Annelids, Ecdysozoans, and Deuterostomes

Annelids

• Polychaetes (Sandworms):

  • Have parapodia: sideways projecting appendages off each segment.

  • Function: gas exchange and regulation.

  • Not actual legs.

• Oligochaetes (Earthworms and Leeches):

  • Segmentations are visible.

  • Leeches:

    • Mostly free-living and carnivorous.

    • Some are parasitic.

  • Earthworms:

    • Soil inhabitants: they eat their way through the soil.

    • Digestive process: extract organic matter as soil passes through their alimentary canal, then excrete the rest.

    • Benefits for soil:

      • Create air and water channels, improving drip and percolation.

      • Mucus binds the soil, improving texture.

      • Redistribute soil nutrients.

• Internal Anatomy of Earthworms:

  • Digestive tract runs through the entire body with the anus at the posterior end.

  • Digestive Tract: pharynx, crop, intestinal portion, and anus.

  • Most structures are segmented along with the body segments.

  • Vascular System: segmental vessels distribute nutrients to each segment.

  • Metanephridia: primordial kidney-type structures for waste removal in each segment.

  • Nerve Cord: Large ganglia (clusters of neurons) in each segment regulate structures within those segments.

• Lophotrochozoans and Lophophores:

  • Lophophore characteristics may not always be displayed in the adult form.

  • Example: Chordates have specific characteristics at some point in their development that aren't always present in adulthood.

Ecdysozoans

• Nematodes (Round Non-segmented Worms) and Arthropods.

• Named for their ability to secrete an external skeleton made of some compound.

• Ectasis: The process of secreting some form of outer layer.

  • Example: Molting in cicadas (shedding the old skeleton and secreting a new one).

  • The outer layer isn't always hard and rigid.

Nematodes

• Round, non-segmented worms.

• Exoskeleton: Tough outer cuticle that they secrete.

  • Shed and regrow the cuticle.

• Typically one end is blunt and the other is tapered.

• Habitats: Aquatic, soil, moist plant areas, and body fluids of animals.

• Have an alimentary canal (complete digestive system).

• Lack a circulatory system

  • Limits size and complexity.

• Ecological roles:

  • Involved in decomposition and nutrient cycling.

  • Some attack pest insects, helping host plants.

  • Some are parasitic to plants and animals.

• Human nematode parasites: Approximately 50 species.

  • Hookworms, pinworms, etc.

  • Trichinella: Causes trichinosis, contracted from undercooked pork.

    • Juvenile worms become encysted in muscle tissue.

    • Once ingested, the worms mature in the intestines.

    • They burrow through the endothelial lining into the muscles and produce more juvenile worms.

    • These worms then travel to other areas, such as skeletal muscles, and re-encyst themselves.

    • Trichinella can affect surrounding muscle tissue by influencing gene expression within the muscle cells, which code for proteins to make the muscle cells more elastic to house the parasites.

    • They also stimulate the development of new blood vessels to feed the worm.

    • Formula: $Muscle\ Cells \rightarrow More \ Elastic \rightarrow Parasite \ Housing$

    • Formula: $Muscle \ Cells \rightarrow New\ Blood \ Vessels \rightarrow Worm \ Feeding$

Arthropods

• Arthropod: Literally means jointed foot.

• Rigid exoskeleton made of chitin.

• Exoskeletons and endoskeletons both provide support and mobility by having muscles pull on them to enable movement.

• Exoskeleton Limitations:

  • Weight: Becomes too heavy for large terrestrial animals to move.

  • Growth: Prevents continuous growth; requires molting (shedding the exoskeleton).

  • Molting: Energy-intensive and leaves the animal vulnerable.

• Highly segmented body organized into discrete elements.

• Most numerous group of animals.

  • Insects, crustaceans, spiders, and scorpions, etc.

  • Two out of every three known animal species are arthropods.

  • Potentially over a billion species.

• Found in diverse habitats (aquatic, terrestrial, etc.).

• Body is covered by an exoskeleton that is periodically shed and regrown (molting).

• Origins traced back to the Cambrian period.

• Lobopods (like Hallucinogenia) are hypothesized to be related to modern arthropods due to segments, jointed legs, and exoskeletons.

• Over time, the amount of segments decreased.

• Arthropods have two specific Hox genes that affect their segmentation.

• Body Segments:

  • Head: Sensory receptors (eyes, antennae, olfaction), and mouth.

  • Thorax: Appendages specialized for defense (claws) or walking legs.

  • Abdomen: Specialization for swimming (in aquatic organisms), digestive organs, and reproductive organs.

• Open Circulatory System:

  • Heart pumps hemolymph (fluid carrying oxygen) through short vessels into sinuses (spaces surrounding organs and tissues).

  • Organs and tissues are bathed in hemolymph for gas exchange, nutrient exchange, and waste exchange before returning to the heart.

• Gas Exchange:

  • Aquatic species: Gills.

  • Insects: Tracheal system (branched series of tubes from the exoskeleton inward); body movements cause air to move through these vessels.

• Four Major Lineages:

  • Formula: $Chelicerates + Myriapods + Hexapods + Crustaceans = Arthropods$

  • Chelicerates: Sea spiders, land spiders, horseshoe crabs, ticks, mites, scorpions.

    • Named for chelicerae (claw-like feeding appendages).

    • Most members are arachnids (spiders, ticks, mites, scorpions).

    • Body Organization:

      • Cephalothorax: Combination of head and thorax, bearing walking appendages and sensory structures.

      • Chelicerae: Fangs that secrete digestive juices to liquefy prey, which is then sucked up.

      • Abdomen: Contains the bulk of the organs, heart, and silk glands (in spiders).

    • Book lungs: Gas exchange areas with capillaries close to the external environment.

  • Myriapods: Millipedes and centipedes (all terrestrial).

    • Millipedes: Two pairs of legs per segment; eat decaying plant matter.

    • Centipedes: One pair of legs per segment; carnivorous and often toxic with poison glands.

  • Hexapods: Insects and close relatives (six legs).

    • More insect species than all other animal species combined.

  • Live in terrestrial and freshwater habitats.

  • Fossils date back to the Devonian period.

  • Diversity explosion during the Carboniferous and Permian periods.

    • Wings: Extensions of the outer cuticle with muscles for flight.

      • First insects may have been dragonflies or their ancestors.

  • Evolved in tandem with angiosperms as pollinators.

  • Metamorphosis: Change post-initial development from larval stage to sexually mature adult.
    *Incomplete Metamorphosis: (Grasshoppers) Young resemble adults but are smaller and wingless; they undergo a series of molts to become more like adults, with the final molt resulting in a sexually mature adult.
    *Complete Metamorphosis:(Butterflies, moths, flies) Larval stage is very different from the adult stage and is specialized for eating; the larva grows and then undergoes a complete body change to emerge as the adult form (caterpillar, maggot, grub).

  • Widespread Ecological Roles:
    *Food source, pollinators, carriers of diseases, crop pests, and parasites.

  • Crustaceans: Crabs, lobsters, shrimps, barnacles, etc.

    • Decapods: Lobsters, crabs, shrimp, and crayfish.

    • Barnacles: Reclassified as crustaceans based on their larval stage; adult form is sessile with a shell.

    • Isopods: Shore bugs that look like pill bugs.

    • Copepods: Planktonic.

    • Found in freshwater and saltwater environments.

Deuterostomes

• Echinoderms (sea stars, sand dollars) and chordates.

• Echinoderms are closely related to chordates.

Echinoderms

• Echinoderm: Literally means spiny skin.

• Endoskeleton made of hard calcareous plates just below the surface skin.

• Includes sea stars, brittle stars, sea urchins, feather stars, sea cucumbers, and sea daisies.

• Water Vascular System: Internal hydraulic system used for suction in tube feet for locomotion and feeding.
  Formula: $Water\ Vascular \ System \rightarrow Hydraulic \ System \rightarrow Suction$

• Sea stars can regrow lost appendages if they include a portion of the central disk.

• Body regrows another starfish: Oyster farmers chop up the species and throw them back into the water, which basically just turns them into two starfish.

• Rudimentary nervous system controls muscles for limb movement and regulates the digestive system.
  Nervous system and Echinoderm pain system: Nerve receptors either have or do not have pathways to conduct pain impulses, so we cannot know if they are in pain.

Chordates

• Notochord: Flexible supporting rod that supports the length of the body between the digestive tract and the nerve cord; replaced by the vertebral column in vertebrates.

• Dorsal Hollow Nerve Cord: Central nervous system develops from this; the anterior portion becomes the brain, and the rest becomes the spinal cord.

• Pharynx: Oral cavity with pharyngeal gill slits at some point in their development.

• Post-Anal Tail

• At some point during their development, all chordates have these four characteristics, but not necessarily all in the adult.

• Formula: $Notochord + Dorsal \ Hollow \ Nerve \ Chord + Pharynx + Post\ Anal \ Tail = Chordates$

Annelids include polychaetes (sandworms) with parapodia for gas exchange and regulation, and oligochaetes (earthworms and leeches) with visible segmentations. Leeches are mostly free-living or carnivorous, while earthworms improve soil quality by creating air channels, binding soil, and redistributing nutrients. Earthworms have a complete digestive tract, segmented vascular system, metanephridia for waste removal, and a nerve cord with ganglia. Lophotrochozoans may not always display lophophore characteristics in adult form, similar to how chordates may not always show all characteristics in adulthood. Ecdysozoans, including nematodes and arthropods, are named for their ability to secrete an external skeleton, such as through ectasis (molting). Nematodes are round, non-segmented worms with a tough outer cuticle, found in various habitats, and play roles in decomposition and nutrient cycling, with some being parasitic. Arthropods have a rigid exoskeleton made of chitin, providing support and mobility but limiting weight and growth. They have highly segmented bodies, diverse habitats, and an open circulatory system. Major lineages include chelicerates, myriapods, hexapods, and crustaceans. Deuterostomes include echinoderms (like sea stars) with an endoskeleton and a water vascular system for locomotion, and chordates, characterized by a notochord, dorsal hollow nerve cord, pharynx, and post-anal tail, though not all features are present in adults.