State the main features used to place organisms into groups within the animal kingdom, limited to: (a) the main groups of vertebrates: mammals, birds, reptiles, amphibians, fish (b) the main groups of arthropods: myriapods, insects, arachnids, crustaceans

Organisms within the animal kingdom are classified into various groups based on specific features that reflect their evolutionary relationships and biological characteristics. Here are the main features used to place animals into groups, focusing on the main groups of vertebrates and arthropods.

(a) Main Groups of Vertebrates

The vertebrate subphylum, part of the phylum Chordata, is divided into several key groups, each characterized by distinct features:

1. Mammals:
   • Characteristics: Mammals possess hair or fur, mammary glands that produce milk for feeding young, and a three-boned middle ear. They are warm-blooded (endothermic) and have a complex brain.
   • Examples: Humans, dogs, whales.
2. Birds:
   • Characteristics: Birds have feathers, beaks without teeth, and lay hard-shelled eggs. They are also warm-blooded and have a lightweight skeletal structure adapted for flight.
   • Examples: Eagles, sparrows, penguins.
3. Reptiles:
   • Characteristics: Reptiles have scaly skin, breathe air through lungs, and typically lay eggs with leathery shells. They are cold-blooded (ectothermic) and have a more advanced circulatory system than amphibians.
   • Examples: Snakes, lizards, turtles.
4. Amphibians:
   • Characteristics: Amphibians generally have moist skin without scales, undergo metamorphosis from larval to adult stages (e.g., from tadpoles to frogs), and can live both in water and on land.
   • Examples: Frogs, salamanders, toads.
5. Fish:
   • Characteristics: Fish are aquatic animals with gills for breathing underwater, fins for swimming, and scales covering their bodies. They can be divided into jawless fish (Agnatha), cartilaginous fish (Chondrichthyes), and bony fish (Osteichthyes).
   • Examples: Salmon (bony fish), sharks (cartilaginous fish), lampreys (jawless fish).

(b) Main Groups of Arthropods

Arthropods are the largest group of invertebrates within the phylum Arthropoda and are characterized by their jointed appendages and exoskeletons. The main groups of arthropods include:

1. Myriapods:
   • Characteristics: Myriapods have elongated bodies with many segments, each bearing one or two pairs of legs. They typically have one pair of antennae.
   • Examples: Millipedes (two pairs of legs per segment) and centipedes (one pair of legs per segment).
2. Insects:
   • Characteristics: Insects have three main body segments (head, thorax, abdomen), six legs, compound eyes, and usually one or two pairs of wings. They undergo various forms of metamorphosis during their life cycle.
   • Examples: Butterflies, beetles, ants.
3. Arachnids:
   • Characteristics: Arachnids have two body segments (cephalothorax and abdomen), eight legs, and no antennae. They typically possess specialized mouthparts such as fangs or pincers.
   • Examples: Spiders, scorpions, ticks.
4. Crustaceans:
   • Characteristics: Crustaceans usually have a hard exoskeleton, two pairs of antennae, and multiple body segments that may be fused into a cephalothorax. They often have gills for respiration.
   • Examples: Crabs, lobsters, shrimp.