forward steadily but slowly, at a speed of

about 15 cm per minute. Sea stars can also climb up

the rock by the combined action of their tube feet.

I f a sea star is accidentally turned upside

down; it can correct its posture by folding or arching

its arms. In folding action, tips of one or two arms

twist to bring their tube feet in contact with the

substratum, thus permitting the whole body to fold

over and right itself. In arching the upturned body is

first raised on its arm tips and then rolled over.

Larval Development:Development of a sea star includes three larval stages __

dipleurula, bipinnaria and branchiolaria.

(1) Dipleurula larva or early bipinnaria:

First larval stage in all echinoderms is called early

bipinnaria. This larva closely resembles a hypothetical dipleurula

larva. It is believed that all modern echinoderms have been

derived from a dipleurula-like ancestor. Early bipinnaria is an

egg-shaped and bilaterally symmetrical organism. An anterior

mid-ventral ectodermal invagination, called stomodaeum,

becomes continuous with the archenteron to form the larval

mouth. Blastopore becomes the larval annus. Archenteron

differentiates into a digestive tract made of oesophagus, stomach

and intestine. Uniform ciliation of gastrula is replaced in

dipleurula by two ciliary bands__ a perioral band surrounding the

mouth. Larva feeds actively on unicellular algae, particularly

diatoms. Food particles are collected from the water current

produced by the aboral or stomodaeal cilia. As larva swims

forward, with the help of perioral band of cilia, It rotates

clockwise.

(2) Bipinnaria larva:

Dipleurula soon forms on its front side a large preoral

lobe, which becomes bordered by a preoral loop of cilia.

Simultaneously, on each lateral side, it forms three lateral lobes

which become bordered by a postoral loop of cilia. Pre and

post-oral loops arise by the sliting of the perioral band of cilia.

Larva thus formed is called bipinnaria. It is bilaterally

symmetrical. It swims and feed freely and after some weeks

transforms into the next larval stage, the branchiolaria larva.

(3) Brachiolaria larva:

The lobes of the bipinnaria develop into long, slender,

ciliated, contractile structures called larval arms. From the

preoral lobes arise three, short and non-ciliated appendages,

each ending in a sucker or adhesive disc. The appendages are

called bipinnarian arms or fixing processes. Larva is now called

bipinnaria. It is also bilaterally symmetrical and swims and

feeds actively. It gradually metamorphoses into a small sea star.

Larval Forms in Echinodermata:

No other group of animals has such

complicated metamorphosis in the course of

development. Development may be direct or

indirect. In direct one, the larval stages are missing

while in indirect one, various types of free

swimming larvae are formed. In each class, a few

members, are viviparous, that is, they brood their

young in a sort of brood pouch on the surface of

their body. The development of larva takes place in

a typical deuterostomous fashion. In most cases

the characteristic free swimming larvae developed

externally which are of phylogenetic significance.

[I] Class 1: Asteroidea

(a) Bipinnaria larva:Two types of development occur in asteroids. The

direct type has large, yolky eggs and a free swimming larval stage.

The indirect stage has homolecithal eggs with little yolk and a free

swimming larval stage. After hatching the larva develops cilia and

begins a free swimming life. The larva feeds on diatoms as an

alimentary canal is formed. The presence of powerful ciliary band on

the stomodaeal walls help in feeding. Two lateral longitudinal

locomotory ciliated bands develop which connect in front of mouth,

forming a preoral loop and in front of the annus, to form a preoral

loop. Preoral loop later separates or in some cases develops

independently into an anterior ciliated ring around the body. Three

lobe sor projections are also develop on each side of the body

bordered by ciliary bands. This larva is known as bipinnaria and

develops in 2 to 7 days.

(b) Brachiolaria larva:Bipinnairia larva transforms into brachiolaria larva which

develops three short arms at preoral lobe, known as brachiolar arms.

They contain coelomic extensions and adhesive cells at their tips. An

adhesive glandular area at their base acts as a sucker. Appearance of

the sucker marks the beginning of metamorphosis.

[II] Class 2: Ophiuroidea

Ophiopluteus larva:

Pluteus is the free swimming larva in brittle star s which

is known as ophiopluteus. It is similar to echinopluteus of

echinoids with the only difference that the former has fewer

arms than the later. The posterolateral arms are the longest

and directed forward. After gastrulation the arms develop

gradually. Posterolateral arms are form first. After 4, 10 and 18

days, anterolateral, postoral and posterodorsal arms develop,

respectively. Ciliated bands accompany the arms edges.

Internally the larva contains coelomic chambers and

archenteron. Internal development proceeds in the same way

as in other classes. While free swimming metamorphosis of the

larva starts, there being no attachment stage. Tiny serpent star

sinks to the bottom to begin its adult existence.

[III] Class 3: Echinoidea:

Echinopluteus larva:

Larva is formed after gastrulation. Gastrula

becomes conical, one side of which flatters to form

the oral surface. Stomodaeal invagination

communicates with archenteron and the gut is

differentiated into mouth, oesophagus, stomach and

intestine. Blastopore remains as larval annus. Larva

begins to form projections which develop into arms.

There are six arms namely, preoral, anterolateral,

anterodorsal, postoral, posterodorsal and

posterolaeral.

[IV] Class 4: Holothuroidea

(a) Auricularia larva:

After gastrulation and formation of coelomic sacs

and gut, the embryo becomes a free swimming larva

called auricularia larva, within three days. It is transparent,

pelagic about 0.5 to 1 mm in length. It swims about by a

ciliated band which forms preoral loop and an anal loop.

Internally, larva has a curved gut with sacciform

stomach, hydrocoel and right and left stomatocoels.

Some giant auricularians of unknown adults

reported from Bermuda, Japan and Canary Islands

measure about 15mm in length and possess a frilly

flagellated band.

(b) Doliolaria larva:

It is a transitional stage from auricularia and appears

barrel-shaped and alike doliolaria of crinoids. Continuous

ciliated band breaks in 3 to 5 flagellated rings. Mouth is

shifted to anterior and annus to posterior pole.

Metamorphosis is gradual during which it acquires 5

tentacles and 1 to 2functional podia. As such it is

sometimes known as pentactula. After appearance of

more podia and tentacles, sea cucumber settles to the sea

bottom and leads an adult mode of life.

[V] Class 5: Crinoidea

(a) Doliolaria larva:

It hatches as a free swimming larva. Body has 4 to 5 ciliated bands with an apical sensory plate at the anterior end provided with a bunch of cilia. There is an adhesive pit over the first ciliated band, near the apical plate in the mid ventral line. Between second and third ciliated band lies the stomodaeum or vestibule. Skeleton also develops at this larval stage. After the differentiation into prospective organs, larva attaches itself and internal organs rotate at an angle of 90 degrees from ventral to posterior position. Larva forms a stalk and is now referred as cystidean or pentacrinoid larva which, after sometime metamorphoses into adult.

Economic Importance of Echinodermata:

The economic importance of Echinoderms is considerable to

man. Of all echinoderms the star fish and sea cucumber are

of much economic value to man.

The dried skeleton of echinoderms have been crushed and

utilized as fertilizer on ground.

Echinoderms are quite rich in calcium and nitrogenous

content so that they are used as food and lime supply four

poultry.

Among the Pacific Island, South Pacific Ocean and China,

sea cucumbers are used as food by man. They are boiled

and dried in the sun and sold in the market. In America sea

cucumber has been used especially in soup for several

years.

The ovaries of sea urchins and the eggs of starfish are eaten, either raw or cooked by the Europeans as well as by the people of tropical regions of South America and the Mediterranean. Sea urchins are also used in research particularly as model in development biology and other sections of science.

Starfish, urchins and cucumber are good scavengers. The latter are perpetually ingesting mud and sediment of the sea bottom the contained organic food, thus helping in the decomposition processed of the organic material continuously accumulating at the sea bottom. Their food includes seaweeds, small crustaceans, mollusks, tube worms, small animals matter and other small organisms and bottom debris. It has been said that an ocean without echinoderms might become a putrid cesspool.

Natives of the Pacific Islands use juices from the body wall of certain sea cucumbers to poison fish in tidal pools.

Negative Aspects:

The star fish are of particular concern to man as

they destroy clams oysters and other marine

mollusks that serve as human food.

A star fish in aquarium devours nearly 10 oysters

or clams in a day. They are universally hated by

oyster and clean fisherman because of

tremendous damages they do to those

commercially important food animals each year.