Kingdom Animalia: Vertebrates and Invertebrates

• Kingdom Animalia consists of multicellular, heterotrophic organisms lacking cell walls, requiring them to consume other living things for sustenance.
• Animals are categorized into vertebrates (possessing a backbone) and invertebrates (lacking a backbone).

Vertebrates

• Animals that have a backbone or spinal column (also called a vertebral column)
• There are only five main categories of vertebrates: fish, amphibians, reptiles, birds, and mammals.
• Humans are vertebrates

Invertebrates

• Animals lacking a backbone or vertebral column
• They were the first animals to evolve
• Comprise over 90% of Earth's animal population, with over 1.3 million species, contrasting with only about 66,000 known vertebrate species.
• They are subdivided into almost 30 phyla; vertebrates have only one phylum, Chordata.
• Ranging from simple organisms to those with complex organ systems mimicking vertebrates.

Phylum Porifera

• 5,500 species
• Known as sponges
• Predominantly sessile (immobile), but larval sponges can migrate
• Filter-feeders, absorbing nutrients from water drawn into their pores and expelling it through an osculum.

Phylum Coelenterata (Cnidaria)

• 10,000 species
• Contains "stinging organisms" with cnidocytes that inject venom
• Includes jellyfish, corals, sea anemones, and hydra
• Has a gastro-vascular system with a single opening serving as both mouth and anus
• Radially symmetrical (circular organisms with no left or right sides).

Phylum Ctenophora

• 100 species
• Contains comb jellies, named for their comb-like cilia used for movement
• Can grow up to 1.5 meters
• Lack cnidocytes (stinging cells)
• Have slightly more complex body and organ systems than jellyfish.

Phylum Platyhelminthes

• 20,000 species
• Flatworms, the first to exhibit bilateral symmetry (left and right sides)
• First organisms with a central nervous system and a simple "brain"
• Mostly marine
• Planaria, a famous flatworm, can regenerate; each cut piece can grow into a separate organism.

Phylum Nemertea

• 900 species
• Ribbon worms, often round and thin, ranging from 1 cm to 30 meters long
• Many can eject a sticky, branched proboscis for capturing prey or for defense.

Phylum Rotifera

• 1,800 species
• Microscopic aquatic organisms, typically 50 µm to 2 mm long
• Named for the circle of cilia around their mouths that draw in food and liquid.

Phylum Gastrotricha

• 800 species
• Worm-like, microscopic animals called hairy bellies due to cilia on their ventral surface
• All aquatic, living in fresh or marine water.

Phylum Nematomorpha

• 325 species
• Similar in appearance to nematodes, also called horsehair worms or hairsnakes
• Usually around 10 cm long
• The longest recorded specimen was over 2 m
• Often parasites of insects.

Phylum Nematoda

• 25,000 species
• Roundworms found globally in soils and waters
• Some are parasitic, infecting humans (e.g., hookworms and pinworms); others infect plants, while some decompose organic material.

Phylum Acanthocephala

• 1,100 species
• Parasitic spiny-headed worms that use hooks to attach to the host's digestive system
• Genetically related to rotifers, leading to suggestions of combining the two phyla.

Phylum Bryozoa (Ectoprocts)

• 4,500 species
• Animals resembling clumps of moss
• Most are colonial, with many individuals living together with a hard exoskeleton and an opening for feeding
• Commonly found in marine reefs.

Phylum Tardigrada

• 800 species
• Microscopic (0.5 mm) water bears found in almost every ecosystem
• Can survive in harsh environments.

Phylum Brachiopoda

• 335 species
• Often mistaken for mollusks
• Possess a lophophore, a row of cilia-like tentacles around their mouth
• Have a stalk-like appendage for anchorage
• Also called lamp shells due to their shape.

Phylum Mollusca

• 100,000 species
• Large and diverse
• Characterized by a soft body (visceral mass), often protected by a shell made by the mantle
• Includes gastropods, bivalves, and cephalopods
  • Gastropods: Largest class, including snails (with shells) and slugs (without shells); name means "stomach-foot."
  • Bivalves: Have a shell made of two hinged halves; includes clams, oysters, and scallops.
  • Cephalopods: Includes octopuses, squids, and nautiluses; name means "head-foot." They are marine predators, and octopuses are considered the most intelligent invertebrates.

Phylum Annelida

• 16,500 species
• Segmented worms with bodies divided into repeating segments
• Includes earthworms and leeches.

Phylum Sipunculoidea

• 162 species
• Peanut worms with a narrow head and bulbous rear
• Genetically related to annelids, suggesting a potential combination of the two phyla.

Phylum Arthropoda

• 1,000,000 species
• Largest phylum in the Animal Kingdom
• Characterized by jointed legs or appendages, hard exoskeletons, and segmented bodies (head, thorax, abdomen)
  • Arachnids: Spiders, scorpions, and ticks, all animals with chelicerae (pincers or fangs) and six pairs of appendages (four walking pairs, one pair of chelicerae, and one pair of pedipalps)
  • Myriapods: Centipedes (carnivores with one pair of legs per segment) and millipedes (herbivores with two pairs of legs per segment)
  • Crustaceans: Mostly aquatic arthropods like crabs, lobsters, and shrimp, with varying numbers of specialized appendages
  • Insects: Largest group of arthropods (also called hexapoda); all have six legs (three pairs) and some have wings. They are essential for pollination, decomposition, and as a food source.

Phylum Chaetognatha

• 200 species
• Arrow worms, marine worms with lateral fins and a tail fin, giving them an arrow-like shape
• All species are predators.

Phylum Echinodermata

• 7,000 species
• Named for their spiny skin
• Includes sea stars, sea urchins, sea cucumbers, and sand dollars
• Have complex organ systems
• Undergo embryonic development similar to vertebrates
• Exhibit radial symmetry and regeneration
• Move via a water-vascular system with tube feet.

Phylum Hemichordata

• 85 species
• The most "vertebrate-like" invertebrate phylum
• Share similar embryonic development, gill slits, and a nerve cord
• Includes acorn worms, marine burrowing worms with an "acorn-shaped" head.