Introduction
What is this section about?
This section discusses the classification of animals based on specific characteristics, such as body organization, symmetry, layers of cells, the presence of body cavities, segmentation, and the notochord. It explains how animals differ and shares the criteria used for grouping them into categories.
Simple Explanation:
1. Basis of Classification:
- Why classify?With over a million animal species, classification helps organize them systematically and makes it easier to study or identify new species.
2. Levels of Organization:
Cellular level: Found in sponges; cells are loosely arranged and perform specific functions.
Tissue level: In coelenterates, cells group into tissues for specialized functions.
Organ level: Seen in animals like Platyhelminthes, where tissues form organs.
Organ-system level: Found in annelids, arthropods, etc., where organs form systems (e.g., digestive or circulatory systems).
Examples of complexity:
Incomplete digestive system: Single opening for food intake and waste removal (e.g., Platyhelminthes).
Complete digestive system: Separate mouth and anus (found in advanced animals).
Circulatory systems:
Open system: Blood bathes tissues directly (e.g., arthropods).
Closed system: Blood flows through vessels (e.g., annelids).
3. Symmetry:
Asymmetry: No equal halves (e.g., sponges).
Radial symmetry: Body can be divided into identical parts through multiple planes (e.g., coelenterates, echinoderms).
Bilateral symmetry: Body has only one plane of symmetry, forming left and right halves (e.g., arthropods, annelids).
4. Diploblastic and Triploblastic Animals:
Diploblastic animals: Have two embryonic layers:
Ectoderm: Outer layer.
Endoderm: Inner layer.
Mesoglea: Gel-like layer between ectoderm and endoderm (e.g., coelenterates).
Triploblastic animals: Have three embryonic layers:
Ectoderm, mesoderm, and endoderm (e.g., platyhelminthes to chordates).
5. Coelom (Body Cavity):
Coelomates: Have a body cavity lined by mesoderm (e.g., annelids, chordates).
Pseudocoelomates: Body cavity partially lined by mesoderm (e.g., aschelminthes).
Acoelomates: No body cavity present (e.g., platyhelminthes).
6. Segmentation (Metamerism):
The body is divided into segments with repeated structures (e.g., earthworms). This is called metamerism.
7. Notochord:
A rod-like mesodermal structure found on the dorsal side during development.
Chordates: Animals with a notochord.
Non-chordates: Animals without a notochord (e.g., porifera to echinoderms).
Simple Conclusion:
Animal classification is based on shared features like body symmetry, organization levels, cell layers, body cavity, segmentation, and the presence of a notochord. These criteria help us group animals systematically and understand their diversity
Broad Classification of Animalia
Phylum : Porifera
What is this section about?
This section provides an overview of the phylum Porifera, which includes sponges. It explains their structure, functions, reproduction, and examples.
Simple Explanation:
1. Name and Habitat:
Common Name: Sponges.
Habitat:
Mostly marine (live in the sea).
A few are freshwater (e.g., Spongilla).
Symmetry: Asymmetrical (no specific shape).
2. Body Structure:
Organization: Cellular level (primitive multicellular animals).
Canal System:
Water enters through small pores called ostia.
Water passes through a central cavity called spongocoel.
Water exits through a larger opening called the osculum.
This system is vital for:
Gathering food.
Exchanging gases (respiration).
Removing waste.
Special Cells: Choanocytes (collar cells) line the canals and spongocoel.
Skeleton: Made of spicules or spongin fibers for support.
3. Digestion:
Digestion occurs inside cells (intracellular digestion).
4. Reproduction:
Sexual Reproduction:
Hermaphrodite: A single sponge produces both eggs and sperm.
Fertilization is internal.
Development involves a larval stage, which looks different from the adult.
Asexual Reproduction:
Happens through fragmentation (breaking into pieces that grow into new sponges).
5. Examples:
Sycon (Scypha): A marine sponge.
Spongilla: A freshwater sponge.
Euspongia: A bath sponge.
Simple Conclusion:
Porifera, or sponges, are simple animals mostly found in the sea. They have a water canal system for food, respiration, and waste removal. They reproduce both sexually and asexually and are supported by a skeleton made of spicules or spongin fibers. Examples include Sycon, Spongilla, and Euspongia.
Phylum : Coelenterata (Cnidaria)
What is this section about?
This section provides an overview of Phylum Coelenterata (Cnidaria), describing their structure, unique features, body forms, reproduction, and examples.
Simple Explanation:
1. Name and Habitat:
Common Name: Cnidarians.
Habitat:
Mostly marine (live in the sea).
Can be sessile (attached to a surface) or free-swimming.
Symmetry: Radial symmetry (body parts arranged around a central axis).
2. Unique Features:
Cnidoblasts/Cnidocytes:
Cells with stinging capsules (nematocysts) present on tentacles and the body.
Functions:
Anchorage (staying attached).
Defense (protection).
Capturing prey.
3. Body Structure:
Level of Organization: Tissue level.
Body Layers: Diploblastic (two layers: ectoderm and endoderm).
Gastrovascular Cavity:
A central cavity for digestion.
Single opening (mouth) on a structure called the hypostome.
Skeleton:
Some have a skeleton made of calcium carbonate (e.g., corals).
4. Digestion:
Both extracellular (outside cells) and intracellular (inside cells).
5. Body Forms:
Polyp:
Cylindrical and sessile (e.g., Hydra, Adamsia).
Medusa:
Umbrella-shaped and free-swimming (e.g., Aurelia or jellyfish).
Alternation of Generation (Metagenesis):
Polyps reproduce asexually to produce medusae.
Medusae reproduce sexually to form polyps.
Example: Obelia.
6. Examples:
Physalia: Portuguese man-of-war.
Adamsia: Sea anemone.
Pennatula: Sea pen.
Gorgonia: Sea fan.
Meandrina: Brain coral.
Simple Conclusion:
Cnidarians are aquatic animals with radial symmetry and unique stinging cells (cnidoblasts). They have two body forms: sessile polyps and free-swimming medusae, with some showing alternation of generations. Examples include Physalia, Adamsia, and Aurelia.
Phylum – Ctenophora
What is this section about?
This section discusses Phylum Ctenophora, which includes marine animals like sea walnuts and comb jellies. It explains their structure, movement, unique features, and reproduction.
Simple Explanation:
1. Name and Habitat:
Common Name: Sea walnuts or comb jellies.
Habitat: Exclusively marine (live only in the sea).
Symmetry: Radial symmetry (body parts arranged around a central axis).
2. Body Structure:
Level of Organization: Tissue level.
Body Layers: Diploblastic (two layers: ectoderm and endoderm).
Unique Feature:
The body has eight rows of ciliated comb plates.
Function: Help in movement (locomotion).
3. Digestion:
Both extracellular (outside cells) and intracellular (inside cells).
4. Bioluminescence:
Ctenophores can produce and emit light, a property called bioluminescence.
5. Reproduction:
Sexes: Not separate (both male and female organs are in one organism).
Type: Sexual reproduction only.
Fertilization: External (occurs outside the body).
Development: Indirect (involves a larval stage).
6. Examples:
Pleurobrachia
Ctenoplana
Simple Conclusion:
Ctenophores, also known as sea walnuts or comb jellies, are marine animals with radial symmetry and tissue-level organization. They have comb plates for movement and exhibit bioluminescence. They reproduce sexually with external fertilization. Examples include Pleurobrachia and Ctenoplana.
Phylum – Platyhelminthes
What is this section about?
This section discusses Phylum Platyhelminthes, commonly known as flatworms. It highlights their structure, habitat, reproduction, and examples.
Simple Explanation:
1. Name and Body Structure:
Common Name: Flatworms.
Body Shape: Dorso-ventrally flattened (flat from top to bottom).
Symmetry: Bilateral symmetry (left and right halves are identical).
Body Organization:
Triploblastic: Three body layers: ectoderm, mesoderm, and endoderm.
Acoelomate: No body cavity (coelom).
Organ Level: Well-developed organs are present.
2. Habitat and Lifestyle:
Mostly endoparasites (live inside the bodies of animals, including humans).
3. Parasitic Features:
Hooks and Suckers: Help attach to the host.
Nutrient Absorption: Absorb nutrients directly through their body surface.
4. Excretion and Osmoregulation:
Flame Cells: Special cells that help in excretion and maintaining water balance.
5. Reproduction:
Sexes: Not separate (hermaphroditic; both male and female organs are in one individual).
Fertilization: Internal (occurs inside the body).
Development: Involves multiple larval stages.
6. Special Ability:
Some, like Planaria, have a remarkable regeneration capacity (can regrow body parts).
7. Examples:
Taenia (Tapeworm).
Fasciola (Liver fluke).
Simple Conclusion:
Flatworms are bilaterally symmetrical, triploblastic, and acoelomate animals. Many are parasitic, with hooks, suckers, and flame cells for nutrient absorption and excretion. They reproduce sexually, and some, like Planaria, can regenerate. Examples include Taenia and Fasciola.
Phylum – Aschelminthes
What is this section about?
This section explains Phylum Aschelminthes, also known as roundworms. It describes their structure, lifestyle, reproduction, and provides examples.
Simple Explanation:
1. Name and Body Structure:
Common Name: Roundworms.
Body Shape: Circular when cut across.
Symmetry: Bilateral symmetry (left and right halves are identical).
Body Organization:
Triploblastic: Three body layers: ectoderm, mesoderm, and endoderm.
Pseudocoelomate: They have a body cavity that is not completely lined with mesoderm.
Organ-System Level: They have well-organized systems and organs for different functions.
2. Habitat and Lifestyle:
They can be free-living (living in water or on land) or parasitic (living in plants or animals).
3. Digestive System:
Alimentary Canal: A full digestive system, with both a mouth and anus.
Muscular Pharynx: A strong muscle helps to suck in food.
4. Excretion:
Excretory Tube: A tube that removes waste through an excretory pore.
5. Reproduction:
Sexes: Male and female roundworms are separate.
Size Difference: Females are usually longer than males.
Fertilization: Takes place inside the body.
Development:
Direct Development: Young worms look like adults.
Indirect Development: Young worms go through different stages before becoming adults.
6. Examples:
Ascaris (Roundworm).
Wuchereria (Filaria worm).
Ancylostoma (Hookworm).
Simple Conclusion:
Roundworms are bilaterally symmetrical, triploblastic, and pseudocoelomate animals with a complete digestive system. They can be free-living or parasitic and reproduce internally. Examples include Ascaris, Wuchereria, and Ancylostoma.
Phylum – Annelida
What is this section about?
This section explains Phylum Annelida, which includes segmented worms like earthworms, leeches, and marine worms. It describes their body structure, organization, habitat, and reproduction.
Simple Explanation:
1. Habitat and Lifestyle:
Habitat: Annelids can be found in water (marine and freshwater), land, or can be parasitic.
Body Organization:
Bilateral Symmetry: Their body can be divided into two identical halves.
Triploblastic: They have three body layers: ectoderm, mesoderm, and endoderm.
Metamerically Segmented: Their body is divided into segments, each of which has repeating organs.
Coelomate: They have a body cavity (coelom) lined with mesoderm.
2. Movement:
Annelids have longitudinal and circular muscles, which help in movement.
Aquatic Annelids: Marine annelids, like Nereis, have lateral appendages (parapodia) that help them swim.
3. Circulatory and Excretory Systems:
Closed Circulatory System: Blood circulates through vessels.
Excretion: They use nephridia (excretory organs) for osmoregulation and waste removal.
4. Nervous System:
Their nervous system consists of paired ganglia (groups of nerve cells) connected by lateral nerves to a double ventral nerve cord.
5. Reproduction:
Sexual Reproduction:
Nereis (marine form) is dioecious (separate sexes).
Earthworms and Leeches are monoecious (hermaphroditic, having both male and female organs in the same individual).
6. Examples:
Nereis (Marine worm).
Pheretima (Earthworm).
Hirudinaria (Leech, a blood-sucking parasitic worm).
Simple Conclusion:
Annelids are segmented, coelomate, bilaterally symmetrical animals with a closed circulatory system and specialized organs. They can live in water or on land and reproduce sexually. Examples include Nereis, earthworms, and leeches
Phylum – Arthropods
What is this section about?
This section explains Phylum Arthropoda, the largest phylum in the animal kingdom, which includes insects, spiders, crustaceans, and other joint-legged animals. It describes their key characteristics, including their body structure, exoskeleton, various organ systems, and examples.
Simple Explanation:
- Body Organization:
Organ-system level: They have complex organ systems.
Bilateral Symmetry: Their bodies can be divided into two similar halves.
Triploblastic: They have three primary germ layers.
Segmented: Their bodies are made up of repeating segments.
Coelomate: They possess a true body cavity.
Chitinous Exoskeleton: They have a hard external covering made of chitin.
Body Divisions: Typically head, thorax, and abdomen.
- Appendages:
Jointed Appendages: They have jointed legs, antennae, and other appendages (hence the name "Arthropoda," meaning "jointed foot").
- Respiratory System:
Variety of Methods: They breathe through gills (aquatic), book gills (aquatic), book lungs (terrestrial), or a tracheal system (insects).
- Circulatory System:
Open Type: Blood flows freely within the body cavity, not enclosed in vessels.
- Sensory Organs:
Diverse Senses: They have antennae for touch and smell, simple and compound eyes for vision, and statocysts for balance.
- Excretory System:
Malpighian Tubules: They excrete waste through specialized tubes called Malpighian tubules.
- Reproduction:
Dioecious: They have separate male and female individuals.
Internal Fertilization: Fertilization usually occurs inside the female's body.
Oviparous: They lay eggs.
Direct or Indirect Development: Development can be direct (young resemble adults) or indirect (with larval stages).
- Examples:
Apis (Honey bee): Economically important for honey production.
Bombyx (Silkworm): Produces silk.
Laccifer (Lac insect): Produces lac.
Anopheles, Culex, Aedes (Mosquitoes): Disease vectors.
Locusta (Locust): A gregarious pest.
Limulus (King crab): A "living fossil."
Simple Conclusion:
Arthropods are the most diverse animal phylum. They have segmented bodies, jointed appendages, a chitinous exoskeleton, and a variety of adaptations for different environments. They exhibit diverse respiratory, circulatory, sensory, and excretory systems. They reproduce sexually and include insects, spiders, crustaceans, and many other familiar creatures.
Phylum – Mollusca
What is this section about?
This section explains Phylum Mollusca, the second-largest animal phylum. It describes the characteristics of molluscs, including their body structure, shell, organ systems, and examples like snails, clams, and squids.
Simple Explanation:
Habitat and Organization:
Habitat: Molluscs live in various environments, including land (terrestrial) and water (both marine and freshwater).
Organ-system level: They have complex organ systems.
Bilateral Symmetry: Their bodies can be divided into two similar halves.
Triploblastic: They have three primary germ layers.
Coelomate: They possess a true body cavity.
Body Structure:
Calcareous Shell: Most molluscs have a protective outer shell made of calcium carbonate.
Unsegmented Body: Unlike arthropods or annelids, their bodies are not divided into repeating segments.
Distinct Head: They have a well-defined head region.
Muscular Foot: They have a large, muscular foot used for movement (e.g., crawling in snails, digging in clams).
Visceral Hump: This contains the internal organs.
Mantle: A soft layer of skin that covers the visceral hump.
Mantle Cavity: The space between the mantle and the visceral hump, which contains gills for respiration.
Respiration and Excretion:
Gills: Feather-like gills within the mantle cavity are used for both respiration (breathing) and excretion (removing waste).
Sensory Organs:
Sensory Tentacles: They have sensory tentacles on their head.
Feeding:
Radula: They have a unique rasping organ called a radula in their mouth, used for scraping food.
Reproduction:
Dioecious: They usually have separate male and female individuals.
Oviparous: They lay eggs.
Indirect Development: They have larval stages in their development.
Examples:
Pila (Apple snail): A common freshwater snail.
Pinctada (Pearl oyster): Produces pearls.
Sepia (Cuttlefish): A marine cephalopod with an internal shell.
Loligo (Squid): A fast-swimming marine cephalopod.
Octopus (Devil fish): A highly intelligent marine cephalopod with no external shell.
Aplysia (Sea hare): A marine gastropod.
Dentalium (Tusk shell): A marine mollusc with a tusk-shaped shell.
Chaetopleura (Chiton): A marine mollusc with a shell composed of eight plates.
Simple Conclusion:
Molluscs are a diverse group of animals, usually characterized by a calcareous shell, a muscular foot, and a visceral hump. They live in various habitats and have specialized organ systems for respiration, excretion, and feeding. They reproduce sexually and include familiar creatures like snails, clams, and squids
Phylum Echinodermata
What is this section about?
This section describes Phylum Echinodermata, a group of marine animals characterized by spiny skin and a unique water vascular system. It covers their body structure, symmetry, organ systems, and examples like starfish and sea urchins.
Simple Explanation:
General Characteristics:
Endoskeleton: They have an internal skeleton (endoskeleton) made of calcareous ossicles (bony plates). This gives them their "spiny" appearance (Echinodermata means "spiny skinned").
Marine Habitat: All echinoderms live in the ocean.
Organ-system Level: They have well-developed organ systems.
Symmetry:
Radial Symmetry (Adults): Adult echinoderms have radial symmetry, meaning their body parts are arranged around a central point (like a pie).
Bilateral Symmetry (Larvae): Their larval forms have bilateral symmetry (like humans, with a left and right side).
Body Organization:
Triploblastic: They have three primary germ layers.
Coelomate: They have a true body cavity.
Complete Digestive System: They have a complete digestive system with a mouth and an anus. The mouth is on the underside (ventral side), and the anus is on the top side (dorsal side).
Water Vascular System:
Unique Feature: This is the most distinctive feature of echinoderms.
Functions: It's a network of fluid-filled canals and tubes that perform several functions:
Locomotion (movement) using tube feet.
Capture and transport of food.
Respiration (gas exchange).
Excretory System:
Absent: Echinoderms do not have a specialized excretory system.
Reproduction:
Separate Sexes: They have separate male and female individuals.
Sexual Reproduction: They reproduce sexually.
External Fertilization: Fertilization usually happens outside the body, in the water.
Indirect Development: They have a free-swimming larval stage that undergoes metamorphosis to become an adult.
Examples:
Asterias (Starfish): A common echinoderm with five arms.
Echinus (Sea urchin): A spiny, globular echinoderm.
Antedon (Sea lily): A stalked, flower-like echinoderm.
Cucumaria (Sea cucumber): An elongated, soft-bodied echinoderm.
Ophiura (Brittle star): An echinoderm with long, slender arms.
Simple Conclusion:
Echinoderms are exclusively marine animals with spiny skin and radial symmetry as adults. They possess a unique water vascular system for movement, feeding, and respiration. They reproduce sexually with external fertilization and have a free-swimming larval stage. Examples include starfish, sea urchins, and sea cucumbers.
Phylum – Hemichordata
Previous Classification:
Earlier considered a sub-phylum under phylum Chordata.
Now classified as a separate phylum under Non-Chordata.
Characteristic Features:
Possess a rudimentary structure called stomochord in the collar region (resembles notochord).
Small group of worm-like marine animals.
Exhibit organ-system level of organization.
Body Symmetry: Bilaterally symmetrical.
Body Structure:
Triploblastic.
Coelomate.
Cylindrical body with three parts:
Anterior proboscis
Collar
Long trunk
Physiological Systems:
Circulatory System: Open type.
Respiration: Through gills.
Excretory Organ: Proboscis gland.
Reproductive Characteristics:
Sexes: Separate.
Fertilization: External.
Development: Indirect.
Examples:
Balanoglossus
Saccoglossus