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erised 46 echinoderms have a simple radial nervous system that consists of a modified nerve net of interconnected neurons with no central brain although some do possess ganglia nerves radiate from central rings around the mouth into each arm or along the body wall the branches of these nerves coordinate the movements of the organism and the synchronisation of the tube feet starfish have sensory cells in the epithelium and have simple eyespots and touch sensitive tentacle like tube feet at the tips of their arms sea urchins have no particular sense organs but do have statocysts that assist in gravitational orientation and they too have sensory cells in their epidermis particularly in the tube feet spines and pedicellariae brittle stars crinoids and sea cucumbers in general do not have sensory organs but some burrowing sea cucumbers of the order apodida have a single statocyst adjoining each radial nerve and some have an eyespot at the base of each tentacle 47 the gonads at least periodically occupy much of the body cavities of sea urchins 48 and sea cucumbers while the less voluminous crinoids brittle stars and starfish have two gonads in each arm while the ancestors of modern echinoderms are believed to have had one genital aperture many organisms have multiple gonopores through which eggs or sperm may be released 49 regeneration edit further information starfish regeneration sunflower star regenerating several arms many echinoderms have great powers of regeneration many species routinely autotomize and regenerate arms and viscera sea cucumbers often discharge parts of their internal organs if they perceive themselves to be threatened regenerating them over the course of several months sea urchins constantly replace spines lost through damage while sea stars and sea lilies readily lose and regenerate their arms in most cases a single severed arm cannot grow into a new starfish in the absence of at least part of the disc 50 51 52 53 however in a few species a single arm can survive and develop into a complete individual and arms are sometimes intentionally detached for the purpose of asexual reproduction 51 52 53 during periods when they have lost their digestive tracts sea cucumbers live off stored nutrients and absorb dissolved organic matter directly from the water 54 the regeneration of lost parts involves both epimorphosis and morphallaxis in epimorphosis stem cells either from a reserve pool or those produced by dedifferentiation form a blastema and generate new tissues morphallactic regeneration involves the movement and remodelling of existing tissues to replace lost parts 55 direct transdifferentiation of one type of tissue to another during tissue replacement is also observed 56 reproduction edit sexual reproduction edit echinoderms become sexually mature after approximately two to three years depending on the species and the environmental conditions almost all species have separate male and female sexes though some are hermaphroditic the eggs and sperm cells are typically released into open water where fertilisation takes place the release of sperm and eggs is synchronised in some species usually with regard to the lunar cycle in other species individuals may aggregate during the reproductive season increasing the likelihood of successful fertilisation internal fertilisation has been observed in three species of sea star three brittle stars and a deep water sea cucumber even at abyssal depths where no light penetrates echinoderms often synchronise their reproductive activity 57 some echinoderms brood their eggs this is especially common in cold water species where planktonic larvae might not be able to find sufficient food these retained eggs are usually few in number and are supplied with large yolks to nourish the developing embryos in starfish the female may carry the eggs in special pouches under its arms under its arched body or even in its cardiac stomach 58 many brittle stars are hermaphrodites they often brood their eggs usually in special chambers on their oral surfaces but sometimes in the ovary or coelom 59 in these starfish and brittle stars development is usually direct to the adult form without passing through a bilateral larval stage 60 a few sea urchins and one species of sand dollar carry their eggs in cavities or near their anus holding them in place with their spines 61 some sea cucumbers use their buccal tentacles to transfer their eggs to their underside or back where they are retained in a very small number of species the eggs are retained in the coelom where they develop viviparously later emerging through ruptures in the body wall 62 in some crinoids the embryos develop in special breeding bags where the eggs are held until sperm released by a male happens to find them 63 asexual reproduction edit see also regeneration comet form of linckia one species of seastar ophidiaster granifer reproduces asexually by parthenogenesis 64 in certain other asterozoans adults reproduce asexually until they mature then reproduce sexually in most of these species asexual reproduction is by transverse fission with the disc splitting in two both the lost disc area and the missing arms regrow so an individual may have arms of varying lengths 53 65 during the period of regrowth they have a few tiny arms and one large arm and are thus often known as comets 52 66 adult sea cucumbers reproduce asexually by transverse fission holothuria parvula uses this method frequently splitting into two a little in front of the midpoint the two halves each regenerate their missing organs over a period of several months but the missing genital organs are often very slow to develop 67 the larvae of some echinoderms are capable of asexual reproduction this has long been known to occur among starfish and brittle stars but has more recently been observed in a sea cucumber a sand dollar and a sea urchin 68 this may be by autotomising parts that develop into secondary larvae by budding or by splitting transversely autotomised parts or buds may develop directly into fully formed larvae or may pass through a gastrula or even a blastula stage new larvae can develop from the preoral hood a mound like structure above the mouth the side body wall the postero lateral arms or their rear ends 68 69 70 cloning is costly to the larva both in resources and in development time larvae undergo this process when food is plentiful 71 or temperature conditions are optimal 70 cloning may occur to make use of the tissues that are normally lost during metamorphosis 72 the larvae of some sand dollars clone themselves when they detect dissolved fish mucus indicating the presence of predators 70 72 asexual reproduction produces many smaller larvae that escape better from planktivorous fish implying that the mechanism may be an anti predator adaptation 73 larval development edit a bilaterally symmetric echino pluteus larva with larval arms starfish late bipinnaria development begins with a bilaterally symmetrical embryo with a coeloblastula developing first gastrulation marks the opening of the second mouth that places echinoderms within the deuterostomes and the mesoderm which will host the skeleton migrates inwards the secondary body cavity the coelom forms by the partitioning of three body cavities the larvae are often planktonic but in some species the eggs are retained inside the female while in some the female broods the larvae 74 75 the larvae pass through several stages which have specific names derived from the taxonomic names of the adults or from their appearance for example a sea urchin has an echinopluteus larva while a brittle star has an ophiopluteus larva a starfish has a bipinnaria larva which develops into a multi armed brachiolaria larva a sea cucumber s larva is an auricularia while a crinoid s is a vitellaria all these larvae are bilaterally symmetrical and have bands of cilia with which they swim some usually known as pluteus larvae have arms when fully developed they settle on the seabed to undergo metamorphosis and the larval arms and gut degenerate the left hand side of the larva develops into the oral surface of the juvenile while the right side becomes the aboral surface at this stage the pentaradial symmetry develops 76 77 a plankton eating larva living and feeding in the water column is considered to be the ancestral larval type for echinoderms but in extant echinoderms some 68 of species develop using a yolk feeding larva 12 the provision of a yolk sac means that smaller numbers of eggs are produced the larvae have a shorter development period and a smaller dispersal potential but a greater chance of survival 12 distribution and habitat edit echinoderms are globally distributed in almost all depths latitudes and environments in the ocean living echinoderms are known from between 0 to over 10 000 meters adults are mainly benthic living on the seabed whereas larvae are often pelagic living as plankton in the open ocean some holothuroid adults such as pelagothuria are pelagic 78 in the fossil record some crinoids were pseudo planktonic attaching themselves to floating logs and debris some paleozoic taxa displayed this life mode before competition from organisms such as barnacles restricted the extent of the behaviour 79 mode of life edit locomotion edit echinoderms use their tube feet to move about colobocentrotus atratus shown further information animal locomotion echinoderms primarily use their tube feet to move about though some sea urchins also use their spines the tube feet typically have a tip shaped like a suction pad in which a vacuum can be created by contraction of muscles this combines with some stickiness from the secretion of mucus to provide adhesion the tube feet contract and relax in waves which move along the adherent surface and the animal moves slowly along 80 brittle stars are the most agile of the echinoderms any one of the arms can form the axis of symmetry pointing either forwards or back the animal then moves in a co ordinated way propelled by the other four arms during locomotion the propelling arms can made either snake like or rowing movements 81 starfish move using their tube feet keeping their arms almost still including in genera like pycnopodia where the arms are flexible the oral surface is covered with thousands of tube feet which move out of time with each other but not in a metachronal rhythm in some way however the tube feet are coordinated as the animal glides steadily along 82 some burrowing starfish have points rather than suckers on their tube feet and they are able to glide across the seabed at a faster rate 83 sea urchins use their tube feet to move around in a similar way to starfish some also use their articulated spines to push or lever themselves along or lift their oral surfaces off the substrate if a sea urchin is overturned it can extend its tube feet in one ambulacral area far enough to bring them within reach of the substrate and then successively attach feet from the adjoining area until it is righted some species bore into rock usually by grinding away at the surface with their mouthparts 84 sea cucumbers like this neothyonidium magnum can burrow using peristaltic movements most sea cucumber species move on the surface of the seabed or burrow through sand or mud using peristaltic movements some have short tube feet on their under surface with which they can creep along in the manner of a starfish some species drag themselves along using their buccal tentacles while others manage to swim with peristaltic movements or rhythmic flexing many live in cracks hollows and burrows and hardly move at all some deep water species are pelagic and can float in the water with webbed papillae forming sails or fins 85 the majority of feather stars also called comatulida or unstalked crinoids and some stalked forms are motile several stalked crinoid species are sessile attached permanently to the substratum movement in most sea lilies is limited to bending their stems can bend and rolling and unrolling their arms a few species can relocate themselves on the seabed by crawling feather stars are unattached and usually live in crevices under corals or inside sponges with their arms the only visible part some feather stars emerge at night and perch themselves on nearby eminences to better exploit food bearing currents many species can walk across the seabed raising their body with the help of their arms or swim using their arms most species of feather stars however are largely sedentary seldom moving far from their chosen place of concealment 86 feeding edit the modes of feeding vary greatly between the different echinoderm taxa crinoids and some brittle stars tend to be passive filter feeders 87 88 enmeshing suspended particles from passing water most sea urchins are grazers 89 sea cucumbers are deposit feeders 90 and the majority of starfish are active hunters 91 crinoids catch food particles using the tube feet on their outspread pinnules move them into the ambulacral grooves wrap them in mucus and convey them to the mouth using the cilia lining the grooves 87 the exact dietary requirements of crinoids have been little researched but in the laboratory they can be fed with diatoms 92 basket stars are suspension feeders raising their branched arms to collect zooplankton while other brittle stars use several methods of feeding some are suspension feeders securing food particles with mucus strands spines or tube feet on their raised arms others are scavengers and detritus feeders others again are voracious carnivores and able to lasso their waterborne prey with a sudden encirclement by their flexible arms the limbs then bend under the disc to transfer the food to the jaws and mouth 93 many sea urchins feed on algae often scraping off the thin layer of algae covering the surfaces of rocks with their specialised mouthparts known as aristotle s lantern other species devour smaller organisms which they may catch with their tube feet they may also feed on dead fish and other animal matter 89 sand dollars may perform suspension feeding and feed on phytoplankton detritus algal pieces and the bacterial layer surrounding grains of sand 94 sea cucumbers are often mobile deposit or suspension feeders using their buccal podia to actively capture food and then stuffing the particles individually into their buccal cavities others ingest large quantities of sediment absorb the organic matter and pass the indigestible mineral particles through their guts in this way they disturb and process large volumes of substrate often leaving characteristic ridges of sediment on the seabed some sea cucumbers live infaunally in burrows anterior end down and anus on the surface swallowing sediment and passing it through their gut other burrowers live anterior end up and wait for detritus to fall into the entrances of the burrows or rake in debris from the surface nearby with their buccal podia 95 nearly all starfish are...