chapter 18

Multicellular
- All animals consist of multiple cells that form complex structures.
Heterotrophic
- Animals cannot create organic molecules from inorganic substances, instead obtaining energy by consuming other organisms.
- Categories of food sources:
- Herbivores: Consume autotrophs (plants).
- Carnivores: Consume other animals.
- Detritivores: Consume decomposing matter.
No Cell Walls
- Animal cells are flexible and lack rigid cell walls, a distinctive feature setting them apart from plant cells.
- Cells are held together by extracellular matrices, notably structural proteins like collagen, which also form unique intercellular junctions.
Active Movement
- Animals have the unique ability to move rapidly and in complex ways due to the flexibility of their cells and the evolution of muscle and nerve tissues.
- Flight is a notable form of movement found in many vertebrates and insects, whereas amphibians are the only major terrestrial vertebrates that do not possess the ability to fly.

Definition: Organisms that obtain their food by ingesting other organisms, in contrast to autotrophs that create their own food.
Animals are obligate heterotrophs, meaning they must consume organic material for energy.

All members of the kingdom Animalia are multicellular, exhibiting complex body structures.
The unicellular organisms that were formerly classified as simple animals are now classified under the kingdom Protista.

The absence of rigid cell walls contributes to the flexibility and mobility of animal cells.

Flexibility in cells allows for advanced locomotion, facilitated by muscle and nerve tissue evolution.

Approximately 99% of animals are invertebrates, lacking a backbone.
Of roughly 10 million animal species estimated, only around 42,500 are classified as vertebrates.
Animals range in size dramatically from microscopic to gigantic forms such as whales and squid.

The animal kingdom has about 35 phyla, with the majority occurring in marine environments, fewer in freshwater, and even fewer on land.
Three dominant marine phyla on land include:
- Arthropoda (insects)
- Mollusca (snails)
- Chordata (vertebrates)

Majority of animals reproduce sexually, involving the fusion of larger, non-motile eggs and smaller flagellated sperm.
During fertilization, gametes form a zygote without undergoing mitosis as seen in plants and fungi.

Most animals exhibit similar patterns during embryo development:
- Cleavage: The zygote undergoes mitotic divisions.
- Morula: A solid ball of cells.
- Blastula: A hollow ball of cells that folds inward to form a gastula.
Cell differentiation varies across animal phyla during gastrulation.

Except sponges, animal cells form specialized tissues:
- Muscle Tissue: Facilitates movement.
- Nervous Tissue: Conducts signals, forming neuromuscular junctions.

Fossil evidence indicates the presence of animals more than 565 million years ago.
Strong consensus exists regarding a single ancestor for the kingdom, most likely a colonial flagellate.
The similarities noted between choanoflagellates (the closest living relatives of animals) and sponge cells further support this.

Parazoa: No tissues (e.g., sponges).
Radiata: Radial symmetry (e.g., jellyfish).
Lophotrochozoa: Includes organisms that grow by increasing body mass, possess ciliary locomotion, and larval stages called trochophores.
Ecdysozoa: Organisms that grow by molting, including arthropods.
Deuterostomes: Coelom forms from the embryonic gut.

Characteristics:
- Multicellular but lack true tissues.
- Sizes range considerably, from a few mm to over 2m.
- Engage in filter feeding and can reproduce both sexually and asexually.
Habitat: Predominantly marine environments.

Examples: Tapeworms and flukes.
Features:
- Possess bilateral symmetry.
- Development of three cellular layers: ectoderm, mesoderm, and endoderm.
- Hermaphroditic; some exhibit asexual regeneration.

True coelomates (organisms with a true body cavity).
Lack segmentation and showcase greater structural complexity.
Noteworthy classes include:
- Gastropoda: Snails and slugs.
- Bivalvia: Clams and oysters.
- Cephalopoda: Squids and octopuses.

Segmented worms like earthworms and leeches.
Features:
- Closed circulatory system and complex nervous system.
- Hermaphroditic reproduction, playing significant ecological roles in soil health.

The largest phylum, includes insects, arachnids, and crustaceans.
Characterized by:
- Segmentation, exoskeleton made of chitin, jointed appendages.
- High diversity; over 1 million species identified.

Marine creatures like starfish and sea cucumbers.
Noted for:
- Deuterostome development and radial symmetry in adults, with bilateral symmetry in larval forms.
- Possess a water vascular system for locomotion and gas exchange.

Distinguished by the presence of a notochord, nerve cord, pharyngeal slits, and a post-anal tail.
Classes within this phylum include:
- Jawless Fish: Such as lampreys.
- Cartilaginous Fish: Sharks and rays.
- Bony Fish: Comprising the majority of fish species.
- Amphibians: Transitioned from water to land, requiring moist environments for reproduction.
- Reptiles: Adapted to terrestrial life with features like scales and lungs.
- Birds: Evolutionarily derived from dinosaurs, equipped with feathers.
- Mammals: Defined by characteristics such as mammary glands, fur/hair, and typically giving live birth.