Animal Phyla and Vertebrate Classes: Arthropods to Mammals

Arthropods

Arthropods: abundant, well-known species (flies, crabs) and representatives in virtually all habitats.

Body plan

The arthropod body plan is based on several key features:

• Body segmented: segments are grouped into regions, such as head and abdomen.
• Exoskeleton: strong and flexible due to chitin; arthropods molt to grow — they emerge from the old exoskeleton and secrete a new one.
• Jointed appendages: some appendages are walking legs, others serve sensory or feeding functions.

Major groups

Characteristics of the major groups of arthropods:

• Arachnids (spiders, scorpions)
• Crustaceans (crabs, shrimp)
• Insects (flies, beetles, butterflies)
• Myriapods (centipedes, millipedes)

Echinoderms

Echinoderms: sea animals such as starfish and sea urchins. They constitute a group of deuterostomes and are evolutionarily closer to vertebrates than to arthropods.

Symmetry and lifestyle

Their radial symmetry is related to an adaptation to a more sedentary or slow-moving lifestyle.

Structural features

• Internal skeleton: an endoskeleton formed by calcareous plates, covered by skin.
• Ambulacral apparatus: consists of a system of internal canals through which water flows to small tube feet (the water vascular system). These tube feet are involved in respiration, locomotion, and feeding.

Chordates

Chordates: this group includes tunicates (sea squirts), lancelets, and all vertebrates. Historically, related groups such as acorn worms were sometimes discussed in comparative contexts, but modern classification places acorn worms in Hemichordata.

Key characteristic

The most characteristic feature is the notochord, a flexible, rod-shaped structure that serves as axial support in chordate embryos and persists in some adult forms.

Vertebrates

Vertebrates: they take their name from the vertebrae of the dorsal spine, which anchor a rigid internal skeleton that provides support and mobility.

Fish

Fish are finned aquatic vertebrates that breathe with gills. They may be cartilaginous (skeleton mostly cartilage) or bony (bone replacing cartilage in much of the skeleton).

Amphibians

Amphibians (salamanders, frogs, toads) generally require a moist environment and typically return to water for reproduction.

Tetrapods

Tetrapods are four-limbed vertebrates (literally "four-footed") and include amphibians, reptiles, birds, and mammals.

Reptiles

Reptiles (lizards, turtles, snakes) have adaptations that reduce water loss and allow them to explore many terrestrial habitats. Features that contributed to their success include the amnion (a fluid-filled sac that surrounds the embryo) and skin that is more waterproof and resistant. Vertebrates that develop these traits are called amniotes.

Birds

Birds occupy almost every available habitat due to two key characteristics: feathers and the modification of the forelimbs into broad wings. They exhibit homeothermy (a form of endothermy), generating internal heat to maintain a relatively high and stable body temperature.

Mammals

Mammals are able to occupy many terrestrial habitats. They differ from other vertebrates in several ways:

• Body partially covered with hair.
• Newborns are fed by a maternal secretion produced by mammary glands (milk).