Arthropods and Major Animal Phyla: Structure, Diversity, and Evolution

Phylogeny and Classification of Major Animal Phyla

Overview of Animal Phylogeny

Animal phyla are classified based on evolutionary relationships, body plans, and developmental patterns. The phylogenetic tree illustrates the connections among major groups, highlighting features such as symmetry, tissue organization, and developmental modes.

• Protostomes: Includes ecdysozoans (e.g., arthropods, nematodes) and lophotrochozoans (e.g., annelids, mollusks).

• Deuterostomes: Includes echinoderms and chordates.

• Key evolutionary traits: Bilateral symmetry, presence of tissues, and modes of development (protostome vs. deuterostome).

Comparison of Major Animal Phyla

Animal phyla differ in their body plans, internal systems, and modes of reproduction. The following tables summarize these differences.

Phylum	Level of Organization	Symmetry	Body Cavity	Segmentation	Digestive System	Circulatory System	Respiratory System	Nervous System	Reproduction	Support	Number of Known Species
Sponges (Porifera)	Cellular	Absent	Absent	Absent	Intracellular	Absent	Absent	Absent	Sexual/asexual	Endoskeleton of spicules	5000
Hydra, Anemones, Jellyfish (Cnidaria)	Tissues	Radial	Absent	Absent	Gastrovascular cavity	Absent	Absent	Nerve net	Sexual/asexual	Hydrostatic	9000
Flatworms (Platyhelminthes)	Organs	Bilateral	Absent	Absent	Gastrovascular cavity	Absent	Absent	Head ganglia with longitudinal nerve cords	Sexual/asexual	Hydrostatic	20,000

Phylum	Body Plan	Symmetry	Body Cavity	Segmentation	Digestive System	Circulatory System	Respiratory System	Nervous System	Reproduction	Support	Number of Known Species
Segmented Worms (Annelids)	Organ system	Bilateral	Present	Present	Complete	Closed	Gills, skin	Head ganglia with ventral nerve cord	Sexual	Hydrostatic	15,000
Snails, Clams, Squid (Mollusca)	Organ system	Bilateral	Present	Absent	Complete	Open	Gills, skin	Head ganglia with ventral nerve cord	Sexual	Hydrostatic	93,000
Insects, Arachnids, Crustaceans (Arthropoda)	Organ system	Bilateral	Present	Present	Complete	Open	Trachea, gills, or book lungs	Head ganglia with ventral nerve cord	Normally sexual	Exoskeleton	1,000,000
Roundworms (Nematoda)	Organ system	Bilateral	Present	Absent	Complete	Absent	Excretory gland	Head ganglia with longitudinal nerve cords	Sexual	Hydrostatic	12,000
Sea Stars, Sea Urchins (Echinodermata)	Organ system	Bilateral larvae, radial adults	Present	Absent	Complete	Absent	Tube feet, gills	Head ganglia absent, nerve ring in skin	Sexual/asexual	Endoskeleton of plates	6,500

Phylum Arthropoda

General Characteristics

Arthropods are the most diverse animal phylum, with millions of species and individuals. Their success is attributed to their unique body structure and adaptability.

• Exoskeleton: A hard external covering made of chitin and proteins, providing protection and flexibility.

• Molting: Arthropods shed their exoskeleton to grow, a process known as ecdysis or molting.

• Segmented bodies: Divided into specialized regions with paired appendages.

• Open circulatory system: Hemolymph circulates through open spaces in the body.

• Sensory systems: Sophisticated organs for detecting light, movement, and chemical signals.

Body Regions of Insects

Insects, a subgroup of arthropods, have bodies divided into three distinct regions, each with specialized functions.

• Head: Responsible for feeding and environmental sensing.

• Thorax: Specialized for locomotion, bearing legs and wings.

• Abdomen: Contains organs for digestion and reproduction.

Insects

Key Features and Diversity

Insects are the only arthropods capable of flight and exhibit a wide range of adaptations.

• Three pairs of legs, one pair of antennae, and 1-2 pairs of wings.

• Respiration: Breathe through spiracles connected to internal tracheae.

• Reproduction: Lay eggs or bear live offspring.

• Metamorphosis: Juveniles undergo dramatic changes in body form to become adults.

• Larval forms: Many insects have worm-shaped larvae (e.g., maggots, caterpillars).

Compound Eyes

Insects possess compound eyes, which consist of multiple lenses and allow for wide-angle vision and detection of movement.

Bees and Colony Collapse Disorder

Bees are essential pollinators in ecosystems and agriculture. Colony Collapse Disorder is a phenomenon where bee populations decline rapidly, threatening food production.

• Causes: Likely due to viral infections weakening immune and orientation systems.

• Implications: Reduced pollination could lead to significant decreases in food supply.

Arachnids

Characteristics of Arachnids

Arachnids include spiders, ticks, scorpions, and mites. They are distinguished by their body structure and feeding habits.

• Eight legs and no antennae.

• Feeding: Predigested prey or blood.

• Spiders: Possess eight simple eyes, venomous fangs, and silk-producing glands.

• Respiration: Use book lungs for gas exchange.

Myriapods

Centipedes and Millipedes

Myriapods are characterized by their elongated bodies and numerous legs.

• Centipedes: Carnivorous, possess a poisonous bite.

• Millipedes: Herbivorous, generally harmless.

• Respiration: Breathe through tracheae.

• Habitat: Found in logs or moist leaf litter.

Crustaceans

Characteristics and Diversity

Crustaceans include lobsters, shrimp, barnacles, crabs, crayfish, and isopods. They are primarily aquatic and exhibit diverse forms.

• Two pairs of antennae and jointed appendages.

• Respiration: Breathe through gills.

• Sexes: Most species have separate sexes.

• Habitat: Freshwater and marine environments.

Summary Table: Arthropod Subgroups

Comparison of Insects, Arachnids, Myriapods, and Crustaceans

Group	Body Segments	Legs	Respiration	Habitat	Reproduction
Insects	Head, thorax, abdomen	6 (3 pairs)	Tracheae	Terrestrial	Eggs/live young
Arachnids	Cephalothorax, abdomen	8 (4 pairs)	Book lungs	Terrestrial	Eggs
Myriapods	Many segments	Many pairs	Tracheae	Terrestrial	Eggs
Crustaceans	Variable	Variable	Gills	Aquatic	Eggs

Additional info:

• Arthropods are a key example of evolutionary success due to their adaptability, segmentation, and exoskeleton.

• Metamorphosis in insects is a critical process for ecological diversity and specialization.

• Colony Collapse Disorder in bees highlights the importance of arthropods in ecosystem services and agriculture.