Key Characteristics of Major Animal Groups

Key Animal Phyla Characteristics

Vertebrates: Backbone & Internal Skeleton

Vertebrates derive their name from the dorsal spine (vertebral column) present in adults, which replaces the embryonic notochord. This structure anchors a rigid internal skeleton, providing essential support and enabling mobility. Vertebrates include several distinct classes:

• Fish: These are aquatic animals that breathe through gills and possess fins for locomotion. While sharks have a skeleton made of cartilage (cartilaginous fish), in most fish species, bone replaces cartilage in adulthood (bony fish).
• Amphibians: Amphibians, such as frogs and salamanders, can inhabit most terrestrial environments. However, they require moist conditions and typically return to water for reproduction. They possess four limbs and are known as tetrapods.
• Reptiles: Reptiles were the first vertebrate group to effectively control water loss, enabling them to thrive in dry terrestrial environments. Key evolutionary adaptations include:
  • The amnion: A fluid-filled sac that encloses and protects the embryo.
  • Waterproof eggs: Their eggs are protected by a tough, waterproof shell, preventing desiccation.
  • Resistant and waterproof skin.
• Birds: Birds have successfully colonized almost every habitat on Earth, largely due to two defining characteristics:
  • Feathers: Unique to birds, providing insulation, aiding in display, and essential for flight.
  • Wings: Their forelimbs are modified into strong wings, enabling flight in most species.

  Birds are also homeothermic (warm-blooded), meaning they generate internal body heat (endothermy) to maintain a stable, high body temperature, a trait they share with mammals.

• Mammals: Mammals primarily occupy terrestrial environments, although some species have adapted to aquatic life. They are distinguished from other vertebrates by several key features:
  • Their bodies are typically covered in hair or fur, which provides insulation.
  • Their offspring are nourished with milk, a secretion produced by the mother's mammary glands.

Arthropods: Segmented Body & Exoskeleton

The body of arthropods is characterized by three fundamental elements:

• Segmented Body: Their bodies are composed of segments, which are often grouped into distinct regions such as the head, thorax, and abdomen.
• Exoskeleton: They possess a strong, yet flexible, external skeleton made primarily of chitin. This exoskeleton is sometimes further hardened with calcium carbonate. To grow, arthropods must periodically shed this exoskeleton in a process called molting.
• Articulated Appendages: Arthropods have jointed limbs (appendages) that are specialized for various functions, including locomotion, feeding, and sensory perception.

Echinoderms: Marine Deuterostomes

Echinoderms, such as starfish and sea urchins, are exclusively marine animals. They belong to the Deuterostomia group, which means they are phylogenetically closer to vertebrates than to arthropods. Their characteristic radial symmetry is often associated with an adaptation to a sedentary or slow-moving lifestyle.

Distinctive Echinoderm Traits

Echinoderms possess several unique features:

• Internal Skeleton: They have an internal skeleton (endoskeleton) composed of calcareous plates (ossicles), which is covered by a thin layer of skin.
• Water Vascular System (Ambulacral System): This is a unique hydraulic system of canals within the body. Water circulates through this internal network, extending into numerous small, external, tube-like projections called tube feet (ambulacral feet). These tube feet are involved in vital functions such as locomotion, feeding, and respiration.

Chordates: The Notochord Feature

The most defining feature of the phylum Chordata is the notochord. This is a flexible, rod-shaped structure that provides skeletal support. It is present in all chordate embryos and is maintained in some adult forms. The original text provides the acorn worm as an example in this context.