star fish.pptx

Habit and Habitat of Asterias:
 Asterias is an exclusively marine and widely distributed member of echinoderm.
 All the species under this genus are benthonic animals, because they inhabit the bottom of the sea.
 They are quite abundant on various types of sea-bottoms, specially at places where bivalves are
available as food.
 They are carnivorous and predacious animals.
 They prefer to stay on rocky or stony places where they can hide very easily and lead a sluggish life.
 Majority of the forms are photonegative and prefer to live in shaded areas.
 A few exceptions are the Asterias rubens, Asterias gibbosa, Asterias panceri, where positive response to
light is observed.

External Structures of Asterias:
 Asterias has a five-pointed star-shaped body.
 The body consists of a central disc and five symmetrically placed arms (or rays).
 Sometimes individuals with more than five arms are found in nature.
 Leptasterias has a six-rayed body.
 The genus Solaster has seven to fourteen-rayed body and Pycnopodia helianthoides possesses fifteen to
twenty-four arms.
 Occurrence of less than five arms may be possibly due to mechanical injury.
 The arms are conspicuously broad at their bases and they gradually taper towards the tips.
 The body has two surfaces, the upper convex and much darker side is called aboral or abactinal side.
 The other side is flat; less pigmented and is designated as oral or actinal side.
 The body exhibits radial symmetry.
 The imaginary lines dividing the central disc and the arms are called the radii and the intervening
regions between the radii are called inter-radii.

 The body is covered by a hard and tough covering containing
numerous calcareous plates or ossicles derived from mesodermal
tissue.
 At the center of the oral surface of the central disc, a five-rayed
aperture called mouth or actinosome is present.
 This opening is surrounded by a membranous peristome and it also
possesses five groups of oral papillae.
 Five narrow grooves, called ambulacral grooves, one on each arm,
run orally from the five-rayed aperture to the extremity of the arms.
 The edges of these grooves are provided with two or three rows of
movable calcareous ambulacral spines.
 The aboral side contains numerous irregular rows of short and stout
spines supported by ossicles.
 The oral and aboral surfaces of the body are separated by a row of
prominent spines.
 Many dermal pores are situated in the spaces between the ossicles
on the aboral side.
 From each dermal pore projects a very small delicate, finger-like,
membranous retractile process, called dermal branchia or papula.
 It is respiratory in function.

A minute aperture, called anus, is situated at the centre of the aboral surface.
On the same side of the body, between the bases of two of the five arms, there lies a structure,
called madreporite.
The madreporite is a flat disc-like body with radiating grooves.
The presence of more than one madreporite in some species is possibly due to increase of the
number of arms beyond the normal number of five.

Body Wall of Asterias:
(i) Cuticle:
The body surface is clothed with a definite cuticle consisting of two layers,
an outer thick homogeneous layer and an inner delicate layer.
(ii) Epidermis:
Just beneath the cuticle lies a layer of ciliated epithelium which extends
over all the external appendages of body such as spines, pedicellariae, tube
feet and gills, etc.
The epidermis is composed of a variety of cells such as ordinary flagellated
or ciliated columnar cells, neurosensory cells, mucous gland cells or goblet
cells having finely granular contents, muriform gland cells filled with
coarse spherules and the pigment granules which provide characteristic
external colouration to the animal.
(iii) Nervous Layer:
Beneath the epidermis lies a nervous layer, varying in thickness in different
areas and penetrated by the attenuated bases of the epidermal cells on
their elastic filaments.

(iv) Basement Membrane:
Just below the nervous layer lies a delicate basement membrane which separates the nervous layer and
epidermis from the underlying dermis.
(v) Dermis:
The dermis is composed of fibrous connective tissue developed from the mesoderm. It is the thickest layer of
body wall and has two regions outer and inner. The outer dermal region secretes and houses the endoskeletal
ossicles and binds them together, while the inner dermal region contains numerous blood containing spaces
called perihaemal spaces.
(vi) Muscular Layer:
The muscular layer consists of smooth muscle-fibres. It is differentiated into an outer circular muscle layer and
inner longitudinal muscle layer. These muscle layers are on the whole weakly developed except in the aboral wall
where stronger longitudinal bundles radiate from the centre of the disc along the mid-dorsal line of each arm, to
bend the arms aborally.
(vii) Coelomic Epithelium:
The innermost layer of body wall lines the coelom and is composed of flagellated cuboidal cells of mesodermal
origin. The innermost layer of body wall is called coelomic epithelium or peritoneum.

Coelom in Asterias:
Asterias possesses a true and spacious coelom which is lined by a coelomic epithelium of ciliated cuboidal cells.
It consists of various compartments, viz.:
(i) A perivisceral coelom extending in central disc and rays and surrounding the visceral organs such as digestive tract
and the gonads,
(ii) Coelom of water vascular system,
(iii) Axial sinus,
(iv) Perihaemal sinus and canals and,
(v) Genital sinuses, etc.
The coelom is filled with a colourless, alkaline coelomic fluid which contains various dissolved nutrients such as amino
acids, fatty acids, glycerol and glucoses, etc. Besides nutrients, the coelomic fluid also contains two main types of
phagocytic amoeboid corpuscles, the amoebocytes or coelomocytes; coelomocytes with ordinary slender pseudopodia
and coelomocytes with petaloid pseudopodia.
The coelomic fluid, like the haemolymph of Arthropoda, bathes the tissue of the body and performs the function of
circulatory system. It distributes the nutrients to various body cells and also performs the respiratory as well as
excretory functions.

Alimentary Canal of Starfish:
In Asterias, the alimentary canal is tubular, straight, short and extends vertically along the oral-aboral axis in the
central disc.
1. Mouth:
The mouth is the anterior most aperture of alimentary canal and it is situated in the center of the peristomial
membrane of the oral surface.
It is provided with a sphincter muscle and radial fibres and is capable of great expansion and retraction.
The mouth leads upward into the oesophagus.
2. Oesophagus:
The oesophagus is a very short, wide and vertical tube.
It opens aborally in the stomach.

3. Stomach:
 The stomach is a broad sac and fills the interior of the disc.
 It is typically divided by a horizontal constriction into a voluminous oral part, the cardiac stomach and a
flattened aboral part, the pyloric stomach.
 The cardiac stomach has a muscular, highly folded wall bulged out to form five lobes, one opposite each
arm.
 The cardiac stomach is connected to the ambulacral ridge of each arm by ligaments of muscles and
connective tissues called mesenteries or gastric ligaments which serve to anchor the cardiac stomach in
place.
 During the feeding process, the cardiac stomach can be everted through the mouth by the contraction of
muscles of body wall.
 The retraction of cardiac stomach is brought about by five pairs of retractor muscles which arise from
the lateral sides of the ambulacral ridge.
 The pyloric stomach is much smaller, flat and pentagonal sac. It communicates with the intestine.

4. Intestine:
 The intestine is a short, narrow, five sided tube that runs straight upward to open out at the anus.
 It gives off two or three little hollow diverticula called intestinal or rectal caeca placed inter-radially.
 The intestinal caeca are brown in colour and each bears several short, irregularly-shaped diverticula.
 5. Anus:
 The intestine opens on the aboral surface by a posterior most aperture of the alimentary canal called
anus.
 The anus is situated eccentrically on the aboral side of central disc.

Digestive Glands:
 To the pyloric stomach are attached ten, long brownish or greenish glandular appendages variously
called pyloric caeca, digestive glands, branchial caeca, hepatic caeca, etc.
 There are two pyloric caeca in each arm, each suspended from the aboral wall of the arm by two
longitudinal mesenteries that enclose between them a coelomic space continuous at its central end with
the general coelom of the disc.
 Each pyloric caecum consists of double series of hollow lobulated sacs that open into a central tube duct.
 The two ducts forming a pair of caeca of each arm unite to form a main pyloric duct that opens into the
pyloric stomach at one of its angles.

Histology of Digestive Gland:
 Histologically the pyloric caeca are complex structures and are lined by ciliated columnar epithelium
which is composed of four types of epithelial cells.
 The current producer cells bear longer flagella and maintain a steady circulation of the fluids and
digested food in the cavities of the caeca; the mucous cells produce mucus; the secretory or granular
cells secrete digestive enzymes to convert proteins into peptones, starch into monosaccharide sugars and
fats into fatty acids and glycerol, and the storage cells store reserve food such as lipids, glycogen and
polysaccharide-protein complex.
 The pyloric caeca function like pancreas of vertebrates.

Water Vascular System
of Asterias:

(i) Madreporite:
 The madreporite is a rounded calcareous plate
occurring on the aboral surface of the central disc in
inter-radial position.
 Its surface bears a number of radiating, narrow,
straight or wavy grooves or furrows.
 Each furrow contains many minute pores at its bottom.
 Each pore leads into a very short, fine, tubular pore
canal which passes inward in the substance of the
madreporite. There may be about 200 pores and pore-
canals.
 The pore-canals unite to form the collecting canals
which open into an ampulla beneath the madreporite.

(ii) Stone Canal:
 The ampulla opens into a S-shaped stone canal.
 The stone canal extends downwards (orally) and opens into a ring canal, around the mouth.
 The walls of stone canal are supported by a series of calcareous rings.
 The lumen of stone canal is lined by very tall flagellated cells.
 In embryonic stages and in young Asterias, the stone canal remains a simple tube but in adult Asterias,
lumen of stone canal possesses a prominent ridge with two spirally rolled lamellae which by branching
become more complicated in structure.
 During its course, the stone canal is en-sheathed by a wide, thin-walled tubular coelomic sac, called
axial sinus.
(iii) Ring Canal:
 The ring canal or water ring is located to the inner side of the peristomial ring of ossicles and directly
above (aboral) to the hypo neural ring sinus.
 It is wide and pentagonal or five sided.

(iv) 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.
 The actual function of Tiedemann’s bodies is still unknown, however, they are supposed to be lymphatic glands to
manufacture the amoebocytes of the water vascular system.
(v) 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.
 They are supposed to regulate pressure inside ambulacral system and to manufacture amoeboid cells of ambulacral
system.
(vi) Radial Canal:
 From its outer surface the ring canal gives off a radial water canal into each arm that runs throughout the length of
the arm and terminates as the lumen of terminal tentacle.
 In the arm the radial water canal runs immediately to the oral side of the ambulacral muscles.

(vii) Lateral Canals:
 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 a tube foot and is provided with a valve to prevent backward
flow of fluid into the radial canal.
(viii) Tube Feet:
 As already mentioned, there are four rows of tube feet in each ambulacral groove.
 A tube foot or podium is a hollow, elastic, thin-walled, closed cylinder or sac-like structure having an
upper sac-like ampulla, a middle tubular podium and a lower disc-like sucker.
 The ampulla lies within the arm, projecting into the coelom above the ambulacral pore which is a gap
between the adjacent ambulacral ossicles for the passage of the podium.
 The tube feet are chief locomotory and respiratory organs of Asterias.

Locomotion of Asterias:
Asterias lacks in head or anterior end, therefore, capable to move in any direction according to its desire. It
can move on horizontal as well as on vertical surfaces by the help of tube feet.
Locomotion on a Horizontal Surface:
When an Asterias desires to move on a horizontal surface in a given direction, the arm or arms pointing in
that direction is lifted.
The ampullae of raised arm contract, the valve in the lateral canals close and the water of the ampullae is
forced into the podia.
The podia of the tube feet become extended, elongated and enlarged in the general direction of movement
due to the hydrostatic pressure produced by influx of water into them.
Subsequently, the terminal suckers of the tube feet become attached to the substratum and their central
parts are withdrawn to form suction cups.

Due to the vacuum so produced, the suckers acquire a firm grip over the substratum.
Mucus secreted by the tips of the tube feet further aids in attachment.
The tube feet now pivot forward on their attached suckers, assuming vertical position and thereby pushing the body
forwards.
The longitudinal muscles of the podia now contract and this forces their fluid back into the ampullae and releases their
suckers. The ampullae then contract again and whole sequence of events is repeated.

Locomotion on a Vertical Surface:
 In climbing a vertical surface, the tube feet pull the body forward.
 By the alternate contraction and expansion of tube feet and by adherence of suckers of tube feet on
surface Asterias climbs on the vertical surface.
 Asterias employs its tube feet, only when, it moves on hard rocky substratum.
 But, on soft mud or sand (substratum) the suckers of tube feet become useless, therefore, on such soft
surfaces the animal literally walks on its extended tube feet which now act like small legs.
 Besides locomotion, tube feet serves many other functions such as clinging of animal body to
substratum, tactile and respiratory function

Circulatory System of Asterias:
The so-called circulatory system includes following two
systems:
1. Perihaemal system
2. Haemal system
1. Perihaemal System:
The perihaemal system, like the water vascular system, is derived
from the coelom and is composed of many tubular coelomic
sinuses such as axial sinus, aboral ring sinus, genital sinuses,
radial perihaemal sinuses, marginal sinuses and peribranchial
sinuses.

(i) Axial Sinuses:
The axial sinus is a thin-walled, vertical, wide tubular coelomic cavity enclosing the stone canal and the axial gland.
The three forming a well developed axial complex.
(ii) Aboral Ring Sinus:
The aboral ring sinus is a tubular, pentagonal channel or sinus around the intestine, lying just inside the aboral wall of
the central disc.
It communicates with the axial sinuses.
(iii) Genital Sinuses:
The aboral ring sinus gives off five pairs of genital branches, one pair in each arm.
The genital sinuses surround the gonads.
(iv) Oral Ring Sinus:
At its oral end, the axial sinus opens into the inner division of a circular channel, the oral, peribuccal, perihaemal, or
hypo neural ring sinus which runs around the mouth.
It is a large tubular sinus and is divisible into an inner narrow and an outer wide ring by an oblique circular septum
called haemal strand.

(v) Radial Perihaemal Sinuses:
The outer division of ring sinus gives out five radial hypo neural or perihaemal sinuses, one of which extends through
each arm between the radial nerve and the radial water canal.
Like oral ring sinus, each radial sinus is also divided longitudinally into two by a vertical partition or septum,
continuous with the haemal strand.
The radial perihaemal sinuses also give out fine channels into the tube feet.
(vi) Marginal Sinuses:
In each arm, two longitudinal marginal sinuses run longitudinally on each side just aboral to the marginal nerve cord.
The fine lateral channels connect the marginal channels with the radial perihaemal sinuses.
(vii) Peribranchial Sinuses:
The sinuses occurring as circular spaces around the basal parts of papulae or gills are called peribranchial sinuses.

2. Haemal System:
The haemal or blood lacunar system of Asterias is reduced and is of open type like the haemocoel of Arthropoda and
Mollusca.
It includes inter-communicating spaces having no coelomic epithelium and are derived embryo-logically from the
blastocoel.
The haemal system is filled with coelomic fluid containing coelomocytes and is enclosed in the coelomic spaces of
perihaemal system.
The main haemal sinuses are as follows:
(i) Oral Haemal Ring:
It is the circular haemal sinus, located around the mouth just below the– ring canal of the water vascular system.
Oral haemal ring is a fine channel or a ring of lacunar tissue which runs in the septum dividing the hyponeural sinus.
The oral haemal ring is connected with aboral haemal ring through axial gland.
(ii) Radial Haemal Sinuses or Strands:
These arise radially from the oral haemal ring and one extends into each arm, along the floor of the ambulacral groove
just below the radial canal of the water vascular system.
The radial haemal sinuses also give off branches into the podia.

(iii) Axial Complex:
The perihaemal and haemal systems of Asterias are intimately connected by a complicated structure called
axial complex. The axial complex comprises the following three parts a thin-walled, tubular coelomic cavity
called axial sinus containing the stone canal of water vascular system and axial gland, both are closely
attached with the wall is of axial sinus by the mesenteries.
(iv) Axial Gland:
This is the principal part of the haemal system. The axial gland is an elongated, fusiform, brownish or
purple coloured spongy body.
It is covered externally by coelomic epithelium and is called variously as heart, ovoid gland, dorsal organ,
septal organ, brown gland, etc.
The axial gland is connected with the oral and aboral haemal sinuses at its oral and aboral ends
respectively.
At its oral end the axial gland becomes thin and terminates in the septum that subdivides the hypo neural
ring sinus. At its aboral end, the axial gland has an aboral extension or terminal process called head
process which is lodged in a separate, closed contractile coelomic sac called dorsal sac.

The dorsal sac is situated below the madreporite, close to the ampulla of the stone canal, but has no communication
with the ampulla.
A pair of gastric tufts also arises from the haemal sinuses in the wall of the cardiac stomach and opens into the axial
gland near its aboral end.
Digested food from the stomach passes into the haemal circulation through the gastric tufts.
Histologically, the axial gland has an external lining of peritoneum and its interior is filled by connective tissue
outlining numerous spaces containing irregularly arranged cells of the nature of coelomocytes.
The axial gland has an intimate relation with the circulation of blood in perihaemal and haemal channels.
(v) Aboral Haemal Ring:
It is a pentagonal ring canal lying beneath the aboral surface of the central disc. From the aboral haemal ring or canal
extend five pairs of genital haemal strands to the gonads.
Function:
The haemal system acts as a pathway for the distribution of food substances carried by the coelomocytes.
The flow of fluid within it is maintained by the contractile activity of the dorsal sac.
The axial gland acts as a genital stolon, producing sex-cells, which reach the gonads through the aboral haemal ring
and its branches.

Reproductive System of Asterias:
 Most species of Asterias are unisexual or dioecious, i.e., sexes are separate except a few
species such as Asterias rubens which is hermaphrodite.
 There is no marked sexual dimorphism, however, during breeding season some sort of
colour difference between both the sexes may occur.
 The reproductive organs of Asterias are of primitive type and lack copulatory organs,
accessory glands, receptacles for storing ova and reservoirs for storing mature sperms.
 There are only gonads which act as reproductive organs.

Gonads:
 The male gonads are testes and female gonads are ovaries.
 Each sexually mature male or female individual contains five pairs of testes or ovaries, one pair is lying free laterally
in the proximal part of each arm between the pyloric caeca and the ampullae.
 The testes and ovaries are morphologically similar.
 Each gonad appears as an elongated feathery tuft or tuft of tubules or bunch of grapes, whose size varies greatly
according to the proximity of spawning time.
 At maturity the gonads occupy the entire perivisceral space.
 The proximal end of each gonad is attached to aboral body wall near the inter-brachial septum by a very short
gonoduct which is ciliated and opens laterally through a small gonopore on the aboral surface almost at the angle of
two adjacent arms.
 Each gonad is enclosed in a genital sac of coelomic nature with a wall of muscle and connective tissue fibres,
covered externally with peritoneum.
 This genital sac is the outgrowth of the genital or aboral coelomic sinus.
 The gonad proper is lined by germinal epithelium, containing the germ cells.
 The mature sperms and ova are discharged by male and female Asterias respectively in sea water.
 The release of sex cells from the gonads is regulated by neuro-hormonal secretion of radial nerve.

Life History and Development of Asterias:
(i) Fertilisation:
The most species of Asterias have only one breeding season in a year. During breeding season, both
types of mature sexes shed their sex cells in the sea and union of male and female sex cells or gametes
(sperms and ova) occurs in sea water. Thus, fertilisation in Asterias is external.
(ii) Embryogeny:
 The embryological development of Asterias is indirect and includes various larval stages.
 The fertilised egg or zygote is spherical, half millimeter in diameter and contains little amount of
yolk.
 The cleavage is holoblastic and equal and it converts the unicellular zygote into a single layered,
hollow, ciliated and spherical structure called coeloblastula.
 The coeloblastula possesses a fluid-filled central space, called blastocoel and it swims about freely.
 The blastula undergoes embolic invagination and becomes two layered cup-like gastrula.
 The gastrulation involves the inward pushing of blastomeres of one side.
 The in-pushing encloses a cavity called archenteron and it occupies the larger part of blastocoel
which ultimately becomes obliterated.

This embryonic stage is called gastrula and it has an outer ectodermal and inner endodermal germinal layers. The
archenteron or gastrocoel communicates to the exterior by a wide aperture called blastopore. The blastopore changes
its relative position with the elongation of gastrula and becomes the anal opening of the larva. Two more openings
appear on the surface of the larva.
On the ventral side, a tubular in-growth of ectoderm forms the larval mouth or stomodaeum. Another opening occurs
on the dorsal side as the dorsal pore. The cilia of general surface of gastrula degenerate and certain definite ciliary
band appears. The mesoderm is formed from two sources.
During the gastrular invagination, the advancing tip of archenteron (endoderm) buds off certain mesenchyme cells
into the blastocoel. The growing archenteron is differentiated into a narrow proximal part and wide terminal part.
The narrow proximal part communicates to the exterior by the blastopore and in later stages forms the stomach, and
intestine, while the wide terminal part of completed archenteron expands and cuts off on each side into a coelomic
pouch, the hydroenterocoel.
These take up their position to the right and the left sides of the archenteron and develop into coelomic pouches. The
latter give rise to coelom, its mesodermal lining and water vascular system. The embryo at this stage becomes a free-
swimming larva.

Larval Development:
Bipinnaria Larva:
 The bipinnaria larva develops from the zygote in about one week.
 It is a bilaterally symmetrical larva which possesses a preoral
and a postoral ciliated band, and a preoral lobe with preoral loop
of ciliated band.
 The various projections emerging out of its body correspond to
the arms.
 Inside the body appears the coelomic apparatus and the
alimentary canal.
 The bipinnaria larva feeds on diatoms, etc., by creating food-
bearing currents by ciliary tracts in the stomodael wall.
 It swims freely by forwarding its anterior end, with a clockwise
rotation, after some time the bipinnaria larva transforms into the
next larval stage, the brachiolaria larva.

Brachiolaria Larva:
 In the brachiolaria larva the side-lobes of bipinnaria
increase in length to become long, slender and ciliated
larval arms.
 The larval arms move and contract.
 The preoral arms also give out processes called the
brachiolar arms.
 The arms of brachiolaria larva have coelomic prolongations
and possess tips of adhesive cells.
 The bases of these arms surround the elevated, adhesive,
glandular area performing the function of a sucker or
fixation disc by which the larva becomes attached at the
time of metamorphosis.

Metamorphosis:
 In about 6 or 7 weeks, the brachiolaria larva settles on the bottom or on some solid object and is
fixed with that by its adhesive arms.
 Now the bilaterally symmetrical larva metamorphoses into a radially symmetrical adult.
 The larval mouth and anus close.
 A new mouth is formed on the left side of the larva and a new anus is developed on the right side.
 The left and right side of the larva, thus, subsequently differentiated into oral and aboral surfaces of
the adult.
 Five lobes called arm rudiments grow out around oral-aboral axis. In later stages, the skeletal
elements appear on the arm rudiments and the radial canals grow into them.
 In each arm two pairs of outgrowths from the coelom form the first tube feet and serve for
attachment.
 Further complex re-organisational changes result in the formation of adult Asterias.
 The newly detached rudiment of the body of sea star is less than 1 mm with short stubby arms.

Dipleurula Larva:
1. The dipleurula form is reached during development and is characterized by its bilaterally symmetrical, egg-shaped
body.
2. The ventral concave side bears the mouth and is encircled by a circumoral ciliated band.
3. The anus is disposed ventrally.
4. The digestive canal is differentiated into oesophagus, stomach and intestine.
5. The pre-oral lobe which is situated at the anterior to mouth, bears an apical sensory plate and a tuft of cilia.
6. The ciliation on the surface of the body becomes reduced to a ciliard band.
This Dipleurula form is regarded by many as the hypothetical ancestral form of Echinoderm, as this form is universally
present. The Dipleurula concept was first propounded by Bather (1900). The major changes involved in other
Echinoderm larvae are due to differential disposition of the ciliated bands.

Pentactula Larva:
This larval stage is regarded as the next evolutionary step of the Dipleurula larva.
This concept has been supported by Semon (1888), Burry (1895), Hyman (1955) and many others.
1. The Pentactula larva (Fig. 21.39B, C) has five tentacles around the mouth.
2. The hydrocoel is separated from the rest of the coelom to form the future warer vascular system.

Auricularia Larva:
1. The externally bilaterally symmetrical larva (Fig. 21.39F) is present in
Holothuroidea and is characterised in having a single longitudinal cili-
ated band.
2. The pre-oral lobe is very well-formed.
3. There are no calcareous rods, being replaced by spheroids or star-
shaped or wheel-like bodies.
4. In certain forms, e.g., Auricularia stelligera and A. sphaerigera, elastic
spheres of unknown consistency are present.

Doliolaria Larva:
1. The larval form is observed in Holothuroidea.
2. The auricularia larva transforms into a barrel-like body with five ciliated bands which subsequently break into
pieces.
This particular stage is also designated as pupa stage.
3. During metamorphosis into an adult form, the ciliated bands disappear and further changes occur.
4. In some species of Holothuroids (Order Dendrochirotida e.g., Cucumaria, Thyone etc.), a non-feeding barrel-
shaped vitellaria larva is seen.
5. This type of larva is also found in crinoids and a few Ophiuroids which possesses ciliated band but no arms.
6. In Cucumaria planci, the auricularia stage is absent and the embryo transforms directly into the doliolaria stage. In
Cucumaria frondosa, C. saxicola, Psolus phantapus, both of the larval stages may be absent. In Holothuria floridana,
there is no larval form and the embryo develops directly into a young Holothuroid.

Pluteus Larva:
1. This larval form can be regarded as a modification of the auricularia
larva of Holothuroid.
2. Like the auricularia larva it has a single ciliated band, but it possesses
long arms with ciliated bands at the margin.
3. It has comparatively smaller pre-oral lobe.
4. The post-anal part of the body is quite well-developed.
5. The arms are also supported by calcareous rods.

The pluteus larvae are of two kinds:
(1) Ophiopluteus—in Ophiuroidea.
(2) Echinopluteus—in Echinoidea.
Both the larval forms possess the post-oral arms, antero-lateral arms, postero-lateral arms and postero-dorsal arms.
But they differ in detail which are summarised in Table 21.2—Echinodermata.
6. The typical ophiopluteus may be absent in certain forms.
7. The arms are small in Ophiopluteus metschnikoffi and O. claparedei.
8. In Ophionotus hexactis the ophiopluteus lacks arms.
9. The larva may be elongated and without ciliated bands.
10. The skeletal rods are usually absent; if present, only one in number. This condition is observed in Ophiopluteus
annulatus, and O. oblongus.

Antedon or Yolk Larva:
1. This particular larva is also called doliolaria larva or Vitellaria larva.
2. This larval stage is present in Antedon and it has many structural pecularities.
3. It has a barrel-shaped body with slightly flattened ventral side.
4. It is free-swimming and exhibits bilateral symmetry.
5. The ciliated bands are in the form of four or five separate transversely placed bands encircling the body.
In Antedon bifida, there are four bands. In Antedon adriatica and A. mediterranea there are five bands.
6. A tuft of cilia with stiff sensory hair springs from a thickened ectodermal patch, called apical neural
plate, which is comparable to that of Tonaria larva of Balanoglossus.
7. The anterior ciliated ring is ventrally incomplete.
8. There is a ciliated depression or larval mouth which is ventrally placed between the second and third
ciliated rings.
9. A small adhesive pit develops between the first and second ciliated rings by which the larva adheres to
the substratum.
10. The internal structures become rotated at an angle of 90° from the ventral to the posterior side.

Cystidean or Pentacrinoid Larva:
1. This larval stage is also present in Crinoids. It is the second larval stage of crinoids.
2. The anterior end of the antedon larva, after attachment, is prolonged into an elongated narrow stalk and
the free end becomes broader (Fig. 21.39L).
3. The ciliated depression becomes a closed ectodermal vesicle which is gradually shifted to the free end.
4. The floor of the depression is perforated by mouth and with the disappearance of the roof; the mouth
and the tentacles become exposed.
5. This particular phase is called Cystidean or Pentacrinoid stage. This stage resembles closely the adult
Pentacrinus. The stalk in this form develops from the pre-oral lobe.
6. This stage is quite similar to that of Asteroidea excepting that it lacks circumoral vessel.

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