MARB 435 1st Edition Lecture 20Current LecturePhylum Echinodermata Approximately 6,000 living species Good fossil record: 13,000 described fossil species Morphology: endoskeleton of calcareous ossicles pentamerous radial symmetry in the adults: body can be divided into five similar parts arranged around a central axis (but bilateral symmetry in larvae) Tricoelomate Catch connective tissue Coelomicwater vascular system forms hydraulic tube feet for locomotion, feedingand gas exchangeCatch connective tissue A.k.a. mutable connective tissue Variable and reversible rigidity of connective tissue Nerves control the stiffness of extracellular matrix Increase in Ca2+ stiffens the ECM (probably by forming bridges between proteins) Allows automy (voluntary casting off) of arms in a sea star or viscera in a sea cucumber as escape mechanismsDevelopment Typical deuterostome development:These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. radial cleavage Blastopore becomes anus, mouth forms secondarily Coelomic cavities develop by enterocoely Mesoderm is derived from archenteron Larva is always bilaterally symmetricalClass StelleroideaSubclass Asteroidea Approximately 1500 species Star-shaped body Occur worldwide, mostly in coastal waters Benthic Can be colorful Usually 5 arms, but sometimes up to 40Asteroidea morphology• Mouth lies in the middle of the oral surface• ambulacra (grooves) extend into the arms• Each ambulacrum contains 2-4 rows of tube feet for locomotion• Movable spines on each side can cover the ambulacrum• A small eyespot lies at the end of each ambulacrumEndoskeleton Consists of ossicles (small calcareous plates secreted by sclerocytes) Can form spines, rods, plates or crosses pedicellariae: claw-shaped ossicles Defense against parasites or small settling organisms Sometimes also used for prey captureNervous system Central nervous system: Circumoral nerves Radial nerves Peripheral nervous system ectoneural system: lies in the epidermis and is mainly sensory hyponeural system: lies in the coelomic lining and is mainly motoric Sense organs: Eyes at tip of arms Sensory tube feetDigestive system and feeding• Many asteroids can evert their cardiac stomach for extra-oral digestion• Stomach releases digestive enzymes and engulfs the prey• Most asteroids are scavengers and carnivoresAsexual Reproduction Some species can automize and regenerate arms Some can reproduce by fission by softening themutable connective tissueSexual Reproduction Gonochoric 10 gonads: 2 in each arm Each gonad has its own gonopore between the bases of the arms Most sea stars are free spawners: a single female can release up to 2,500,000 eggs! Most have planktonic larvae but some brood their offspringSea Star Wasting Syndrome First discovered in the ochre star, Pisasterochraceus, in June 2013 in Olympic National Park Affects at least 20 species Has been observed on the north American west coast from Alaska to northern Mexico inintertidal and subtidal habitatsSubclass Ophiuroidea About 2000 species Mostly cryptic lifestyles Five jointed arms that are clearly set off from the central disc Arms lack ambulacral groove Tube feet have no suckersReproduction and development Asexual reproduction common (by autotomy of arms) Sexual Reproduction: Have small pockets at the bases of the arms called bursae Gonads open into the bursae Larvae are often brooded in the bursae Planktonic
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