Invertebrates: Poriferans (Sponges)

• Invertebrates lack a backbone or spinal column.
• They are a diverse group of animals, with varying characteristics.
• The classification of invertebrates is artificial due to their lack of evolutionary relationship.
• Major invertebrate groups include Poriferans, Cnidarians, Nematodes, Platyhelminthes, Annelids, Molluscs, Arthropods, and Echinoderms.

Poriferans (Sponges)

• Sponges are the simplest invertebrates.
• They are aquatic animals, living mainly in marine environments, attached to rocks.
• Poriferan bodies are bag-shaped and hollow, with an inner space called the atrium.
• The body wall contains many pores and a large opening called the osculum.
• Sponges lack organs or systems.
• They have spicules, small pieces of calcium carbonate or silica among their body cells that act as a skeleton.
• They feed through filtration, with water entering pores and exiting through the osculum.
• Choanocytes, cells lining the atrium, capture nutritive particles from the water, and have a flagellum to create water flow.
• Asexual reproduction occurs through fragmentation or budding, while sexual reproduction occurs through eggs.

Cnidarians

• This phylum includes jellyfish, anemones, hydra, and corals.
• They are aquatic animals, primarily marine.
• Cnidarians have radial symmetry and lack organs or systems.
• Their body is soft with a single opening (mouth-anus) surrounded by tentacles.
• The gastrovascular cavity is a hollow space inside their body.
• Cnidarians have two basic body forms:
  • Medusa: Umbrella-shaped and free-swimming.
  • Polyp: Bag-shaped and attached to a substrate (sessile).
• Coral species form colonies composed of numerous individual organisms.
• They are carnivorous and use tentacles to capture prey.
• Stinging cells called cnidocytes on their tentacles contain poisonous liquid and a filament to inject into prey.
• Cnidarian life cycle involves alternation of generations, including medusae for sexual reproduction and polyps for asexual reproduction.

Worms

• This group includes Annelids, Nematodes, and Platyhelminthes.
• Worms share bilateral symmetry and a soft, elongated body without a skeleton or limbs.

Platyhelminthes (Flatworms)

• Some are free-living and aquatic, like planarians, while others, like tapeworms (Taenia), are parasitic.
• They possess a flat body divided into rings.
• Platyhelminthes lack digestive or respiratory systems.
• They reproduce sexually and are hermaphroditic, possessing both male and female sexual organs in one individual.
• Self-fertilization occurs in this group.

Nematodes

• Some are free-living in aquatic or terrestrial environments, others are parasitic, such as hookworms.
• They have a cylindrical body that is not divided into segments or rings.
• Nematodes lack a respiratory system.
• Nematodes reproduce sexually and have separate sexes (male and female individuals).

Annelids (Ringed Worms)

• They have a cylindrical body divided into segments called metameres, each similar and containing the same organs.
• Bristles (setae) between metameres aid in movement.
• Annelids reproduce sexually, with separate sexes or hermaphroditic species.
• Cross-fertilization occurs in this phylum.
• The phylum is divided into three classes:
  • Polychaetes: Free-living aquatic animals with many bristles, breathing through gills, and are filter feeders or carnivores.
  • Oligochaetes: Free-living terrestrial animals with few bristles, breathing through the skin, and are detritus feeders.
  • Hirudineans: Parasitic leeches that are bloodsuckers.

Molluscs

• Most molluscs are aquatic, living in marine or freshwater environments, but some are terrestrial.
• They exhibit bilateral symmetry and have a soft body divided into three parts:
  • Head
  • Visceral mass containing organs
  • Muscular foot adapted for different functions depending on the group
• The body is covered by the mantle, a thick membrane that produces a protective shell.
• Nutrition: Molluscs contain a digestive system with a mouth, anus, and glands and can be herbivorous or carnivorous.
• Aquatic species breathe through gills, while terrestrial species breathe through lungs.
• Molluscs have an open circulatory system.
• Interaction: Molluscs have a well-developed brain and sense organs.
• Reproduction: Most molluscs are hermaphroditic and oviparous; larvae undergo metamorphosis to reach adulthood.
• Classification:
  • Gastropods like snails, slugs and sea snails, are terrestrial and aquatic, most having a single spiral-shaped shell.
    • Gastropods have a well-developed head with four tentacles (two with eyes) and a rasping tongue (radula) for herbivorous feeding, and a single muscular foot for movement.
  • Bivalves, including clams, cockles, and mussels, are aquatic and have a shell with two valves, lacking a head.
    • Bivalves are filter feeders that use a single axe-shaped foot to excavate.
  • Cephalopods, such as octopuses, squids, and cuttlefish, are aquatic and possess an internal shell (except octopuses).
    • Cephalopods have a well-developed head with large eyes, use tentacles around the mouth to capture prey, are carnivorous, and possess a beak.

Arthropods

• Arthropods are made up of nearly 90% of all animal species, totaling over a million and are the largest and most varied group of animals.
• Most are terrestrial, but some live in aquatic environments, both marine and freshwater.
• Morphologic characteristics:
  • Arthropods have bilateral symmetry and a segmented body covered by rigid exoskeleton (chitin).
  • The arthropod body is divided into the head, thorax, and abdomen.
    • The cephalothorax is present in some groups where the head and thorax are joined.
  • They possess jointed legs for walking, swimming, hopping, digging, catching prey.
    • Arthropods also have other appendages like mouthparts, antennae, and wings.
• Vital functions:
  • Arthropods have a digestive system with a mouth, anus, and glands adapted for different feeding styles.
    • In general, arthropods have mouthparts for cutting, chewing, piercing, sucking and licking.
  • Spiders have chelicerae to inject digestive juices into prey, and insects have pedipalps.
  • Aquatic species use gills to breathe, while terrestrial species use tracheae.
    • Tracheae form an exclusive respiratory system with tiny tubules branching throughout body to deliver oxygen directly to cells.
  • Arthropods have an open circulatory system.
• Interaction:
  • They have a well-developed brain and sense organs such as simple and compound eyes, ears, pedipalps, and antennae.
    • Antennae are sensory organs that can detect smells, flavors, heat, or touch.
• The exoskeleton provides protection and attachment points for muscles; they will shed their old exoskeleton (moulting) to permit further growth.
• Reproduction:
  • Most arthropods have male and female sexes, internal fertilization and are oviparous.
    • Larva hatches and undergoes metamorphosis to become adult stage.
• Classification utilizes the number of legs, antennae, and body parts:
  • Arachnids: Terrestrial carnivores with cephalothorax and abdomen, eight legs, and no antennae.
  • Crustaceans: Aquatic filter feeders, carnivores, or herbivores with cephalothorax and abdomen, usually ten or more legs (often with first pair transformed into claws), and four antennae.
  • Myriapods: Terrestrial carnivores with head and long segmented trunk, more than ten legs, and two antennae.
  • Insects: Terrestrial carnivores or herbivores with head, thorax, and abdomen, six legs, and two antennae, with some having four wings.

Echinoderms

• All Echinoderms are aquatic, and live on the sea bed, some fixed to substrate or others moving slowly.
• Morphologic characteristics:
  • Echinoderms exhibit radial symmetry.
  • The body can have several shapes formed by five equal sections.
  • They have an internal skeleton underneath the skin made of calcium carbonate (dermal skeleton), and many have spines.
  • Echinoderms don't have a differentiated head.
• Vital functions
  • Echinoderms have a digestive system with mouth, anus, and glands and are carnivorous.
  • Echinoderms have an exclusive system called the ambulacral apparatus, consisting of tubules filled with sea water that end in ambulacral feet and have several functions: locomotion, breathing, circulation and excretion.
• Interaction: They have a poorly developed nervous system without a brain or complex sense organs.
• Reproduction:
  • Echinoderms have sexual reproduction, with separate sexes (mostly) or hermaphrodites.
  • Fertilization is external, and they are oviparous.
  • Larvae hatch and undergo metamorphosis to become adults.
  • Some species can regenerate lost body parts:
• Classification:
  • Asteroideans (Starfish): Star-shaped with five arms and carnivorous.
  • Echinoideans (Sea Urchins): Rounded with long spines and herbivorous.
  • Ophiuroideans (Brittle Stars): Star-shaped with long, flexible arms and filter feeders.
  • Crinoideans (Sea Lilies): Attached to substrate, feather-shaped, and filter feeders.
  • Holothurideans (Sea Cucumbers): Cylindrical-shaped with tentacles around the mouth that eat plankton.