Echinoderms

 Relationships to other animals

Echinoderms share a common ancestry with hemichordates and chordates because of the deuterostome characteristics. These characteristics include: 

•       Anus develops in the region of the blastopore. 

•       Coelom that forms from out pockets of the embryonic gut tract.  

•       Radial, indeterminate cleavage.  

•       Adult echinoderms are radially symmetrical Larval stages are bilateral symmetrical. This evidence indicates that echinoderms evolved from bilaterally symmetrical ancestors.

 

   Echinoderm characteristics

There are approximately seven thousand species of living echinoderms. They are exclusively marine and occur at all depths in all oceans.  

Radial Symmetry and its importance 

Modern echinoderms have a form of radial symmetry, called pentaradial symmetry, in which body parts are arranged in fives, or a multiple of five, around an oral-aboral axis. Radial symmetry is an advantage for echinoderms because it allows a uniform distribution of sensory, feeding, and other structures around the animal.  

Skeleton 

The echinoderm skeleton consists of calcium carbonate plates called ossicles. These plates are derived from mesoderm, held in place by connective tissues, and covered by an epidermal layer. In some echinoderms the skeleton is modified into spines that project from the body surface. 

Water-Vascular system

The water-vascular system of echinoderms is a series of water-filled canals. This system is derived from coelom during development. The water-vascular system includes a ring canal that surrounds the mouth. The ring canal usually opens to the outside through a stone canal and a pore called the madreporite. The madreporite serves as an inlet to replace water lost from the water-vascular system. Swellings associated with ring canal are called as Tiedemann bodies. Phagocytic cells known as coelomocytes are produced in these swellings. Polian vesicles are sacs that are also associated with the ring canal and function in fluid storage for the water-vascular system. Five radial canals branch from the ring canal and are associated with arms of echinoderms. Many lateral canals branch off each radial canal and end at the tube feet. Internally, tube feet usually terminate in a bulblike, muscular ampulla. When an ampulla contracts, it forces water into the tube foot. Valves prevent the backflow of water from the tube foot into the lateral canal. A tube foot often has a suction cup at its distal end which can create a vacuum against a substrate. In some taxa, tube feet have a pointed or blunt distal end.  

Functions of water vascular system: 

•      Locomotion 

•      Feeding 

•      Respiratory gas exchange 

•      Nitrogenous waste removal  

                                                           

                                                              

Hemal system

A hemal systemconsists of strands of tissue that encircle the ring canal of the water-vascular system and run into each arm near the radial canals.The hemal system is a vestigial circulatory system. It may aid in the transport of large molecules, hormones, or coelomocytes, which are cells that engulf and transport waste particles within the body. 

  Echinoderm classification

       

                                   Class Asteroidea (Sea stars)

➢Asteroidea mean “star like” organisms. 

➢There are about 1500 species of asteroids. 

➢They often live on hard substrates in marine environments. 

➢Sea stars may be brightly colored with red, orange, blue, or gray. 

Morphology

Sea stars have a central disc and have five arms that arise from a central disk. The oral opening, or mouth, is in the middle of one side of the central disk which is normally downward. Spines are present on the aboral surface. Thin folds of the body wall, called dermal branchiae are present between ossicles and function in gas exchange. In some sea stars, the aboral surface has structures called pedicellariae, which clean and protects the body surface.  

A series of ossicles in the arm form an ambulacral groovethat runs the length of the oral surface of each arm. The ambulacral groove contains the radial canal, and two rows of tube feet. Tube feet of sea stars move in a stepping motion. Alternate extension, attachment, and contraction of tube feet move sea stars across their substrate. The suction disks of tube feet are effective attachment structures, allowing sea stars to maintain their position, or move from place to place, in spite of waves. 

Digestive system

Sea stars feed on snails, bivalves, crustaceans, polychaetes, corals, detritus, and a variety of other food items. The mouth opens to a short esophagus and then to a large stomach that fills most of the coelom of the central disk. The stomach is divided into two regions. The larger, oral stomach also called the cardiac stomach, receives ingested food. A smaller, aboral stomach also called the pyloric stomach joins with oral stomach. The aboral stomach gives rise to ducts that connect to secretory and absorptive structures called pyloric cecae. Two pyloric cecae extend into each arm. A short intestine leads to rectal cecae and to the anus which is nonfunctional. 

Feeding

Some sea stars ingest whole prey, which are digested extracellularly within the stomach. Undigested material is expelled through the mouth.  

Many sea stars feed on bivalves by forcing the valves apart. When a sea star feeds on a bivalve, it wraps itself around the bivalve’s ventral margin. Tube feet attach to the outside of the shell, and the body-wall muscles open the valves. When the valves are opened about 0.1 mm, the oral (cardiac) portion of the sea star’s stomach is everted into the bivalve shell. Digestive enzymes are released, and partial digestion occurs in the bivalve shell. This digestion further weakens the bivalve’s adductor muscles, and the shell eventually opens completely. Partially digested tissues are taken into the aboral (pyloric) portion of the stomach, and into the pyloric cecae for further digestion and absorption.  

Gas exchange and excretion

Gas exchange and excretion of metabolic wastes (ammonia) occur by diffusion across dermal branchiae, tube feet, and other membranous structures.  

Hemal system

A sea star’s hemal system consists of strands of tissue that encircle the mouth near the ring canal, extend aborally near the stone canal, and run into the arms near radial canals.

Nervous system

The nervous system of sea stars consists of a nerve ring that encircles the mouth and radial nerves that extend into each arm. Radial nerves lie within the ambulacral groove. Radial nerves coordinate the functions of tube feet. Other nervous elements are in the form of a nerve net associated with the body wall. 

Sensory structures

Most sensory receptors are distributed over the surface of the body and tube feet. Sea stars respond to light, chemicals, and various mechanical stimuli. They often have specialized photoreceptors at the tips of their arms. These are actually tube feet that lack suction cups. 

 

Reproduction and development

 Sea stars are well known for their powers of regeneration. They can regenerate any part of a broken arm. In a few species, an entire sea star can be regenerated from a broken arm if the arm contains a portion of the central disk. Regeneration is a slow process, taking up to a year for complete regeneration. Asexual reproduction involves division of the central disk, followed by regeneration of each half.  

Sea stars are dioecious. Two gonads are present in each arm. Gonopores open between the bases of each arm. External fertilization is the rule. Because gametes cannot survive long in the ocean, maturation of gametes and spawning must be coordinated if fertilization is to take place. The photoperiod (the relative length of light and dark in a 24-hour period) and temperature are environmental factors used to coordinate sexual activity. In addition, gamete release by one individual is accompanied by the release of spawning pheromones, which induce other sea stars in the area to spawn, increasing the likelihood of fertilization. 

Embryos are planktonic, and cilia are used in swimming. After gastrulation, a bilaterally symmetrical larva, called a bipinnaria larva, forms. The larva usually feeds on planktonic protists. Larval arms are developed in it and it is now called as brachiolaria larva, which settles to the substrate, attaches, and metamorphoses into a juvenile sea star. 

 

Class Ophiuroidea (Brittle stars)  

Morphology

The class Ophiuroidea includes the basket stars and the brittle stars. With over two thousand species, this is the most diverse group of echinoderms. The arms of ophiuroids are long and, unlike those of asteroids, are sharply set off from the central disk, giving the central disk a pentagonal shape. Brittle stars have unbranched arms, and most have a central disk that ranges in size from 1 to 3 cm. Basket stars have arms that are branch. Neither dermal branchiae nor pedicellariae are present in ophiuroids. The tube feet of ophiuroids lack suction disks and ampullae. Unlike the sea stars, the madreporite of ophiuroids is on the oral surface. 

                                                      

 

Water vascular system

The water-vascular system of ophiuroids is not used for locomotion. Superficial ossicles, which originate on the aboral surface, cover the lateral and oral surfaces of each arm. The ambulacral groove containing the radial nerve, hemal strand, and radial canal is thus said to be “closed.” Ambulacral ossicles are in the arm, forming a central supportive axis.  

Locomotion

Ambulacral ossicles articulate with one another and along with the large muscles produce snakelike movements. During locomotion, the central disk is held above the substrate, and two arms pull the animal along, while other arms extend forward or trail behind the animal.  

Feeding

Ophiuroids are predators and scavengers. They use their arms and tube feet in sweeping motions to collect prey which are then transferred to the mouth. Some ophiuroids are filter feeders that wave their arms and trap plankton on mucus covered tube feet. Trapped plankton is passed from tube foot to tube foot along the length of an arm until it reaches the mouth.  

Digestive system

The mouth of ophiuroids is in the center of the central disk, and five triangular jaws form a chewing apparatus. The mouth leads to a saclike stomach. There is no intestine, and no part of the digestive tract extends into the arms.  

Coelom

The coelom of ophiuroids is reduced and is mainly confined to the central disk, but it still serves as the primary means for the distribution of nutrients, wastes, and gases. Coelomocytes are special cells that aid in the distribution of nutrients and the excretion of particulate wastes.  

Excretion

Ammonia is the primary nitrogenous waste product, and it is excreted by diffusion across tube feet and membranous sacs, called bursae, that are present in central disk. Slits in the oral disk, near the base of each arm, allow cilia to move water into and out of the bursae 

Reproduction and development

 Like sea stars, ophiuroids can regenerate lost arms. The process called autotomy is used in escape reactions when brittle star is grasped by its arm. The ophiuroid later regenerates the arm. Some species also have a fission line across their central disk. When an ophiuroid splits into halves along this line, two ophiuroids regenerate. 

Ophiuroids are dioecious. Males are usually smaller than females, who often carry the males. The gonads are associated with each bursa, and gametes are released into the bursa. Eggs may be shed to the outside or retained in the bursa, where they are fertilized and held through early development. Embryos are protected in the bursa and are sometimes nourished by the parent. A larval stage, called an ophiopluteus larva, is planktonic. Its long arms bear ciliary bands used to feed on plankton, and it undergoes metamorphosis, attaches to the substrate and develops into an adult. 

 

Class Echinoidea

The sea urchins, sand dollars, and heart urchins make up the class Echinoidea. The approximately one thousand species are widely distributed in nearly all marine environments. Sea urchins are specialized for living on hard substrates. Sand dollars and heart urchins usually live in sand or mud. They use tube feet to catch organic matter settling on them or passing over them. Sand dollars often live in dense beds, which favours efficient reproduction and feeding. 

                

Morphology

Sea urchins are rounded, and their oral end is oriented toward the substrate. Their skeleton, called a test, consists of 10 closely fitting plates that arch between oral and aboral ends. Five rows of ambulacral plates have openings for tube feet, and the other five are called interambulacral plates. Interambulacral plates contain spines. Spines are often sharp and sometimes hollow, and they may contain venom dangerous to swimmers. The pedicellariae of sea urchins have either two or three jaws and connect to the body wall by a relatively long stalk. They clean the body of debris and capture planktonic larvae, which provide an extra source of food. Pedicellariae of some sea urchins contain venom sacs which contains venom. 

Water vascular system

 The water-vascular system is similar to that of other echinoderms. Radial canals run along the inner body wall between the oral and the aboral poles. Tube feet possess ampullae and suction cups, and the watervascular system opens to the outside through many pores in one aboral ossicle that serves as a madreporite. 

Locomotion

 Echinoids move by using spines for pushing against the substrate and tube feet for pulling. Sand dollars and heart urchins use spines to help burrow in soft substrates. Some sea urchins burrow into rock to minimize the action of waves and strong currents. They form cup-shaped depressions and deeper burrows, using the action of their chewing apparatus called the Aristotle’s lantern. 

Feeding

 Echinoids feed on algae, bryozoans, coral polyps, and dead animal remains. Oral tube feet surrounding the mouth manipulate food. A chewing apparatus, called Aristotle’s lantern,can be projected from the mouth. It consists of about 35 ossicles and attached muscles and cuts food into small pieces. 

                                                                                               

Digestive system

The mouth cavity leads to a pharynx, an esophagus, and a long, coiled intestine that ends aborally at the anus. 

Coelom

Echinoids have a large coelom, and coelomic fluids are the primary circulatory medium.  

Gas exchange

Small gills, found in a thin membrane surrounding the mouth, are out pockets of the body wall and are lined by ciliated epithelium. Gas exchange occurs by diffusion across this epithelium and across the tube feet. Ciliary currents, changes in coelomic pressure, and the contraction of muscles associated with Aristotle’s lantern move coelomic fluids into and out of gills. 

 

                                                      

Reproduction and development

 Echinoids are dioecious. Gonads are on the internal body wall of the interambulacral plates. During breeding season, they nearly fill the coelom. One gonopore is in each of five ossicles, called genital plates, at the aboral end of the echinoid. Gametes are shed into the water, and fertilization is external. Development eventually results in a pluteus larva that spends several months in the plankton and eventually undergoes metamorphosis to the adult.