# 10.3 DIVERSITY IN ANIMALS This section covers the major phyla of the Kingdom Animalia, focusing on the vast structural differences between invertebrates (animals without a backbone) and vertebrates (animals with a backbone), and their key evolutionary adaptations. --- - **Invertebrates:** Animal species that lack a backbone. They account for approximately 95% of all known animal species and are grouped into several major phyla. - **Vertebrates:** Animals that possess a backbone or vertebral column. They are part of the Phylum Chordata. - **Symmetry:** The arrangement of body parts around a central point or axis. - *Asymmetrical:* No symmetry (e.g., Sponges). - *Radial Symmetry:* Body parts arranged around a central axis, like spokes on a wheel (e.g., Cnidarians, adult Echinoderms). - *Bilateral Symmetry:* Body can be divided into two mirror-image halves along a single plane (e.g., Flatworms, Humans). - **Germ Layers:** Embryonic layers of cells that differentiate into tissues and organs. - *Diploblastic:* Having two germ layers: ectoderm and endoderm (e.g., Cnidarians). - *Triploblastic:* Having three germ layers: ectoderm, mesoderm, and endoderm (e.g., Platyhelminthes to Chordates). - **Body Cavity (Coelom):** A fluid-filled space between the digestive tract and the outer body wall. - *Acoelomate:* Lacks a body cavity (e.g., Platyhelminthes). - *Pseudocoelomate:* Has a "false" coelom not completely lined by mesoderm (e.g., Aschelminthes). - *Coelomate:* Has a true coelom completely lined by mesoderm (e.g., Annelids, Chordates). - **Chordate Characteristics:** All chordates share four key features at some point in their life cycle: a **notochord**, a **dorsal hollow nerve cord**, **pharyngeal gill slits**, and a **post-anal tail**. --- ### 10.2.1 Invertebrates #### 1. Phylum Porifera (Pore-bearers) *Sponges* are the simplest multicellular animals. - **Habitat:** Aquatic; mostly marine, some freshwater. - **Mobility:** *Sessile* (attached to a substrate) as adults, with motile larvae. - **Body Plan:** Multicellular but lack true tissues or organs. Body is *asymmetrical*. - **Structure:** - Body perforated by pores called ***ostia***. - A central cavity called the ***spongocoel***. - A large opening called the ***osculum*** for water to exit. - **Cell Layers:** - *Pinacoderm:* Outer layer of flat cells (*pinacocytes*). - *Choanocytes:* Inner layer of flagellated collar cells that create water currents and trap food. - *Mesenchyme:* Gelatinous middle layer with amoeboid cells. - **Skeleton:** Composed of needle-like ***spicules*** (calcareous or siliceous) or flexible ***spongin*** fibres. - **Reproduction:** - *Asexual:* By budding or formation of *gemmules*. - *Sexual:* By eggs and sperm; can be hermaphroditic. - **Regeneration:** Possess a remarkable ability to regenerate from small fragments. - **Examples:** *Spongilla*, *Sycon*. - **Importance:** Used for bathing (bath sponges), absorbing fluids in surgery, and sound absorption in buildings. #### 2. Phylum Cnidaria (Stinging-celled organisms) - **Habitat:** Entirely aquatic, mainly marine. - **Body Plan:** *Diploblastic* (ectoderm and endoderm with *mesoglea* in between) with *radial symmetry*. Possess a sac-like ***gastrovascular cavity*** with a single opening (mouth). - **Key Feature:** Presence of stinging cells called ***cnidocytes***, which contain stinging organelles called ***nematocysts*** used for defense and prey capture. - **Body Forms (Polymorphism):** - ***Polyp:*** Sessile, cylindrical form (e.g., *Hydra*, sea anemone). - ***Medusa:*** Free-swimming, bell-shaped form (e.g., Jellyfish). - **Reproduction:** Asexual (budding) and sexual (gametes). - **Examples:** *Hydra*, Jellyfish, Sea anemone, Corals (*Obelia*). - **Importance:** Coral reefs protect shores from erosion and provide critical habitats. #### 3. Phylum Platyhelminthes (Flatworms) - **Habitat:** Free-living (aquatic) or parasitic. - **Body Plan:** *Triploblastic*, *acoelomate* (no body cavity), and exhibit *bilateral symmetry*. Body is soft and flattened dorsiventrally. - **Reproduction:** Asexual (fission) and sexual (most are *monoecious* - hermaphroditic). - **Examples:** *Dugesia* (planaria), *Fasciola* (liver fluke), *Taenia* (tapeworm). #### 4. Phylum Aschelminthes / Nematoda (Roundworms) - **Habitat:** Free-living in soil and water, or parasitic in plants and animals. - **Body Plan:** *Triploblastic*, *pseudocoelomate* (false body cavity), and have *bilateral symmetry*. Body is cylindrical and tapers at both ends. - **Reproduction:** Mostly *dioecious* (separate sexes) with internal fertilization. - **Examples:** *Ascaris lumbricoides* (intestinal parasite), *Enterobius vermicularis* (pinworm). #### 5. Phylum Mollusca (Soft-bodied animals) - **Habitat:** Freshwater, marine, and terrestrial (moist places). - **Body Plan:** *Triploblastic*, *coelomate*, unsegmented soft body with *bilateral symmetry*. - **Key Features:** - **Muscular Foot:** Used for locomotion. - **Visceral Mass:** Contains the internal organs. - **Mantle:** A fold of tissue that covers the visceral mass and secretes a shell (if present). - **Radula:** A tongue-like structure with teeth for scraping food (absent in bivalves). - **Shell:** Can be external (snail), internal (cuttlefish), or absent (octopus). - **Examples:** Snail, Slug, Oyster, Octopus, Cuttlefish. #### 6. Phylum Annelida (Segmented worms) - **Habitat:** Soil, freshwater, and marine. - **Body Plan:** *Triploblastic*, *coelomate*, with *bilateral symmetry*. Body is *metamerically segmented*. - **Key Features:** - **Closed Circulatory System.** - **Setae (chaetae):** Chitinous bristles used for locomotion. *Nereis* has paddle-like appendages called **parapodia**. - The coelom acts as a *hydrostatic skeleton*. - **Reproduction:** Mostly sexual; can be hermaphroditic (*earthworm*) or have separate sexes (*Nereis*). - **Examples:** *Pheritima* (Earthworm), *Hirudo* (Leech), *Nereis*. #### 7. Phylum Arthropoda (Animals with jointed legs) This is the largest and most successful animal phylum. - **Habitat:** Found in all types of habitats. - **Body Plan:** *Triploblastic*, *coelomate* (reduced), segmented body (head, thorax, abdomen) with *bilateral symmetry*. - **Key Features:** - **Jointed Appendages:** Specialized for various functions (walking, feeding, sensing). - **Chitinous Exoskeleton:** Provides protection and support; must be shed to grow (molting). - **Haemocoel:** The main body cavity, connected to an open circulatory system. - **Reproduction:** Separate sexes with internal fertilization. Development often involves *metamorphosis*. #### 8. Phylum Echinodermata (Spiny-skinned animals) - **Habitat:** Exclusively marine. - **Body Plan:** *Triploblastic*, *coelomate*. Adults exhibit *radial symmetry*, while larvae are *bilaterally symmetrical*. - **Key Features:** - **Endoskeleton:** Composed of calcium-rich plates, often with spines. - **Water Vascular System:** A network of water-filled canals used for locomotion (via **tube feet**), feeding, and gas exchange. - **Reproduction:** Separate sexes with external fertilization. High capacity for *autotomy* (shedding parts) and *regeneration*. - **Examples:** Starfish, Sea urchin, Sea cucumber, Brittle star. ### Transitional Phyla #### 9. Phylum Hemichordata (Acorn worms) - Considered a link between echinoderms and chordates. - **Habitat:** Exclusively marine. - **Body Plan:** Soft, unsegmented, worm-like body divided into a **proboscis**, **collar**, and **trunk**. They are triploblastic, coelomate, and bilaterally symmetrical. - **Examples:** *Balanoglossus*, *Saccoglossus*. ### 10. Phylum Chordata This phylum includes all vertebrates and some invertebrate relatives. #### Characteristics of Chordates All chordates share four fundamental features at some stage of their development: 1. ***Notochord:*** A flexible, supportive rod along the dorsal side. In most vertebrates, it's replaced by the vertebral column. 2. ***Dorsal Hollow Nerve Cord:*** A tube of nerve tissue located above the notochord. 3. ***Pharyngeal Gill Slits:*** Paired openings in the pharynx. Used for filter-feeding in invertebrates and respiration in aquatic vertebrates. 4. ***Post-anal Tail:*** A tail that extends beyond the anus. #### Classification of Chordates | Group | Subphylum | Key Characteristics | Examples | | --------------------- | ------------------ | ---------------------------------------------------------------------------------------- | ---------------- | | **Protochordata (Acrania)** | Urochordata | Notochord present only in the tail of the larva. | *Ascidia* | | (No cranium) | Cephalochordata | Fish-like body; notochord extends the entire length of the body and persists throughout life. | *Amphioxus* | | **Craniata** | **Vertebrata** | Notochord replaced by a vertebral column; brain enclosed in a cranium. | Fishes, Mammals | ### Subphylum Vertebrata #### a. Superclass Agnatha (Jawless Fishes) - Slender, eel-like body with naked skin. - Lack jaws and paired fins. - **Examples:** Hagfish, Lamprey. #### b. Superclass Gnathostomata (Jawed Vertebrates) Includes all other vertebrate classes. Jaws are believed to have evolved from the first pair of gill arches. ##### 1. Class Chondrichthyes (Cartilaginous Fishes) - **Skeleton:** Composed entirely of cartilage. - **Body:** Spindle-shaped (fusiform) with a ventral mouth. - **Skin:** Covered in placoid scales. - **Examples:** Sharks, Rays, Skates. ##### 2. Class Osteichthyes (Bony Fishes) - **Skeleton:** Endoskeleton made of bone. - **Gills:** Covered by a protective flap called the *operculum*. - **Swim Bladder:** Many possess a gas-filled sac for buoyancy control. - **Examples:** Labeo (Rohu), Sea horse, *Latimeria* (a lobe-finned fish). ##### 3. Class Amphibia - The first tetrapods; mark the transition from aquatic to terrestrial life. - **Limbs:** Most have two pairs of legs (pentadactyl). - **Skin:** Smooth, moist, glandular skin used for gas exchange. - **Heart:** Three-chambered heart (two atria, one ventricle). - **Lifecycle:** Undergo *metamorphosis* from an aquatic larval stage (gills) to a terrestrial/semi-aquatic adult stage (lungs). - **Temperature:** *Poikilothermic* (cold-blooded/ectothermic). - **Examples:** Frogs, Toads, Salamanders. ##### 4. Class Reptilia - First truly terrestrial vertebrates, adapted for dry environments. - **Skin:** Covered with dry, cornified scales to prevent water loss. - **Reproduction:** Internal fertilization and lay an ***amniotic egg*** with a protective leathery shell. - **Heart:** Incompletely four-chambered (two atria, a partly divided ventricle). Crocodiles have a complete four-chambered heart. - **Temperature:** *Poikilothermic* (cold-blooded/ectothermic). - **Examples:** Tortoise, Lizard, Snake, Crocodile. ##### 5. Class Aves (Birds) - **Adaptations for Flight:** - Forelimbs modified into **wings**. - Body covered in **feathers**. - **Lightweight skeleton** with air spaces. - Horny **beak** instead of teeth. - **Heart:** Four-chambered heart (efficient double circulation). - **Respiration:** Lungs supplemented by a system of **air sacs**. - **Temperature:** *Homoeothermic* (warm-blooded/endothermic). - **Excretion:** Excrete uric acid (semi-solid urine); no urinary bladder. - **Examples:** Ostrich, Sparrow, Penguin, Eagle. ##### 6. Class Mammalia - **Key Characteristics:** - Body covered with **hair** or fur. - Females possess **mammary glands** to produce milk for young. - A muscular **diaphragm** separates the thoracic and abdominal cavities. - Three middle ear bones (*incus, malleus, stapes*). - **Teeth** are differentiated (heterodont). - **Heart:** Four-chambered heart with a left aortic arch. - **Temperature:** *Homoeothermic* (warm-blooded/endothermic). **Sub-classes of Mammalia** 1. **Prototheria (Monotremes):** Egg-laying mammals. A link between reptiles and mammals. *Examples: Duckbill platypus, Echidna.* 2. **Metatheria (Marsupials):** Pouched mammals. Give birth to immature young that complete development in a pouch (*marsupium*). *Examples: Kangaroo, Koala, Opossum.* 3. **Eutheria (Placental Mammals):** Young develop inside the mother's uterus, nourished via a **placenta**. *Examples: Human, Whale, Bat, Elephant.* --- - Animal diversity ranges from simple, asymmetrical sponges to complex vertebrates. - Invertebrates lack a backbone and are classified into major phyla based on body plan, symmetry, germ layers, and presence of a coelom. - Chordates are defined by the presence of a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. - Vertebrates are a subphylum of Chordata characterized by a cranium and a vertebral column. - Key evolutionary trends in vertebrates include the development of jaws, the transition to land (amphibians), the amniotic egg (reptiles), and endothermy (birds and mammals). **Comparison of Major Invertebrate Phyla** | Phylum | Symmetry | Germ Layers | Body Cavity | Key Distinguishing Feature | |---------------|------------------|--------------|------------------|----------------------------------------------------------| | **Porifera** | Asymmetrical | None (cellular) | Acoelomate | Pores (ostia), choanocytes, spicules | | **Cnidaria** | Radial | Diploblastic | Acoelomate | Nematocysts (stinging cells), polyp/medusa forms | | **Platyhelminthes**| Bilateral | Triploblastic| Acoelomate | Dorsoventrally flattened body | | **Aschelminthes** | Bilateral | Triploblastic| Pseudocoelomate | Cylindrical body, "false" coelom | | **Mollusca** | Bilateral | Triploblastic| Coelomate | Soft body, mantle, muscular foot, radula | | **Annelida** | Bilateral | Triploblastic| Coelomate | Metameric segmentation, setae | | **Arthropoda** | Bilateral | Triploblastic| Coelomate (reduced)| Chitinous exoskeleton, jointed appendages | | **Echinodermata** | Radial (adult) | Triploblastic| Coelomate | Water vascular system, spiny skin, endoskeleton |