Lecture #13

1. Lecture #13 Phylum Echinodermata

2. Features common to Phylum Echinodermata • 1. triploblastic • 2. pentaradial symmetry in adults; bilateral symmetry in larvae • 3. coelomate • 4. endoskeleton made of calcerous plates called ossicles • 5. water vascular system for locomotion • 6. complete digestive tract • 7. hemal system derived from the coelom – fluid filled canals, function unknown • 8. nervous system consisting of a nerve net, a nerve ring and radial nerves

3. Phylum Echinodermata • pentaradialsymmetry • body parts are arranged in fives or multiples of fives around an oral-aboral central axis • probably developed during the evolution of sedentary lifestyles • however, most echinoderms are now mobile • adult form is pentaradial • larvae have bilateral symmetry • internal/endoskeleton made of calcium carbonate • consisting of calcium carbonate plates called ossicles • derived from the mesoderm • covered by an epidermal layer • skeleton can be modified to form articulating or fixed spines that project from the body surface

4. Water vascular system • water-filled canals • extensions are called tube feet • derived embryologically from the coelom • ciliated internally – movement of water • includes a ring canal that surrounds the mouth • ring canal opens to the outside through a stone canal and a sieve-like pore called the madreporite • madreporite may serve as an inlet to replace water lost from the system • may also help equalize pressure differences between the water-vascular system and the outside • five (or a multiple of five) radial canals branch from the ring canal and run down each arm or arch down the body wall toward the aboral end • multiple lateral canals branch off of the radial canals and end in tube feet madreporite

5. -tube feet • extensions off the lateral canals • emerge to the outside through openings between the ossicles • inside the body they end as a bulblike, muscular ampulla • when the ampulla contracts- forces water into the tube which then extends • outside the body - tube feet often have suction cups at its distal end • function of tube feet: locomotion • also functions in feeding and diffusion of respiratory gases and nitrogenous wastes

6. Phylum Echinodermata • approximately 7,000 species • all are marine • living echinoderms are classified into 6 classes • 1. Asteroidea– sea stars • 2. Ophiuroidea– brittle stars and basket stars • 3. Echinodea– sea urchins and sand dollars • 4. Holothuroidea– sea cucumbers • 5. Crinoidea– sea lilies and feather stars • 6. Cocnentricycloidea– sea daises

7. Class Asteroidea • sea stars • about 1,500 species • live on hard substrates in marine environments • may be brightly colored • five arms radiating from a central disc • mouth is found on the ventral side (oral surface) of this disc • tube feet located on the ventral side/oral surface • moveable spines project from the endoskeleton – on the dorsal or aboral surface • endoskeleton comprised of ossicles • dermal branchiae – thin folds of the body wall extend between these ossicles • function in gas exchange and excretion of metabolic wastes

8. Class Asteroidea • in some species – the aboral surface has pincerlikeappendages called pedicelleriae • clean the body surface of debris • have a protective function • series of ossiclesfound on the oral surface of each arm - form the ambulacral groove • each groove houses aradial canal • paired rows of tube feet protrude through the body wall on either side of the groove • locomotion: tube feet move in a stepping motion • alternate extension, attachment, and contraction of these feet move the sea star across its substrate • suction discs on the tube feet allow effective adhesion in strong currents ambulacral groove

9. Class Asteroidea • nervous system: • consists of a nerve ring that encircles the mouth • plus radial nerves down each arm or running down the body wall toward the aboral end • responsible for coordination of the tube feet • body wall also houses a nerve net • sensory receptors are distributed over the surface of the body and the tube feet • sea stars respond to light, chemicals and various mechanical stimuli • often have specialized photoreceptors at the tip of each arm

10. Class Asteroidea • digestion and feeding: • feed on snails, bivalves, small crustaceans, polycheates, corals and detritus • mouth surrounded by moveable oral spines • mouth opens to a short esophagus and then into a large stomach that fills most of the central disc • esophagus first leads into a larger cardiac stomach(oral stomach) for the receipt of food • then into a smaller pyloric stomach (aboral stomach) • pyloric stomach leads into secretory and absorptive digestive glands - also called pyloric cecae – 2 per arm • a short intestine leads to an non –functional anus that opens on the aboralsurface • large, undigested materials are expelled out through the mouth NOT the anus mouth

11. sea stars feed on bivalves by prying open the shell • wraps itself around the outside of the shell • tube feet attach to the outside of the shell and forces the valves apart • changes tube feet when others begin to tire • once open slightly - evertsthe cardiac portion of the stomach into the mollusc • releases digestive enzymes and partial digestion digestionweakens the bivalves adductor muscles - the sea star then pulls the bivalve open completely • takes in the partially digested mollusc for continued digestion in the cardiac stomach

12. reproduction: • dioecious • sex is indistinguishable externally • two gonads present in each arm – open via gonoporesat the base of the arm • external fertilization • gamete release is timed with pheromone release that induces other sea stars to spawn – increases chances of fertilization • embryology has been studied extensively – easy to spawn and fertilize in the lab • form a bilateral embryo = bipinnaria larva • float around in the plankton • development of radial arms results in a branchiolarialarva – develops into a juvenile sea star • regenerative powers are remarkable – can regenerate entire radial arms

13. Class Echinoidea • sea urchins and sand dollars • about 1,000 species • nearly all marine • sea urchins specialized for living on hard substrates – wedging themselves into cracks and crevices • sand dollars – burrow into sand or mud • use their tube feet to catch organic matter settling on them or passing over them

14. Class Echinoidea • sea urchin: • skeleton is called a test • 10 closely fitted plates that arch between oral and aboral ends • five rows of ambulacral plates have openings for tube feet • alternate with 5 rows of interambulacral plates with spines • moveable spines - base of each spine has muscles to control it • some spines are hollow and contain venom to kill prey • can have pedicelleriae for prey capture and cleaning • water vascular system is similar to sea stars • radial canals run along the inner wall – between the oral and aboral ends • move by pushing spines against the substrate and using their tube feet to pull them along

15. mouth • feeding and digestion: • feed on algae, bryozoans, corals and other echinoderms • oral end with a mouth – faces toward the substrate • mouth is surrounded by oral tube feet • possess a chewing apparatus called Aristotle’s lantern • can be projected from the mouth • 35 ossicles and attached muscles • cuts food into small pieces for ingestion • have a large coelom • coelomic fluid is used as their primary circulatory fluid

16. gas exchange: diffusion across the tube feet • small gills are also found surrounding the mouth • similar nervous system to the sea stars • reproduction and development: • dioecious • five gonads along the body wall of interambulacral plates • the gonads open at ossicles found at the aboral end - called genital plates • sand dollars have four gonads and four plates • external fertilization

17. Class Holothuroidea • sea cucumbers • about 1,500 species • found at all depths of the ocean • have no arms • elongated along the oral-aboralaxis • lie on one side which is flattened – called the ventral side • thick and muscular body wall • lacks spines or pedicelleriae • outer epidermis covering a dermis • dermis contains embedded ossicles • microscopic and do not function in shape • 10 larger ossiclesform a ring or water canal that surrounds the oral end • site of muscle attachment • below the dermis – layer of circular muscle overlying longitudinal muscles • used for locomotion http://webs.lander.edu/rsfox/invertebrates/cucumaria.html

18. Class Holothuroidea • water-vascular system is filled with coelomic fluid • ring canal – surrounds the oral end of the digestive tract • gives rise to pouches called Polianvesicles • function is unknown – may function as a fluid reservoir • 5 radial canals also arise from the ring canal – give rise to tube feet that run from oral to aboral poles • ventral side has 3 rows of tube feet used for attachment • dorsal surface has two rows of tube feet – may be absent in some • tube feet surrounding the mouth – can elongate and are often referred to as tentacles • internal madreporite • connected to the ring canal via a stone canal tube feet

19. Class Holothuroidea • respiratory trees for gas exchange • attach at the rectum and branch throughout the body • pumping action of the rectum circulates water through these trees • water is drawn into these trees – gases diffuse into the colemic fluid • fluid is circulated through the coelom for distribution of respiratory gases, wastes and nutrients • digestive system – ingest matter using their tentacles • mouth leads to a stomach plus a long, looped intestine • then leads to a rectum and an anus • tentacles are thrust into their mouths to “wipe off” the food trapped in mucus • extracellular digestion • hemal system has a function – food distribution • nervous system – similar to other echinoderms