Deuterostomia: Echinoderms

Superphylum Deuterostomia

• Consists of sea stars, sea urchins, sea cucumbers, and sea feathers.
• Exclusively marine species.
• Triploblastic and eucoelomate tissues.
• The first appearance of an endoskeleton in the animal kingdom.
• Calcareous ossicles are formed by mesoderm tissue underneath the epidermis.
• Ossicles can be individual and attached to each other rigidly or hinged.
• Echinoderms are the most ancestral of all deuterostomes.
• An ancestor of this phylum was a sessile filter-feeding animal.
• Larval body plan is bilateral, while the adult body plan is radial.
• Hox genes control the shift in body symmetry from bilateral larvae to radial adults.

Echinoderm Water Vascular System

• Central coelomic cavity transports seawater within the animal.
• The ventral surface contains tube feet, which are thin-walled extensions of the epidermis.
• Water enters through the madreporite (sieve plate), passes through the stone canal into the ring canal.
• Radial canals extend into each arm and connect with the ring canal.
• Ampullae, connected to the radial canals, fill with seawater.
• Contraction of ampullae extends the tube feet, and release retracts them, allowing coordinated movement.
• Gas exchange occurs through the tube feet, serving as the respiratory system.
• Oxygen diffuses into the tube feet and travels along the water vascular system.
• Carbon dioxide travels along the water vascular system and diffuses out of the tube feet.

Echinoderm Morphology

• Each arm/section contains digestive glands, gonads, a nerve cord, and tube feet.
• The digestive system is complete, with a mouth on the ventral side and an anus on the dorsal side.
• Ingested food moves into the cardiac stomach, then into the pyloric stomach, which moves food into radial canals to each arm.
• Each arm has digestive glands that digest the food.
• A simple nerve ring aligning with the ring canal serves as the nervous system.
• Radial nerve cords branch from the nerve ring, with one cord extending into each arm, but there is no organized brain.
• Metabolic waste is transported via the water vascular system and diffuses out of the tube feet, serving as the excretory system.
• Osmoregulation is unnecessary as body fluid is very similar to seawater.
• Larvae have metanephridia excretory systems.
• Animals are dioecious, and reproduction occurs via broadcast spawners with external fertilization.

Echinodermata Classes

Class Asteroidea (Sea Stars)

• Large animals with five thick arms extending from a central body.
• Predators or scavengers.
• Some species suck food into their stomachs, while others evert their stomachs and begin digestion externally.
• Use tube feet for walking.
• Glands excrete adhesive carbohydrates and proteins.
• Move in a coordinated way with one arm as the leading arm, due to coordination through the nerve ring.
• Light-sensing eyespot at the end of each arm.

Class Ophiuroidea (Brittle Stars)

• Small animals with five long, thin arms extending from a central body.
• Harder endoskeleton with hinged ossicles, allowing for greater flexibility of the arms.
• Predators that grab food with their arms, aided by mucus excreted from tube feet.
• Tube feet do not have ampullae.
• Coordinate movement by having one leading arm while two arms on each side sweep back and forth, creating a jerky walking/swimming motion
• Arms have spine projections for traction
• Arms have nerve ganglia along each arm, allowing better coordination between arms.
• Most species have light intensity sensing cells, sensing light/shadows from a direction.

Class Echinoidea (Sea Urchins and Sand Dollars)

• No arms, spherical shape with tube feet.
• Flattened spherical shape with a fixed lattice of ossicles forming a fused body called a "test."
• The test is covered in spines.
• Sea urchins are browsing herbivores that scrape seaweed from rocks using movable ossicles called "Aristotle's lantern."
• Aristotle’s lantern has 5 jaws and 1 tooth in each jaw, connected by a ligament
• Sand dollars are burrowing deposit feeders with tube feet modified for burrowing.
• Five ambulacral grooves in place of arms, where tube feet are located.
• Tube feet have ampullae and are used for walking, but the water vascular system isn't as important due to the rigid test.

Class Crinoidea (Sea Lilies and Sea Feathers)

• The oldest surviving group of echinoderms.
• Filter feeders, like their ancestors, with the mouth facing upwards.
• Most are sessile.
• Sea lilies are entirely sessile, connected to the substrate via a stalk.
• Arms have increased surface area via feather-like modifications
• Tube feet collect food and carry it to the mouth.
• Sea feathers can switch between being sessile and being motile.
• They swim by waving their arms up and down.

Class Holothuroidea (Sea Cucumbers)

• Exhibit secondary bilateral symmetry as adults.
• They have an elongated body axis extending from the mouth to the anus.
• The endoskeleton is very reduced with only a few internal ossicles and no external spines.
• Tube feet are located in grooves along the main body axis.
• Specialized tube feet at the anterior end form long tentacles.
• Deposit feeders that use tentacles to gather food.
• Respiration takes place in the respiratory tree, which branches from the intestines.
• They breathe through the anus by taking water into their anus to fill the respiratory tree.

Phylum Chordata

• All are bilateral deuterostomes with a complete digestive system.
• Triploblastic eucoelomates.
• Kidneys remove metabolic waste.
• Closed circulatory system with a heart.
• Brain with a central nervous system.
• Respiratory system varies depending on the group (gills, lungs, skin).
• Most species fall into subphylum vertebrata (also called craniata), but Hemichordates, Cephalochordata (lancelets), and Urochordata (tunicates) are exceptions.

Five Defining Characteristics of all Vertebrates

• Notochord:
  • Flexible but stiff rod structure derived from mesoderm tissue.
  • Runs from anterior to posterior of the body, dorsal of the digestive tube and ventral of the nerve cord.
  • Provides rigid support and muscle attachment sites.
  • Longitudinally incompressible but laterally and dorsally flexible.
  • In true vertebrates, replaced with the vertebral column (spine).
• Hollow dorsal nerve cord:
  • Hollow tube derived from ectoderm.
  • Runs down anterior to posterior of the body, consolidating into an anterior brain.
  • Located just dorsal of the notochord.
  • Develops into the brain and spinal cord (central nervous system).
• Pharyngeal slits (gills):
  • Originated in filter feeders, allowing water to enter the mouth and leave through gill slits.
  • In aquatic vertebrate fishes, develop into gill support in jawless fishes and jaw support in jawed fishes.
  • In land vertebrates, develop into parts of the ear, tonsils, and thymus.
• Post-anal tail:
  • Extends beyond the anus.
  • Used for locomotion in aquatic species.
  • Adapted for balance in terrestrial species (reduced to the coccyx in great apes and humans).
• Endostyle/thyroid gland:
  • Mucus-producing tissue on the floor of the pharynx, producing substances similar to thyroid hormones.
  • Important in iodine metabolism.
  • In most vertebrates, the endostyle is present at the beginning of embryological development but develops into the thyroid gland as development continues.
  • Urochordata (tunicates) and cephalochordata (lancelets) have an endostyle as adults.

Chordate Clades that are "Invertebrates"

• Cephalochordata (Lancelets):
  • Adults have all five characteristics.
• Urochordata (Tunicates/Sea Squirts):
  • Larvae have all five defining characteristics
  • Adults have pharyngeal slits and an endostyle only.

Subphylum Vertebrata (Craniata)

• Largest group of Chordata.
• Cranium: bony, cartilaginous, or ligamentous structure surrounding the brain, including jaw and facial structures.
• Has the five defining characteristics of Chordata.
• Vertebral column (spine): irregular-shaped bones joined together to form a backbone.
• Embryos have a notochord which develops into the vertebral segments of a true backbone.
• Segments can be cartilaginous or osseous.

Agnathostomes - Jawless Vertebrates

• Two classes:
  • Myxini (Hagfish):
    • Scavengers.
    • Blind but have sensory barbels around the mouth.
    • Cartilaginous cranium.
    • Have a notochord for body support, not vertebrae.
    • Have keratin teeth on a cartilaginous base for rasping food from dead animals.
  • Petromyzontidae (Lampreys):
    • Parasitic.
    • Have eyes and a cerebellum.
    • Cartilaginous cranium.
    • Simple vertebrae.
    • Have a suction mouth with a rasping tongue.

Gnathostomes - Jawed Vertebrates

• Divided into Fishes and Tetrapods.
• Tetrapods (four limbs): Amphibians, reptiles, birds, mammals.
• Tetrapods are divided into Amphibians and Amniotes.
• Amniotes: animals with eggs that contain extraembryonic membranes (yolk, amnion, chorion, allantois) adapted to terrestrial life (mammals, birds, reptiles).

Gnathostomes - Jaw mouth

• True, hinged jaw:
  • Allows for biting, grasping, and tearing food.
  • Bigger bites means increased food ingestion, leading to more energy available & large body size and faster speed
• Paired pectoral and pelvic fins:
  • Allowing Accurate stable control of movement.
• The jaw & paired fins allowed fish to become active feeders.

Gnathostomes - Two Classes

Chondrichthyes (Sharks, Rays, Skates)

• Almost exclusively marine species with cartilaginous bone.
• Characterized by dorsolaterally flattened, sleek fusiform bodies.
• They are generally fast, active swimmers.
• Almost exclusively carnivorous, but some are filter feeders.
• Intestine has a spiral valve that slows food passage for greater nutrient absorption.
• Rectal gland secretes high concentrations of sodium chloride, aiding kidney osmoregulation.
• Have placoid scales that are pointed/cone-shaped and abrasive.
• Chondrichthyes' teeth evolved from placoid scales.
• Scales do not grow as the animal grows; the animal produces more scales.
• Cartilaginous bodies are denser than water; have large livers that contain buoyant oils and large pectoral fins that provide lift.
• Have a heterocercal caudal (tail) fin.
• Sensory System:
  • Excellent eyesight but are colorblind.
  • Some have binocular vision.
  • Acute sense of smell; water flows into each nostril, through a nasal sac, and then out the nostril, lined with olfactory epithelium cells.
  • Ampullae of Lorenzini allow for rapid electromagnetic field detection.
  • Lateral line detects water motion; receptors are the same as hair cells found in ears of terrestrial animals.
• Respiratory System:
  • Gills are the respiratory organs.
  • Thin tissue filaments with extensive folding increase surface area for oxygen absorption from the water.
  • Diffusion occurs from high to low concentrations; oxygen-poor blood flow countercurrent (opposite) to the flow of water across the gills.
  • Carbon dioxide is removed via diffusion from high concentrations in the blood to low concentrations in the water.
• Reproduction:
  • Sexual reproduction with internal fertilization; males have claspers.
  • Produce relatively few offspring at a time with no parental care.
  • Offspring development via:
    • Oviparous (all skates, some sharks): female lays fertilized eggs; the embryo receives nutrition from the egg yolk; eggs are covered in a leathery case.
    • Viviparous (all rays, most sharks): the egg sac with the embryo attaches to the mother's oviduct, and the embryo gets nutrition from the mother; live birth.
    • an are Ovoviviparous: fertilized egg develops in the mother's body; the embryo receives nutrition from the egg yolk (not from the mother's body), hatches, and completes development in utero, then the mother gives birth.
• Subclass Elamobranchii: includes sharks, skates, and rays; subclass Holocephali includes chimeras.
  • Mouth armed with rows of serrated, pointed enameled teeth (phosphatized mineralization).
  • Teeth in the first row are the current functional teeth; there are rows of replacement teeth behind the row of functional teeth.
  • If a tooth is broken or lost, a replacement tooth moves into position.
  • Replacement rate is rapid; young elasmobranchs can replace teeth weekly.
  • Most sharks go through upwards of 50,000 teeth in a lifetime.
• Superorder Batoidea (Rays):
• Bottom-dwellers and have spiracles that allow them to bury in the sand and still breathe.
• Manta rays and devil rays are large and pelagic filter feeders.
• Pectoral fins are enlarged and fused with the head, providing propulsion/thrust.
• The caudal fin is adapted into a whip-like tail with a jagged edge and sharp spines with venom glands, used for defense.
• Teeth are modified into flattened, spiky bars that crush crustaceans, mollusks, and small fish.
• Continuously replace teeth

Osteichthyes (Osseous Bone, Jawed Fish)

• Have an endoskeleton made of osseous bones.
• Both marine and freshwater species with laterally flattened bodies.
• Have overlapping teleost scales.
• The scales grow larger as the fish grows larger.
• Are carnivorous, herbivorous, and omnivorous.
• Osseous bodies are denser than water and use a swim bladder to control buoyancy.
• Sensory system:
  • Excellent eyesight; most can see color (individual species perceive the colors typical of their environment).
  • Acute sense of smell; water flows into each nostril through a nasal sac lined with olfactory epithelium cells.
  • Lateral line to detect water motion
• Subclasses:
  • Actinopterygii (Ray-finned fish): predominant subclass of Osteichthyes.
  • Have long, thin endoskeletal bones in fins; muscles that move the fins are located in the body.
  • Sarcopterygii (Lobe-finned fish): only a few extant species, very ancient.
  • Have short, fleshy fins with thick bones and muscle in the fins; thought to have given rise to terrestrial tetrapods.
• Reproduction:
  • Fertilization is almost exclusively via courtship spawning.
  • Embryos develop within eggs.
  • Spherical eggs with yolk for nourishing the embryo.
  • Some species guard their eggs; most do not offer parental care.
  • Sea horses are an example of internal fertilization and are ovoviviparous.
  • The female lays eggs in the male's pouch, and he releases sperm into the pouch.
  • Embryos develop within the egg inside the father’s pouch.
• When embryos hatch, the father releases them from his pouch

Gnathostome Fish Circulatory System

• Two-chambered heart consisting of one atrium and one ventricle.
• Blood flows from the heart to the gills to the body and back to the heart in a single circuit.
• Deoxygenated blood from the body enters the atrium, flows from the atrium into the ventricle.
• The ventricle pumps blood to the gills, where it is re-oxygenated (gill circulation); blood flows from the gills to the body and returns to the atrium deoxygenated (systemic circulation).