Invertebrate Diversity - Annelids, Nematodes, Arthropods and Deuterostomes

Annelids (Phylum Annelida)
- Segmented worms characterized by:
- Segmentation: Body divided into repeated segments.
- Nervous System: Complex; consists of a central nerve cord and ganglia in each segment.
- Closed Circulatory System: Blood contained in vessels.
- Coelomates: Organisms with a true coelom (body cavity).

Errantians:
- Mostly mobile marine organisms (e.g., sandworms).
- Characterized by paddle-like structures called parapodia on each segment.
- Chaetae: Bristles on parapodia that function in locomotion and respiration (gills).
Sedentarians:
- Less mobile; include earthworms and leeches.
- Live in protective tubes or burrow into sediment.

Class Polychaeta:
- Contains errantians (e.g., ragworms, sandworms).
Class Oligochaeta:
- Earthworms and relatives characterized by sparse chaetae.
- Earthworms consume soil to extract nutrients, are hermaphrodites, and typically mate during wet conditions.
Class Hirudinea:
- Leeches, often found in freshwater, some are marine or terrestrial.
- Predatory and parasitic; secrete hirudin to prevent blood coagulation.

Digestive System:
- Food ingested through mouth, processed through the alimentary canal, and ejected via the anus.
Muscle Layers:
- Longitudinal Muscles: Aid in lengthwise contraction.
- Circular Muscles: Contribute to body elongation.
Body Cavities:
- Body cavity lined with mesoderm-derived tissue.

General Characteristics:
- Animals with a protective cuticle that undergo ecdysis (molting).
Major Phyla:
- Nematoda (roundworms)
- Arthropoda (insects, arachnids, crustaceans).

Characteristics:
- Bilateral symmetry with three tissue layers.
- Nonliving cuticle, complete digestive tract (mouth to anus).
- Can be free-living or parasitic.
- Diverse habitats and ecological roles.

General Features:
- Largest phylum: Over a million species.
- Key characteristics:
- Hard exoskeleton (cuticle) made of chitin.
- Jointed appendages and segmentation.
- Open circulatory system.

Clade Chelicerata:
- Includes horseshoe crabs, arachnids (spiders, scorpions).
- Characterized by chelicerae (claw-like appendages).
- Gas exchange through book lungs.
Clade Myriapoda:
- Includes millipedes (herbivores) and centipedes (carnivores).
- Differentiated by the number of legs per trunk segment.
Clade Crustacea:
- Includes lobsters, crabs, barnacles; exhibit specialized appendages for movement and feeding.
- Gills for larger species; smaller species gas exchange through the cuticle.

Hexapoda:
- Include insects and their relatives.
- Nearly 75% of all identified animal species.
- Body plan includes head, thorax, abdomen, and usually wings.
Reproductive Adaptations:
- Life cycles often include metamorphosis (complete/incomplete).

Echinoderms (Phylum Echinodermata):
- Includes sea stars, sea cucumbers; exhibit radial symmetry as adults.
- Possess a water vascular system for movement and feeding.
Chordates (Phylum Chordata):
- Defined by dorsal nerve cord, notochord, pharyngeal slits, and post-anal tail.

Invertebrates play crucial roles in ecosystems and contribute significantly to human economies (e.g., pollination, environmental health).

This structured overview provides a comprehensive summary of invertebrate diversity, specifically focusing on annelids, nematodes, arthropods, and deuterostomes. Each section elucidates their respective anatomy, physiology, and ecological significance while highlighting distinctions among classes and phyla.