Detailed notes-2 : Chapter4-Animal Kingdom

4.2 CLASSIFICATION OF ANIMALS

Kingdom Animalia:
- All animals belong to the kingdom Animalia, which encompasses a vast diversity of multicellular, eukaryotic organisms.
Phyla:
- Animals are further classified into different phyla based on shared characteristics such as body plan, symmetry, presence of tissues, and other structural features.
- Some major phyla include Porifera (sponges), Cnidaria (jellyfish, corals), Platyhelminthes (flatworms), Annelida (segmented worms), Arthropoda (insects, spiders, crustaceans), Mollusca (snails, clams, octopuses), Echinodermata (starfish, sea urchins), and Chordata (vertebrates and some invertebrates).
Subphyla, Classes, Orders, Families, Genera, and Species:
- Within each phylum, animals may be further divided into subphyla, classes, orders, families, genera, and species based on increasingly specific characteristics.
- These taxonomic ranks help to organize and categorize animals into increasingly specific groups.
Hierarchy of Classification:
- The classification hierarchy typically progresses from broader, more inclusive groups (like kingdom and phylum) to narrower, more specific groups (like species).
- This hierarchical system allows scientists to organize and study the diversity of life in a systematic and structured manner.

Characterstic Features Of Different Phyla

Porifera (Sponges):
- Lack true tissues and organs.
- Filter-feeders, with specialized cells called choanocytes.
- Simple body organization with pores for water intake and canals for water circulation.
- Often asymmetrical or radially symmetrical.
Cnidaria (Jellyfish, Corals):
- Radially symmetrical body plan.
- Possess specialized stinging cells called cnidocytes.
- Two basic body forms: polyp (sessile) and medusa (free-swimming).
- Simple nerve net for coordination.
Platyhelminthes (Flatworms):
- Bilaterally symmetrical.
- Flattened body shape.
- Lack a body cavity (acoelomates).
- Many are parasitic, while others are free-living.
Annelida (Segmented Worms):
- Segmented body with repeated units (metamerism).
- Bilaterally symmetrical.
- Closed circulatory system (in most).
- True coelom providing space for organs.
Arthropoda (Insects, Spiders, Crustaceans):
- Segmented bodies with jointed appendages.
- Exoskeleton made of chitin.
- Largest phylum with incredible diversity.
- Highly specialized respiratory, circulatory, and nervous systems.
Mollusca (Snails, Clams, Octopuses):
- Soft-bodied animals often protected by a hard shell.
- Radially symmetrical (in some) or bilaterally symmetrical.
- Muscular foot for locomotion.
- Well-developed digestive, nervous, and circulatory systems.
Echinodermata (Starfish, Sea Urchins):
- Unique water vascular system for locomotion and feeding.
- Radially symmetrical as adults, but bilaterally symmetrical in larval stage.
- Endoskeleton made of calcareous plates.
- Regenerative abilities, capable of regrowing lost body parts.
Chordata (Vertebrates and Some Invertebrates):
- Possess a notochord at some stage of development.
- Dorsal hollow nerve cord.
- Pharyngeal gill slits or pouches.
- Post-anal tail (at least during some stage of development).
- Segmented muscles (in most).

4.2.1 Phylum – Porifera

Asymmetrical Body Plan:
- Sponges typically exhibit an asymmetrical body plan, lacking a distinct symmetry.
- This lack of symmetry is one of the primitive features of sponges.
Cellular Level of Organization:
- Sponges have a cellular level of organization, meaning their bodies are composed of loosely aggregated cells without true tissues or organs.
Water Transport System:
- Sponges possess a water transport or canal system.
- Water enters the sponge through minute pores called ostia in the body wall and moves into a central cavity called the spongocoel.
- From the spongocoel, water exits through an opening called the osculum.
- This water flow helps in various functions such as food gathering, respiratory exchange, and waste removal.
Choanocytes:
- Choanocytes, also known as collar cells, line the spongocoel and canals.
- These cells have flagella surrounded by a collar-like structure, which helps in creating water currents and capturing food particles.
Intracellular Digestion:
- Digestion in sponges is intracellular, meaning it occurs within individual cells.
- Food particles captured by choanocytes are engulfed and digested within specialized cells.
Skeleton:
- Sponges are supported by a skeleton made up of spicules or spongin fibers.
- Spicules are small, needle-like structures made of calcium carbonate or silica.
Reproduction:
- Sponges are hermaphrodites, meaning they possess both male and female reproductive organs.
- Eggs and sperm are produced by the same individual sponge.
- Sponges reproduce asexually by fragmentation, where a piece of the sponge breaks off and grows into a new individual.
- Sexual reproduction involves the formation of gametes, internal fertilization, and development of larvae that are morphologically distinct from the adult sponge.
Examples:
- Examples of sponges include Sycon (Scypha), Spongilla (Freshwater sponge), and Euspongia (Bath sponge).

4.2.2 Phylum – Coelenterata (Cnidaria)

Aquatic Habitat:
- Cnidarians are mostly aquatic animals, primarily found in marine environments.
- They can be sessile (attached to a substrate) or free-swimming.
Radial Symmetry:
- Cnidarians exhibit radial symmetry, meaning their body parts are arranged symmetrically around a central axis, similar to the spokes of a wheel.
Cnidoblasts and Nematocysts:
- Cnidarians possess specialized cells called cnidoblasts or cnidocytes, which contain stinging capsules called nematocysts.
- Nematocysts are used for anchorage, defense, and capturing prey.
Tissue Level of Organization:
- Cnidarians exhibit tissue level of organization, meaning they have distinct tissues but lack organs.
- They are diploblastic, consisting of two embryonic germ layers: ectoderm and endoderm.
Central Gastrovascular Cavity:
- Cnidarians have a central gastrovascular cavity with a single opening, the mouth located on a structure called the hypostome.
- This cavity serves both digestive and circulatory functions.
Extracellular and Intracellular Digestion:
- Digestion in cnidarians is both extracellular (occurring in the gastrovascular cavity) and intracellular (occurring within individual cells).
Polyp and Medusa Body Forms:
- Cnidarians exhibit two basic body forms: polyp and medusa.
- Polyps are cylindrical and sessile, attached to a substrate. Examples include Hydra and Sea Anemones.
- Medusae are umbrella-shaped and free-swimming. Examples include jellyfish like Aurelia.
- Some cnidarians exhibit alternation of generations (metagenesis), where polyps produce medusae asexually and medusae produce polyps sexually.
Calcium Carbonate Skeleton:
- Some cnidarians, such as corals, have a skeleton composed of calcium carbonate.
Examples:
- Examples of cnidarians include Physalia (Portuguese Man-of-War), Adamsia (Sea Anemone), Pennatula (Sea Pen), Gorgonia (Sea Fan), and Meandrina (Brain Coral).

4.2.3 Phylum – Ctenophora

Marine Habitat:
- Ctenophores are exclusively marine organisms, found in various oceanic habitats around the world.
Radial Symmetry:
- Like cnidarians, ctenophores exhibit radial symmetry, with body parts arranged symmetrically around a central axis.
Diploblastic Organization:
- Ctenophores are diploblastic organisms, composed of two embryonic germ layers: ectoderm and endoderm.
Tissue Level of Organization:
- They have a tissue level of organization, with distinct tissues but no organs.
Comb Plates:
- The body of ctenophores bears eight external rows of ciliated comb plates.
- These comb plates are used for locomotion, propelling the animal through the water.
Extracellular and Intracellular Digestion:
- Digestion in ctenophores is both extracellular (occurring in the digestive cavity) and intracellular (occurring within individual cells).
Bioluminescence:
- Ctenophores are known for their bioluminescent abilities, meaning they can emit light.
- Bioluminescence is well-marked in ctenophores and serves various functions including defense and prey attraction.
Reproduction:
- Ctenophores are hermaphroditic, meaning they possess both male and female reproductive organs.
- Reproduction occurs only by sexual means, with external fertilization.
- Development is indirect, involving larval stages before reaching adulthood.
Examples:
- Examples of ctenophores include Pleurobrachia and Ctenoplana.

4.2.4 Phylum – Platyhelminthes

Dorso-Ventrally Flattened Body:
- Flatworms have a flattened body shape, which gives them their common name.
- This flattened body allows them to move efficiently in their environment and aids in surface area-to-volume ratio.
Endoparasitic Lifestyle:
- Many flatworm species are endoparasites, meaning they live inside the bodies of other organisms, including humans.
- Examples include tapeworms (Taenia) and liver flukes (Fasciola).
Bilateral Symmetry:
- Flatworms exhibit bilateral symmetry, with a distinct left and right side.
Triploblastic and Acoelomate:
- They are triploblastic organisms, meaning they have three embryonic germ layers: ectoderm, mesoderm, and endoderm.
- Flatworms are acoelomates, lacking a body cavity between their digestive tract and body wall.
Organ Level of Organization:
- They have organ-level organization, with distinct organs performing specific functions.
Hooks and Suckers:
- Parasitic flatworms often possess specialized structures such as hooks and suckers for attachment to their host’s tissues.
Direct Nutrient Absorption:
- Some flatworms absorb nutrients directly through their body surface, especially parasitic forms.
Flame Cells for Osmoregulation:
- Specialized cells called flame cells are involved in osmoregulation and excretion in flatworms.
Reproduction:
- Flatworms are hermaphroditic, meaning they possess both male and female reproductive organs.
- Fertilization is internal, and development typically involves multiple larval stages.
Regeneration Capacity:
- Some flatworms, like Planaria, have a high regeneration capacity, meaning they can regrow lost body parts.
Examples:
- Examples of flatworms include Taenia (Tapeworm) and Fasciola (Liver fluke).

4.2.5 Phylum – Aschelminthes

Circular Body Cross-Section:
- Aschelminthes have a circular body cross-section, which gives them their common name, roundworms.
- This circular shape contrasts with the flattened body shape of flatworms.
Ecological Diversity:
- Roundworms can be free-living, found in aquatic and terrestrial environments, or parasitic in plants and animals.
- They exhibit a wide range of lifestyles and ecological roles.
Organ-System Level of Organization:
- Roundworms have organ-system level of body organization, meaning they possess distinct organ systems that perform specific functions.
Bilateral Symmetry:
- They exhibit bilateral symmetry, with a distinct left and right side.
Triploblastic and Pseudocoelomate:
- Roundworms are triploblastic organisms, consisting of three embryonic germ layers: ectoderm, mesoderm, and endoderm.
- They are pseudocoelomates, meaning they have a body cavity (pseudocoelom) that is not completely lined by mesoderm.
Complete Alimentary Canal:
- Roundworms have a complete alimentary canal, including a well-developed muscular pharynx for feeding.
Excretory System:
- An excretory tube removes body wastes from the body cavity through an excretory pore.
- This system helps in osmoregulation and waste removal.
Separate Sexes (Dioecious):
- Roundworms have separate sexes (dioecious), meaning males and females are distinct individuals.
- Females are often longer than males.
Internal Fertilization:
- Fertilization is internal, occurring inside the female reproductive tract.
Development:
- Development may be direct, where the young ones resemble the adult, or indirect, involving larval stages.
Examples:
- Examples of roundworms include Ascaris (Roundworm), Wuchereria (Filaria Worm), and Ancylostoma (Hookworm).

4.2.6 Phylum – Annelida

Habitat Diversity:
- Annelids can be found in various habitats, including aquatic (marine and freshwater) environments as well as terrestrial habitats.
- They may be free-living or parasitic.
Organ-System Level of Organization:
- Annelids exhibit organ-system level of body organization, with distinct organ systems that perform specific functions.
Bilateral Symmetry:
- They display bilateral symmetry, with a distinct left and right side.
Triploblastic and Coelomate:
- Annelids are triploblastic organisms, consisting of three embryonic germ layers: ectoderm, mesoderm, and endoderm.
- They are coelomate animals, meaning they possess a true body cavity (coelom) completely lined with mesoderm.
Metameric Segmentation:
- Annelids are characterized by metamerism, where the body is divided into distinct segments or metameres.
- This segmentation is reflected in their common name, “Annelida,” derived from the Latin word “annulus,” meaning “little ring.”
Muscular System for Locomotion:
- Annelids possess longitudinal and circular muscles that aid in locomotion.
- Aquatic annelids, such as Nereis, may also have lateral appendages called parapodia, which assist in swimming.
Closed Circulatory System:
- Annelids have a closed circulatory system, meaning blood is confined to vessels and pumped by a heart.
Excretory System:
- Nephridia (sing. nephridium) are specialized structures involved in osmoregulation and excretion in annelids.
Neural System:
- The neural system of annelids consists of paired ganglia (sing. ganglion) connected by lateral nerves to a double ventral nerve cord.
- This nerve cord runs along the length of the body and helps coordinate movement and sensory perception.
Reproductive System:
- Annelids exhibit sexual reproduction.
- Aquatic annelids like Nereis are dioecious (separate sexes), while earthworms and leeches are monoecious (hermaphroditic).
Examples:
- Examples of annelids include Nereis, Pheretima (Earthworm), and Hirudinaria (Blood-Sucking Leech).