Arthropoda Summary

Dominant animal phylum with ~1 million documented species; estimates of 3-30 million potential species.
Ecdysozoa clade includes Nematoda and Arthropoda.

Originated during the Cambrian explosion.
Body plan: segmented body, jointed appendages, hard exoskeleton.
Evolutionary trends: decrease in body segments, increase in appendage specialization, alterations in Hox gene expression.

Segmented Body
- All embryonic stages are segmented; functional units are called tagmata (e.g., head, thorax, abdomen).
Jointed Appendages
- Versatile with specialized functions (e.g., feeding, locomotion).
Exoskeleton
- Sclerotized cuticle composed mainly of chitin; serves as protection, muscle attachment, and may include minerals for strength.
Cephalization
- Centralized nervous system with complex eyes and antennae.
Reduced Coelom
- Hemocoel supports internal organs; acts as a hydrostatic skeleton.
Open Circulatory System
- Haemolymph circulates in the hemocoel, with various respiratory adaptations (e.g., gills, book lungs).

Three main lineages based on morphological and molecular data:
- Chelicerata (spiders, scorpions);
- Myriapoda (millipedes, centipedes);
- Pancrustacea (includes Crustacea and Hexapoda).

Originated in the Early Paleozoic; main features include:
- Cephalothorax and abdomen structure;
- Presence of chelicerae (feeding appendages).

Terrestrial arthropods with less tagmatised body plan; primarily use tracheal systems for respiration.

Includes Crustacea and Hexapoda;
- Crustacea: diverse aquatic lifestyles, characterized by a three-part body plan and specialization of appendages;
- Hexapoda: over 1 million species; insects capable of flight with adaptations for various environments.

Tagmata arrangement; gas exchange via a tracheal system; presence of wings as a significant evolutionary adaptation.