Invertebrates Overview

Invertebrates

• Invertebrates comprise 95% of all animal species.
• They lack backbones.
• They are classified into various phyla, including Porifera, Cnidaria, Platyhelminthes, Nematoda, and Annelida.
• Ectothermic animals regulate their body temperature through external factors. Endothermic animals have internal mechanisms for regulating body temperature.

Invertebrate Life Processes

• Movement: Some invertebrates are sessile (attached and immobile) while others exhibit various forms of locomotion.
• Support: They may have exoskeletons, endoskeletons, or hydrostatic skeletons.
• Body Covering: Invertebrates can have varied body coverings, with structures and coloration playing a role in protection and identification.
• Nutrition: Involves ingestion, digestion, egestion, and excretion—removal of waste products through diffusion.
• Respiration: All invertebrates are aerobic, meaning their respiration requires oxygen.
• Circulation: Methods of transporting materials throughout the body vary.
• Response: Invertebrates exhibit stimuli-response behaviors.
• Reproduction: Primarily, sexual reproduction occurs through fertilization (internal or external). Several have also developed asexual methods.
• Development: Invertebrates experience diverse types of development, including oviparous (egg-laying), viviparous (live birth), and ovoviviparous (egg development within the parent).
• Larval Stages: Many invertebrates have larval stages that differ significantly from adult forms.

Phylum Porifera (Sponges)

• Sponges are filter feeders.
• They have incurrent pores (ostia) and an excurrent pore (osculum).
• Collar cells (choanocytes) with flagella circulate water through the sponge, capturing food particles.
• Sponges have two cell layers and support structures called spicules composed of calcium carbonate, silica, or spongin.
• Reproduction can be asexual through budding or regeneration, or sexual through the release of sperm and eggs.

Phylum Cnidaria (Hydra, Jellyfish, Coral)

• Cnidarians have specialized cells called cnidocytes that contain nematocysts (stinging structures).
• Polyp form is typically sessile, while medusa form is motile.
• They possess a hydrostatic skeleton.
• This phylum displays radial symmetry.
• The hydra has two cell layers, ectoderm, and gastrodermis, separated by mesoglea.
• The gastrovascular cavity plays a role in digestion (both intracellular and extracellular).
• Some cnidarians reproduce asexually through budding or regeneration, and sexually via separate sexes.

Phylum Platyhelminthes (Flatworms)

• Free-living flatworms, like planarians, exhibit bilateral symmetry and cephalization.
• Planarians move using cilia and have a simple nervous system.
• Other members, like parasitic tapeworms, lack external cilia and have suckers and/or hooks for attachment.
• Tapeworms have a scolex (head) and proglottids (segments).

Phylum Nematoda (Roundworms)

• Roundworms are unsegmented with a complete digestive tract.
• Ascaris is an example of a human intestinal parasite.

Phylum Annelida (Segmented Worms)

• Segmented worms, like earthworms, exhibit bilateral symmetry.
• They have setae that aid in locomotion.
• They possess longitudinal and circular muscles for movement.
• Earthworms have a closed circulatory system with a dorsal blood vessel, ventral blood vessel, and aortic arches (helping to regulate blood pressure).
• They breathe through their moist epidermis.
• They have nephridia for excretion.
• Earthworms are hermaphroditic.

Phylum Mollusca (Clams, Snails, Octopuses)

• Mollusks exhibit bilateral symmetry and cephalization.
• They often have a shell secreted by a mantle.
• They typically possess a radula (tongue-like structure).
• Siphon for water intake/exit.
• Some have chromatophores for camouflage.

Phylum Echinodermata (Starfish, Sea Urchins)

• Echinoderms are exclusively marine invertebrates.
• They exhibit radial symmetry, often in multiples of five.
• They have a water vascular system with tube feet used for locomotion and feeding.
• They demonstrate significant regeneration capability.