The phylum Echinodermata is said to be a connection between invertebrates and chordates due to certain similarities that two phyla share. Echinoderms and chordates are called as deuterostomes as in both groups, blastopore of embryo grows in anus and mouth forms at opposite end. In protostomes conversely, blastopore of embryo gives rise to mouth of adult. It is thought that echinoderms branched from same deuterostome evolutionary stem as vertebrates.
Diversity of echinoderms:
There are approx 6,250 - 6,500 species of echinoderms distributed between 3 subphyl and 5 classes. They are exclusively marine.
The early larva exhibits evidence of metameric segmentation; adults are unsegmented. Adult is secondarily radially symmetrical. There are two major kinds of body form among adults: radiate and globular. They are not cephalized (without any distinguished head) but have oral - aboral axis. Their body surface has five fields with podia (ambulacra) alternating with 5 that do not have podia (interambulacra). There are different forms of gut and it might not have anus. Their nervous system is well grown but uncentralized. Body wall is composed of ciliated epidermis underneath which is a dermis which contains calcareous ossicles that give rise to spines. They possess simple circulatory or haemal system which runs through system of coelomic compartments. Tube feet (podia) are offshoots of radial canals of water vascular system and suckers at their ends facilitate attachment to substrate and are also utilized for locomotion. They have different forms of free-swimming and free-living larvae each related with different classes.
Despite diversity of echinoderms observed above, most of them look very much alike and have related modes of life. Almost all of them are scavengers and predators which feed on variety of animals as they move slowly along sea shore or bottom of ocean.
Body form of Asterias:
Asterias is star-shaped; it is powerfully flattened along oral-aboral axis and comprises of central disc from which project five blunttipped arms. Central disc bears mouth in centre of lower (oral) surface, and inconspicuous anus on upper (aboral) face. Another opening, the madreporite, located inter-radially near centre of aboral face and opens in stone canal, an extension of water vascular system. Aboral surface is brownish and covered with many irregularly arranged blunt spines; oral surface is paler and has approx 200 tube feet that project in two double rows in grooves (ambulacral grooves) extending from mouth along each arm. Pedicellariae (small pincer-like structures), happen on both oral and aboral surfaces. Each pedicellaria is formed from 3 ossicles: basal supporting structure, and pair of ossicles that form pincers or jaws themselves. One pair of muscles (abductors) swing them open, and another pair (adductors), close them. Pedicellariae may be fixed or stalked with the long reach and significant flexibility. Stalked pedicellariae are mainly characteristic of ambulacral grooves while those of the aboral suface are more frequently fixed and are arranged in rings round spines. Pedicellariae are utilized to remove detritus from body surface, and also to kill and remove any small animals which touch them. Without pedicellariae slow moving animal with firm surface, like a starfish, would be susceptible to colonizers, and to small predators.
The body wall:
The outermost layer of body wall is thin and has mucous gland cells, ciliated cells, and sensory cells enclosed by cuticle. Mucous cells secrete the protective coating while epidermal cilia sweep off detritus. Immediately below epidermis is nerve plexus; and below this is a tissue in which ossicles are embedded. These are created of mixture of calcium carbonate (90%) and magnesium carbonate (10%) and are 3-dimensional matrices, with spaces filled by soft tissue, and are attached to each other by network of collagen fibres. Oral wall ossicles are closely but flexibly fitted and those of ambulacral grooves are mainly large for muscle insertion. On the contrary, aboral wall ossicles form irregular lattice. Body wall is completed by inner longitudinal and outer circular smooth muscle fibres and by the layer of peritoneum.
The water vascular system:
Madreporite opens in small space, ampulla that leads via vertical stone canal to ring canal. Lumen of stone canal is subdivided so that fluid can pass both orally and aborally. Five ciliated radial canals (one per arm) expand from ring canal, and each ends in the sensory structure, optic cushion, that is a modified tube foot. Lateral canals extend from each side of radial canal, each opening in aboral bulb (ampulla) and oral tube foot (podium). Opening is secured by valve. Canals are alternately long and short on each side so that tube feet on either side are set in two rows. Radial canals and tube feet lie outside main part of body in ambulacral groove, but ampullae project in body cavity. Total of nine Tiedemann's bodies arise inter-radially from ring canal; position of tenth (expected from pentaradial symmetry) is occupied by stone canal.
Podia possess broad sucker-like tips and function hydraulically. Rings of smooth muscle in wall of ampulla contract, lateral canal valve closes, and fluid is therefore forced in podium causing it to elongate. When sucker of podium touches the solid surface its centre is withdrawn producing slight vacuum and thus causing it to stick by suction. Epidermis of sucker region has several sensory cells, and gland cells that produce a copious adhesive secretion. The madreporite is said to balance internal and external pressure over remainder of water vascular system.
Asterias crawls gradually, with one arm leading; at times it may extend 2 arms in front or even lead with inter-radial region. Any of the arms can be utilized to lead. Propulsion is given by tube feet that move in series of steps (3-10 per minute). Each foot in turn is extended, fixed to substratum, and utilized as a lever to push body forwards. Muscles in podial walls are utilzied to retract foot, and to orientate it during use, and give the essential suction for attachment. Lot of these originates on ambulacral ossicles. Additionally tube contains layer of connective tissue that prevents foot from bulging rather than extending.
Feeding and digestion:
Alimentary canal is aligned in vertical axis, expanding through disc from mouth on lower side to anus on upper side. Mouth that is closed by sphincter muscle opens in short narrow oesophagus and then large, pouched, cardiac stomach. Walls of this stomach are joined to ambulacral ossicles of arms by ten pairs of triangular gastric ligaments. Cardiac stomach opens in smaller pyloric stomach; this is flattened and looks star-shaped due to 10 blind-ending pyloric caeca, or digestive glands, that extend from it, one pair down each arm. From pyloric stomach, short intestine, from which arise short, blind-ending rectal sacs, extends to anus. Asterias is predaceous carnivore and feeds primarily on bivalve molluscs, particularly Mytilus. It holds bivalve so that non-hinged surface is held definitely pressed against mouth, and joins its arms to valves by tube feet. Retraction of tube feet then forces valves to open a little. Once shell is open Asterias everts part of its cardiac stomach into prey.
Gaseous exchange, excretion, and circulation:
Gaseous exchange occurs across tube feet and small evaginations of body wall known as papulae. Tube feet and papulae are also site for removal of waste product, ammonia; echinoderms do not have specialized excretory system. As body fluids are isotonic with sea water, osmoregulation is needless. Haemal system in Asterias is badly developed and comprises of oral and aboral rings, radial sinus paralleling water vascular canal in each arm and canal that parallels stone canal and is enclosed by mass of spongy tissue, axial gland. Axial gland is said to demolish any micro-organisms that get in body.
Nervous system and sense organs:
Additionally to sub-epidermal nerve net, that extends all over body, Asterias illustrates some concentration of nerve fibres that form nerve ring around mouth. 5 radial nerve cords arise from nerve ring and extend along each arm; they are engaged in coordination of podia. Sub-epidermal plexus thickens to create marginal nerve cords which extend along sides of ambulacral grooves. Asterias contains large numbers of epidermal tactile, chemical and light sensitive receptors scattered over general body surface, but concentrated mainly along margins of ambulacral groove and on podia.
Reproduction and growth:
The sexes are separate. Asterias contains 5 pairs of gonads (ovaries or testes) that are suspended in perivisceral coelom by strands of mesentery; they open by ciliated gonoducts, at cluster of gonopores near junction of arms. Site and extent of gonads differs seasonally; near spawning, they may almost fill arms. Presence of eggs in water stimulates shedding of sex cells by other males and females. Shed sperm stick to ova. After fertilization, dipleurula larva grows, with ventral mouth, and with anus derived from blastopore (the deuterostome condition). At first cilia are extensively distributed over surface of larva, but this soon decreases to definite ciliary band, comprising of two longitudinal bands related in front of mouth and behind anus. The features larval stage of Asterias is a bipinnaria larva arrived at by growth of two groups of arms along that ciliated bands extend. It is planktonic for several weeks and feeds on pelagic organisms like diatoms.
Economic significance of echinoderms:
Many species of sea urchins are exploited in some European countries like France, Spain or Croatia, because of high commercial value of their tasty gonads. Toxicity tests using embryos and larvae of sea urchin in assessment and monitoring of marine pollution. Starfish and brittle stars prevent development of algal mats on coral reefs that would obstruct filter-feeding constituent organisms. Echinoderms are also staple diet of several organisms, most particularly otter. Several sea cucumbers give habitat for parasites, and also crabs, worms and snails. Sea cucumbers are also regarded as a delicacy in some countries of south East Asia. Calcareous tests or shells of echinoderms are utilized as source of lime by farmers in areas where limestone is unavailable. Trade in shells is frequently performed in conjunction with shellfish farmers, for whom starfish pose major irritation by eating stocks.
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