Invertebrate Animals: Characteristics and Classification

Porifera and Cnidarians: Structure

Porifera (sponges) and Cnidarians (anemones, corals, and jellyfish) are simple, aquatic invertebrates.

Porifera (Sponges)

• Mostly marine, some freshwater species
• Bag-shaped body with pores that communicate with the atrial cavity
• The atrial cavity connects with the exterior through the osculum
• High capacity for regeneration

Cnidarians

• Anemones, corals, and jellyfish (all marine), hydra (freshwater)
• Radial symmetry
• Soft body with a single opening surrounded by tentacles
• Stinging cells (cnidocytes)
• Gastrovascular cavity that functions as a stomach
• Carnivorous
• Two forms:
  • Polyps: Tubular, bag-shaped with the opening at the top
  • Medusae: Umbrella-shaped, actively swim or float in the water

Worms: Platyhelminthes, Nematodes, and Annelids

Worms have bilateral symmetry, a soft body, and no skeleton.

Platyhelminthes (Flatworms)

• Flat, thin, soft body, not divided into segments
• Simple sense organs
• No specialized organs for locomotion
• Aquatic or live in moist environments
• No digestive or respiratory system
• Hermaphrodites

Nematodes (Roundworms)

• Cylindrical, soft body, not divided into rings, pointed at the ends
• Free-living, aquatic, and terrestrial environments
• Some are parasites

Annelids (Segmented Worms)

• Soft body, divided into cylindrical rings
• Rings are similar and have the same organs (metamerism)
• Some have a thickened package called the clitellum, which is involved in reproduction
• Aquatic or terrestrial
• Some are parasites
• Respiration through skin or gills
• Hermaphrodites or separate sexes
• Can regenerate

Mollusks: Gastropods, Bivalves, and Cephalopods

• Aquatic (marine or freshwater), some terrestrial
• Bilateral symmetry
• Body divided into three regions: head, visceral mass, and foot
• Foot used for swimming or movement
• Body covered by a mantle
• Some have a shell with one or two valves
• Respiration through gills
• Varied diet
• Hermaphrodites or separate sexes
• Internal or external fertilization
• Undergo metamorphosis
• Classified as:
  • Gastropods
  • Bivalves
  • Cephalopods

Arthropods: Insects, Arachnids, Crustaceans, and Myriapods

• Live in all environments
• Bilateral symmetry
• Body covered by an external skeleton (exoskeleton) made of chitin
• Articulated parts, allowing them to adapt to different environments
• Jointed appendages
• Body divided into three areas: head, thorax, and abdomen
• Varied diet: carnivorous, herbivorous, or scavengers
• Separate sexes
• Oviparous
• Internal fertilization
• Growth involves shedding the external skeleton (molting)
• Classified as:
  • Insects
  • Arachnids
  • Crustaceans
  • Myriapods

Insects

• Three pairs of jointed legs and one or two pairs of wings on the thorax
• Segmented abdomen without appendages
• Respiration through branched tubes called tracheae
• Separate sexes
• Internal fertilization
• Oviparous

Arachnids

• Mostly terrestrial, living in warm, dry areas
• Body divided into cephalothorax and abdomen
• Two chelicerae and two pedipalps on the cephalothorax
• Pedipalps have defensive and tactile functions
• Four pairs of legs
• Respiration through tracheae
• Mostly carnivorous
• Some have claws, hooks, and venom glands
• Separate sexes
• Oviparous, viviparous, or ovoviviparous

Crustaceans

• Marine, freshwater, and terrestrial
• Body divided into cephalothorax and abdomen
• Respiration through gills or the body surface
• Varied diet: carnivorous, filter feeders, or parasites
• Separate sexes
• Oviparous
• Undergo metamorphosis

Myriapods

• Terrestrial, live in damp and dark places
• Elongated body with a head and trunk
• Many segments, each with one or two pairs of legs
• One pair of antennae on the head
• Simple eyes
• Chewing jaws
• Respiration through tracheae
• Varied diet: carnivorous or herbivorous
• Separate sexes
• Oviparous
• No metamorphosis

Echinoderms: Characteristics

• Exclusively marine, live at the bottom, attached to the substrate
• Radial symmetry
• Rounded, cylindrical, or star-shaped body
• Internal skeleton
• Mouth on the underside
• Move using the ambulacral system
• Respiration through the skin, some have gills
• Carnivorous
• Separate sexes, some hermaphrodites
• External fertilization
• Some undergo metamorphosis
• Some can regenerate