2Evolutionary Perspective Flourished in 400 million year-old seasMany were attached suspension feeders.12 of 18 classes now extinctRemain a major component of marine ecosystemsCharacteristicsCalcareous endoskeleton (ossicles)Adults with pentaradial symmetryWater-vascular systemComplete digestive tractHemal systemNervous system consisting of nerve net, nerve ring, and radial nerves
3Evolutionary Perspective DeuterostomesClosest relativesHemichordataChordataEvolved from bilaterally symmetrical ancestors?Larval stages are bilaterally symmetrical.Some extinct forms bilaterally symmetrical.Evolved from radially symmetrical ancestors?Arkarua tentatively identified as oldest extinct echinoderm.Pentaradially symmetrical
4Figure 16.1 Evolutionary relationships of the echinoderms to other animals.
5Echinoderm Characteristics Pentaradial symmetryBody parts arranged in fives around an oral-aboral axis.Some secondarily bilateralEvolution of the skeleton may account for pentaradial form(figure 16.2).
6Figure 16. 2 Pentaradial symmetry Figure Pentaradial symmetry. (a) Body parts are arranged in fives around an oral-aboral axis.(b) Arrangement of the body in fives means skeletal joints are not directly opposite one another. This arrangement may make the skeleton stronger than if joints were opposite one another.(a)(b)
7Echinoderm Characteristics Water vascular system (figure 16.3)Water-filled canal and tube feetRing canal opens to outside via stone canal and madreporite.Polian vesicles function in water storage.Tube feetMuscular ampullaOften suction cup at distal end (may also be blunt or pointed)FunctionsLocomotionAttachmentFeedingExchanges of respiratory gases and wastesSensory functionsHemal systemDistributes nutrients and large molecules
8Figure 16.3 The water-vascular system of a sea star.
9Class Asteroidea Sea stars Hard or sandy substrates Moveable and fixed spines roughen body surface.Dermal branchiae (papulae)Gas exchangePedicellariaePincerlikeClean and protect body surfaceTube feet with suction disks
10Class Asteroidea Maintenance Functions Predators and detritus feeders Ingest whole preyMany are bivalve predatorsInternal transport of gases, nutrients, and metabolic wastes by diffusion and hemal systemGas exchange and excretion by diffusion across dermal branchiaeNervous systemNerve ring, radial nerves, nerve netSensory receptorsWidely distributed over body surfacePhotoreceptors at tips of arms (specialized tube feet)
11Figure 16.4 Body wall and internal anatomy of a sea star.
13Regeneration, Reproduction, and Development Broken arm replacedEntire sea star from portion of central diskAsexual reproduction in someRegeneration after division of central diskSexual reproductionDioeciousTwo gonads per arm (figures )External fertilization and planktonic larval development (figure 16.6)
15Class Asteroidea Sea Daisies Previously class Concentricycloidea Highly modifies AsteroideaLack arms1 cm diameterDigestion and absorption of decaying organic matterFigure A sea daisy (Xyloplax medusiformis).
16Class Ophiuroidea Basket stars and brittle stars Arms long, sharply set off from central disk (highly branched in basket stars)No dermal branchiae or pedicellariaeTube feet lack suction disks.Madreporite on oral surfaceMuscles and articulating ossicles produce snake-like movements of arms.Water vascular system is not used in locomotion.
17Figure 16. 8 Class Ophiuroidea Figure Class Ophiuroidea. (a) A brittle star (Ophiopholis aculeata). (b) A basket star (Gorgonocephalus arcticus).
18Class Ophiuroidea Maintenance functions Predators and scavengers Arms sweep substrate.Basket stars are suspension feeders.Wave arms and trap plankton on mucus-covered tube feetCoelom confined to central disk.Distribution of nutrients, gases, wastesAmmonia lost by diffusion across tube feet and bursae.
19Figure 16.9 Oral view of the disk of the brittle star Ophiomusium.
20Regeneration, Reproduction, and Development Lost armsAutotomy commonFission across central diskSexual ReproductionDioeciousGonads associated with bursaGametes released into bursaEggs may be retained and fertilized within bursa or released for external fertilization.DevelopmentWithin bursa or as planktonic larvaeOphiopluteus is planktonic and metamorphoses to adult.
21Class Echinoidea Sea urchins, sand dollars, heart urchins Sea urchins—hard substratesSand dollars and heart urchins—sand or mud just below surfaceSea urchin skeletonTest of 10 sets of closely fitting platesAmbulacral plates with openings for tube feetInterambulacral plates articulate spinesPedicellaria with long stalk (may be venomous)
22Figure 16.10 (a) A sea urchin (Strongylocentrotus). (b) A sand dollar.(a)(b)
23Class Echinoidea Water-vascular system Spines Radial canal runs along inner body wall.Tube feet with ampullae and suction disksMadreporite opens at aboral surface.SpinesLocomotion and burrowing
24Figure 16.11Internal anatomy of a sea star.(b) Aristotle’s lantern.
26Reproduction and Development DioeciousGonads on internal body wallOne gonopore in each of 5 genital ossiclesSand dollars have 4 gential ossicles and gonopores.External fertilization and planktonic larvaeMetamorphosis to adult
27Class Holothuroidea Sea cucumbers Hard and soft substrates in all oceansElongate along oral-aboral axisOne side flattened and “ventral”Secondary bilateral symmetryOral tube feet enlarged and modified as tentacles.Body wall thick and muscular with microscopic ossicles.
28Figure 16.12 Class Holothuroidea (Parastichopus californicus).
29Class Holothuroidea Water-vascular system Madreporite internal Filled with coelomic fluidRing canal encircles oral end.1-10 Polian vesiclesRadial canals run between oral and aboral poles.Tube feed with ampulae and suction cups3 of 5 rows on flattened “ventral” surface used for attachment.
30Figure 16.13 Internal structure of the sea cucumber, Thyone.
31Class Holothuroidea Maintenance Functions Feed on particulate organic matter by sweeping substrate with mucus-covered tentaclesLong, looped intestineCirculationCoelomic fluidGas exchangeRespiratory trees attach to rectum.DefenseToxins in body wallEvert Cuverian tubules
32Class Holothuroidea Reproduction and Development Asexual Dioecious Single gonad and single gonoporeFertilization externalPlanktonic larvaeEggs may be trapped in female’s tentacles and transferred to body surface for larval brooding.AsexualTransverse fission and regeneration
33Class Crinoidea Sea lilies and feather stars Most primitive living echinodermsExtensive fossil recordSea liliesAttach permanently to substrate by stalkCrownCalyx and arms with pinnulesMouth and anus open to upper (oral) surface.Feather starsLack stalkSwimming and crawlingCling to substrate by cirri when at rest
36Class Crinoidea Maintenance functions Arms used in suspension feeding Plankton trapped by tube feetTransported with cilia to mouthOriginal function of water-vascular system (?)Cup-shaped nerve mass with radial nerves to armsLack nerve ring
37Class Crinoidea Reproduction and Development Development Regeneration Many dioeciousOthers monoeciousProtandry commonGametes released through ruptures in walls of arms.DevelopmentPlanktonic larvae metamorphose to adults.Some brood larvae on outer surface of armsRegenerationAs in other echinoderms
38Further Phylogenetic Considerations Crinoids most closely resemble oldest fossils.Mouth-up suspension feeding probably original orientation and function of water-vascular systemCalcium carbonate endoskeleton may have evolved to support filtering arms.Mobile, mouth-down, free-living lifestyle probably secondarily derivedAmpullae, suction disks, tentacles, and secondary bilateral symmetry adaptations for this mobile life style
39Figure 16.16 Evolutionary relationships among the echinoderm classes.