ECHINODERMSZLY 303:

Biology of Free-living Non-Arthropod Invertebrates

Delivered by

SHITTU, OlalereDept. of Zoology,

University of Ilorin,Nigeria

General Characteristics

• Adults exhibit pentamerous radial symmetry

• Radially symmetry is secondary; larvae are bilaterally symmetrical and undergo metamorphosis to become radially symmetrical adults.

• Poorly ganglionated; possess few sensory structures

• Body wall contains an endoskeleton of calcareous plates - ossicles

General Characteristics

• Possess a network of canals throughout the body - water vascular system.

• The canals are connected to extensions called tube feet (=podia), located on the oral surface

• The water vascular system is important for locomotion, feeding, and gas exchange.

• Sexes are separate; gametes shed into the water; fertilization is external

General Body Wall

Morphology

Diversity

Class Asteroidea• Sea stars, or starfish, vary in shape from

nearly circular, to pentagonal, to the familiar star-like and flower-like forms

• Typically have 5 arms which merge with a central disc with ambulacral area with holes for the tube feet

• Mouth is located in the center of oral surface which is directed downward

Diversity

Class Asteroidea• The tip of each arm bears a tube foot

that functions as a sensory receptor for chemical and vibratory stimuli, and some have a red pigment spot that serves as a simple eye

• The outer surface consists of a latticework of lime ossicles, or plates, between which project thin-walled fingerlike extensions called papulae.

• The papulae and the tube feet are the principal sites of respiratory exchange.

Diversity

Class Asteroidea• In some groups of sea stars there are

also body wall projections called pedicellaria, equipped with tiny pinchers that are operated by muscles and are used to clean the body surface and capture very small prey.

• They crawl about on rocks or muddy bottoms, feeding on a variety of living and dead animals.

• Many feed largely on bivalve mollusks and are notorious as destroyers of commercial oyster beds.

Diversity

Class Asteroidea• There are two or more gonads in each

arm; at spawning time these may nearly fill the arms. The swimming larva settles and goes through a sessile (attached) stage while changing to the adult form.

Morphology

Water Vascular System

• On the aboral surface is the opening of the water vascular system the madreporite (=sieve plate)

• Water enters the madreporite and goes through the stone canal to the ring canal and then passes through a radial canal extending into each arm

• All along the length of these canals are lateral canals that terminate in bulb-like structures called ampullae equipped with tube feet

• Tube feet line the grooves on the oral surface - ambulacral grooves

Water Vascular System in Sea Star

Cross Section of Sea Star showing the Arm

Groups of Starfish

• Brisingida - There are about 111 species of Brisingida alive today. Members of this group have between 6 and 20 long arms that they use for suspension feeding.

• Forcipulatida - There are about 270 of Forcipulatida alive today. Members of this group have thick arms and body and includes many of the starfish that inhabit temperate waters.

• Notomyotida - There are about 74 species of Notomyotida alive today. Members of this group live in deep sea habitats.

Groups of Starfish

• Paxillosida - There are about 370 species of Paxillosida alive today. Members of this group are among the more ancient of the living starfish and lack more advanced features such as suckers on their tube feet and an anus.

• Spinulosida - There are about 120 species of Spinulosida alive today. Members of this group have small spines on the upper surface of their body.

Groups of Starfish

• Valvatida - There are about 700 species of Valvatida alive today. Members of this group include cusion stars and sea daisies.

• Velatida - There are about 140 species of Velatida alive today. Members of this group are deep sea species and live in cold water. They have between five and fifteen arms.

Class Ophiuroidea

• The brittle stars, or serpent stars, are so called for their long, slender, fragile arms, which are set off sharply from the circular, pentagonal, or slightly star-shaped body disk.

• The arms of brittle stars are flexible and appear jointed because of the conspicuous plates of the outer surface.

• They bear a row of spines along each edge. In one group, the basket stars, they are repeatedly branched, forming a large mass of tentacle-like limbs.

Class Ophiuroidea

• Each arm contains a radial canal (or one of its branches), but no body organs.

• They feed on detritus and small organisms. The mouth leads to a large saclike stomach that fills most of the body cavity.

• There is no intestine or anus, and solid waste is extruded through the mouth.

• The stomach is folded into ten pouches, between which lie ten respiratory sacs that open by slits onto the oral surface.

Class Ophiuroidea

• The cells lining the sacs have flagellate, creating a current of water moving in and out. Respiratory exchange occurs chiefly through the thin lining of the sacs.

Class Echinoidea

• They include sea urchins, heart urchins, and sand dollars—are echinoderms without arms and with a spiny shell, or test, formed of tightly fused skeletal plates.

• The sea urchins (regular echinoids) are hemispherical in shape, round on top and flat on the lower surface. They have very long, prominent spines and are often brightly colored.

• The test of a sea urchin is divided into ten parts from pole to pole, like the sections of an orange.

Structure of Sea Urchins

Class Echinoidea

• Five of these are ambulacra, with openings for tube feet; these alternate with wider sections, called inter ambulacra that lack tube feet.

• Spines and pedicellaria are found over the entire surface of the test.

• Urchins move by pushing against the substratum with the spines and extending the tube feet in the direction of movement.

Class Echinoidea

• The mouth, located in the center of the undersurface, is surrounded by a thickened region bearing five pairs of short, heavy tube feet and sometimes five pairs of bushy gills.

• Within the mouth is an elaborate five-sided jaw structure called Aristotle's lantern extrudes.

• It is able to grind up calcareous exoskeletons of plants and animals.

• The anus is at the center of the aboral surface and is surrounded by a thin-walled area without skeletal plates.

Internal Structure of Sea Urchin

Irregular Echinoids

• Sand dollars and heart urchins have a dense covering of short spines, and locomotion is exclusively by movement of the spines.

• There are two groups of podia-bearing ambulacra, one arranged in a petal-like pattern on the upper surface and the other forming a similar pattern on the lower surface.

• The upper tube feet function as respiratory organs (there are no gills around the mouth), and the lower ones are specialized for gathering food particles.

Irregular Echinoids

• Sand dollars are extremely flattened and oval in outline; the anus is on the oral surface.

• Heart urchins are somewhat flattened and are heart-shaped; a deep ambulacral groove running from top to bottom creates a secondary bilateral symmetry.

• The anus is on the aboral surface, opposite the groove.

Sea Urchins

• Sea Urchins• Spherical body• Ambulacral plates

bearing tube feet that radiate out toward the aboral surface

• Use podia and spines during locomotion

• The spines are moveable and articulate with the with the calcareous ossicles

Sea Urchins

• complex chewing apparatus called Aristotle's lantern

• feed by scraping algae off rocks

Class Holothuroidea

• The sea cucumbers are long-bodied echinoderms with the mouth at or near one end and the anus at or near the other.

• Because of their elongation along the oral-aboral plane, they lie on their sides rather than on the oral surface.

• In nearly all sea cucumbers the skeleton is reduced to microscopic ossicles imbedded in the leathery skin.

Class Holothuroidea

• Sea cucumbers have no arms, but tube feet around the mouth have been modified to form a circle of 10 to 30 tentacles of varying lengths and shapes that function in gathering food particles from the ocean bottom.

• The gut of the sea cucumber terminates in a chamber called the cloaca that opens into the anus. Two unique structures called respiratory trees, found in most sea cucumbers, also terminate in the cloaca.

Anatomy of Holothuroidea

Respiratory Trees of Holothuroidea

• These are systems of highly branched tubes, one on either side of the body.

• The animal pumps water into the respiratory trees by contracting the cloaca, and oxygen diffuses through from the walls of the trees into the fluid of the body cavity.

• The madreporite in most sea cucumbers opens into the body cavity rather than to the outside and receives its fluid from the cavity.

Respiratory Trees of Holothuroidea

• In a few sea cucumber species there is a large mass of tubules at the base of the respiratory tree that can be shot out of the anus if the animal is irritated.

• The extruded tubules, which may engulf and incapacitate an intruder, break off; they are then regenerated by the sea cucumber.

• In other species the respiratory trees, gonads, and part of the digestive tract are shot out through the anus; this evisceration is followed by regeneration of the lost organs.

Dermal Ossicles of Holothuroidea

• Dermal ossicles are usually microscopic bodies buried in the leathery dermis.

• They can be extracted from tissues with commercial bleach and are of important taxonomic characteristics.

• They illustrate the meshwork Stereom structure at some developmental stage in Echinoderms.

Class Crinoidea

• The sea lilies and feather stars are members of an ancient group of stalked, sessile, detritus-feeding echinoderms.

• Most of the sea lilies remain stalked throughout life; their movements include bending the stalk and the arms and crawling.

• Feather stars break off the stalk and become free-living as adults.

• Crinoids, whether free or stalked, always have the oral side upward, and the ring of arms about the mouth gives them a flowerlike appearance.

Class Crinoidea

• They have at least 10 arms, but some sea lilies have up to 40 and some feather stars up to 200 arms.

• The stalk and the arms have a jointed appearance, and each arm has a row of projections, the pinnules, on either side, giving a feathery appearance.

• A ciliated ambulacral groove runs along each arm and branches into the pinnules; the groove contains feathery, tube feet arranged in triads.

Class Crinoidea

• The feet react to the presence of minute food particles in the water by bending inward, sweeping the particles into the groove, where they are trapped in mucus and swept by the cilia toward the mouth.

• Gametes develop in some of the pinnules, which rupture at spawning time.

• The free-swimming larva eventually settles and develops a stalk and a crown.

Anatomy of Crinoidea

Class Concentricycloidea

• The sea daisies, which were discovered in 1986, have disk-shaped flat bodies and are less than 0.39 in. (1 cm) in diameter.

• There are two known species, viz: Xyloplax and Dendrocystites.

• They have a water-vascular system, with tube feet on the body surface around the edge of the disk.

• They have no obvious arms or mouth, and appear to absorb nutrients through the membrane surrounding their bodies.