Chapter Notes

Basis of Classification

To classify the vast diversity of animals, scientists look for fundamental common features despite differences in appearance and structure. These core features form the basis for grouping animals together. Key characteristics include the arrangement of cells, body symmetry, the nature of the body cavity (coelom), and patterns in the digestive, circulatory, and reproductive systems.

Levels of Organisation

All animals are multicellular, but their cells are organized in different ways, showing increasing complexity.

• Cellular Level: This is the simplest level, where cells are arranged as loose clumps or aggregates. There's some division of labour, but the cells don't form organized tissues.
  • Example: Sponges (Phylum Porifera).
• Tissue Level: In this level, cells that perform the same function are grouped together to form tissues.
  • Example: Coelenterates (Phylum Cnidaria).
• Organ Level: Here, different tissues are grouped together to form organs, each specialized for a specific function.
  • Example: Platyhelminthes (Flatworms).
• Organ System Level: This is the highest level of organization, where different organs work together in functional systems, like the digestive or circulatory system.
  • Examples: Annelids, Arthropods, Molluscs, Echinoderms, and Chordates.

Patterns in Organ Systems

Organ systems can also show varying levels of complexity.

• Digestive System:
  • Incomplete: Has only a single opening that serves as both mouth and anus. Example: Platyhelminthes.
  • Complete: Has two separate openings: a mouth for ingestion and an anus for egestion.
• Circulatory System:
  • Open Type: Blood is pumped by the heart into open spaces or cavities, directly bathing the cells and tissues.
  • Closed Type: Blood is always contained within a network of vessels (arteries, veins, and capillaries) and circulates throughout the body.

Symmetry

Symmetry refers to how the body parts of an animal are arranged.

• Asymmetrical: The body cannot be divided into two equal halves by any plane passing through the center.
  • Example: Most sponges.
• Radial Symmetry: The body can be divided into two identical halves by any plane passing through the central axis. This is common in animals that are sessile (fixed in one place).
  • Examples: Coelenterates, Ctenophores, and adult Echinoderms.
• Bilateral Symmetry: The body can be divided into identical left and right halves by only one specific plane. This arrangement is associated with active movement.
  • Examples: Annelids, Arthropods, and Chordates.

Diploblastic and Triploblastic Organisation

This classification is based on the number of embryonic cell layers (germ layers) from which all body tissues develop.

• Diploblastic: Animals in which the cells are arranged in two embryonic layers: an outer ectoderm and an inner endoderm. A non-cellular, jelly-like layer called mesoglea is present between them.
  • Example: Coelenterates.
• Triploblastic: Animals in which the developing embryo has a third germinal layer, the mesoderm, located between the ectoderm and endoderm. This allows for the development of more complex organs.
  • Examples: All phyla from Platyhelminthes to Chordates.

Coelom

The coelom is a body cavity located between the body wall and the gut wall. Its presence or absence is a crucial feature for classification.

• Coelomates: These animals have a true coelom, which is a body cavity lined on all sides by mesoderm.
  • Examples: Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates, and Chordates.
• Pseudocoelomates: In these animals, the body cavity is not fully lined by mesoderm. Instead, the mesoderm is present as scattered pouches between the ectoderm and endoderm. This type of cavity is called a pseudocoelom.
  • Example: Aschelminthes (Roundworms).
• Acoelomates: These animals have no body cavity at all.
  • Example: Platyhelminthes (Flatworms).

Segmentation

In some animals, the body is divided both externally and internally into a series of repeating segments. This pattern is called metameric segmentation, and the phenomenon is known as metamerism. This allows for specialization of different body regions.

Notochord

The notochord is a flexible, rod-like structure derived from the mesoderm. It forms on the dorsal (back) side of the animal during embryonic development and provides support.

• Chordates: Animals that possess a notochord at some stage of their life.
• Non-chordates: Animals that do not have a notochord. This group includes all phyla from Porifera to Echinoderms.

Classification of Animals

Based on the fundamental features described above, the animal kingdom is broadly classified into different phyla.

Phylum - Porifera (Sponges)

• Common Name: Sponges.
• Habitat: Mostly marine.
• Symmetry: Mostly asymmetrical.
• Organisation: Primitive multicellular animals with a cellular level of organisation.
• Key Feature: Presence of a water transport or canal system. Water enters through tiny pores called ostia, flows into a central cavity called the spongocoel, and exits through a larger opening called the osculum. This system helps in food gathering, respiration, and waste removal.
• Specialised Cells: Choanocytes or collar cells line the spongocoel and canals.
• Digestion: Intracellular (occurs inside the cells).
• Skeleton: Made of spicules or spongin fibres.
• Reproduction: Hermaphrodites (both male and female gametes produced by the same individual). They reproduce asexually by fragmentation and sexually by forming gametes. Fertilisation is internal, and development is indirect (involves a larval stage).
• Examples: Sycon, Spongilla (freshwater sponge), Euspongia (bath sponge).

Phylum - Coelenterata (Cnidaria)

• Habitat: Aquatic, mostly marine. They can be sessile (fixed) or free-swimming.
• Symmetry: Radially symmetrical.
• Organisation: Tissue level of organisation and are diploblastic.
• Key Feature: Presence of cnidoblasts or cnidocytes, which are stinging cells found on the tentacles and body. These are used for anchorage, defense, and capturing prey.
• Body Cavity: A central gastro-vascular cavity with a single opening (mouth).
• Digestion: Both extracellular and intracellular.
• Body Forms: Exhibit two main body forms:
  • Polyp: A sessile, cylindrical form. Examples: Hydra, Adamsia.
  • Medusa: An umbrella-shaped, free-swimming form. Example: Aurelia (jellyfish).
• Alternation of Generations (Metagenesis): Some cnidarians, like Obelia, exist in both forms, where polyps produce medusae asexually, and medusae produce polyps sexually.
• Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), Meandrina (Brain coral).

Phylum - Ctenophora (Comb Jellies)

• Common Name: Sea walnuts or comb jellies.
• Habitat: Exclusively marine.
• Symmetry: Radially symmetrical.
• Organisation: Tissue level of organisation and are diploblastic.
• Key Feature: The body has eight external rows of ciliated comb plates, which help in locomotion.
• Special Property: Bioluminescence, the ability to emit light, is a well-marked feature.
• Digestion: Both extracellular and intracellular.
• Reproduction: Only sexual reproduction. They are hermaphrodites. Fertilisation is external with indirect development.
• Examples: Pleurobrachia, Ctenoplana.

Phylum - Platyhelminthes (Flatworms)

• Common Name: Flatworms, due to their dorso-ventrally flattened body.
• Habitat: Mostly endoparasites in animals, including humans.
• Symmetry: Bilaterally symmetrical.
• Organisation: Organ level of organisation and are triploblastic and acoelomate.
• Parasitic Adaptations: Parasitic forms have hooks and suckers for attachment.
• Excretion: Specialised cells called flame cells help in osmoregulation and excretion.
• Reproduction: Hermaphrodites. Fertilisation is internal, and development involves many larval stages.
• Special Property: Some members like Planaria have a high capacity for regeneration.
• Examples: Taenia (Tapeworm), Fasciola (Liver fluke).

Phylum - Aschelminthes (Roundworms)

• Common Name: Roundworms, as their body is circular in cross-section.
• Habitat: Can be free-living (aquatic/terrestrial) or parasitic.
• Symmetry: Bilaterally symmetrical.
• Organisation: Organ-system level of organisation and are triploblastic and pseudocoelomate.
• Digestive System: Complete alimentary canal with a muscular pharynx.
• Excretion: An excretory tube removes waste through an excretory pore.
• Reproduction: Sexes are separate (dioecious). Often, females are longer than males. Fertilisation is internal. Development can be direct or indirect.
• Examples: Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).

Phylum - Annelida (Segmented Worms)

• Habitat: Aquatic (marine and freshwater) or terrestrial; free-living or parasitic.
• Symmetry: Bilaterally symmetrical.
• Organisation: Organ-system level and are triploblastic, metamerically segmented, and coelomate.
• Key Feature: The body is distinctly divided into segments or metameres (like little rings).
• Locomotion: Have longitudinal and circular muscles. Aquatic annelids like Nereis have lateral appendages called parapodia for swimming.
• Circulatory System: Closed type.
• Excretion: Nephridia help in osmoregulation and excretion.
• Nervous System: Consists of paired ganglia connected to a double ventral nerve cord.
• Reproduction: Sexual. Nereis is dioecious, while earthworms and leeches are monoecious (hermaphrodites).
• Examples: Nereis, Pheretima (Earthworm), Hirudinaria (Blood-sucking leech).

Phylum - Arthropoda (Jointed-Leg Animals)

• Diversity: This is the largest phylum in the animal kingdom, including insects. Over two-thirds of all named species are arthropods.
• Symmetry: Bilaterally symmetrical.
• Organisation: Organ-system level and are triploblastic, segmented, and coelomate.
• Key Features:
  • Chitinous Exoskeleton: The body is covered by a tough, protective exoskeleton made of chitin.
  • Jointed Appendages: The name "Arthropoda" means jointed feet.
• Body Division: Body consists of a head, thorax, and abdomen.
• Respiration: Organs include gills, book gills, book lungs, or a tracheal system.
• Circulatory System: Open type.
• Sensory Organs: Have antennae, eyes (simple and compound), and statocysts (balancing organs).
• Excretion: Takes place through malpighian tubules.
• Reproduction: Mostly dioecious. Fertilisation is usually internal, and they are mostly oviparous (egg-laying).
• Examples:
  • Economically important: Apis (Honey bee), Bombyx (Silkworm).
  • Vectors: Anopheles, Culex, Aedes (Mosquitoes).
  • Pest: Locusta (Locust).
  • Living fossil: Limulus (King crab).

Phylum - Mollusca

• Diversity: The second largest animal phylum.
• Habitat: Terrestrial or aquatic (marine or freshwater).
• Symmetry: Bilaterally symmetrical.
• Organisation: Organ-system level and are triploblastic and coelomate.
• Body Plan: Body is unsegmented and covered by a calcareous shell. It has a distinct head, a muscular foot, and a visceral hump.
• Mantle: A soft, spongy layer of skin forms a mantle over the visceral hump. The space between the mantle and the hump is the mantle cavity, which contains feather-like gills for respiration and excretion.
• Feeding: The mouth contains a file-like rasping organ called the radula.
• Reproduction: Usually dioecious and oviparous with indirect development.
• Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Octopus (Devil fish), Dentalium (Tusk shell).

Phylum - Echinodermata (Spiny-Skinned Animals)

• Name Meaning: "Echinodermata" means spiny-bodied, referring to their endoskeleton of calcareous ossicles.
• Habitat: Exclusively marine.
• Symmetry: Adults are radially symmetrical, but their larvae are bilaterally symmetrical.
• Organisation: Organ-system level and are triploblastic and coelomate.
• Key Feature: Presence of a water vascular system. This system is used for locomotion, capturing and transporting food, and respiration.
• Digestive System: Complete, with a mouth on the ventral (lower) side and an anus on the dorsal (upper) side.
• Excretory System: Absent.
• Reproduction: Sexual. Sexes are separate. Fertilisation is usually external, with indirect development (free-swimming larva).
• Examples: Asterias (Starfish), Echinus (Sea urchin), Cucumaria (Sea cucumber), Ophiura (Brittle star).

Phylum - Hemichordata

• Classification Status: Previously considered a sub-phylum of Chordata, but now placed as a separate phylum under non-chordates.
• Key Structure: They have a rudimentary structure in the collar region called a stomochord, which is similar to a notochord.
• Habitat: A small group of worm-like marine animals.
• Symmetry: Bilaterally symmetrical.
• Organisation: Organ-system level and are triploblastic and coelomate.
• Body Plan: The cylindrical body is composed of an anterior proboscis, a collar, and a long trunk.
• Circulatory System: Open type.
• Respiration: Through gills.
• Excretion: The proboscis gland is the excretory organ.
• Reproduction: Sexes are separate. Fertilisation is external with indirect development.
• Examples: Balanoglossus, Saccoglossus.

Phylum - Chordata

Chordates are defined by the presence of four key features at some stage in their life:

1. A notochord.
2. A dorsal, hollow nerve cord.
3. Paired pharyngeal gill slits.
4. A post-anal tail.

• Symmetry & Organisation: Bilaterally symmetrical, triploblastic, coelomate with an organ-system level of organisation.
• Circulatory System: Closed type.

Phylum Chordata is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata.

• Protochordates: Urochordata and Cephalochordata are often grouped as protochordates and are exclusively marine.
  • Urochordata: Notochord is present only in the larval tail. Examples: Ascidia, Salpa.
  • Cephalochordata: Notochord extends from head to tail and persists throughout life. Example: Branchiostoma (Amphioxus).

Subphylum - Vertebrata

In vertebrates, the notochord is present during the embryonic period and is replaced by a cartilaginous or bony vertebral column in the adult.

Vertebrates also have:

• A ventral muscular heart with 2, 3, or 4 chambers.
• Kidneys for excretion and osmoregulation.
• Paired appendages (fins or limbs).

Vertebrata is divided into several classes.

Class - Cyclostomata (Jawless Fish)

• Habitat: All are ectoparasites on some fishes. They are marine but migrate to freshwater for spawning, after which they die.
• Body: Elongated body with 6-15 pairs of gill slits.
• Mouth: Sucking and circular mouth without jaws.
• Fins and Scales: Body lacks scales and paired fins.
• Skeleton: Cranium and vertebral column are cartilaginous.
• Circulation: Closed type.
• Examples: Petromyzon (Lamprey), Myxine (Hagfish).

Class - Chondrichthyes (Cartilaginous Fishes)

• Habitat: Marine animals with a streamlined body.
• Skeleton: Endoskeleton is made of cartilage.
• Mouth: Located ventrally (on the underside).
• Gills: Gill slits are separate and not covered by an operculum (gill cover).
• Skin: Tough, with minute placoid scales. Teeth are modified placoid scales.
• Air Bladder: Absent, so they must swim constantly to avoid sinking.
• Heart: Two-chambered (one auricle, one ventricle).
• Body Temperature: Cold-blooded (poikilothermous).
• Special Features: Some have electric organs (Torpedo) or a poison sting (Trygon).
• Reproduction: Internal fertilisation. Many are viviparous (give birth to live young).
• Examples: Scoliodon (Dogfish), Pristis (Sawfish), Carcharodon (Great white shark).

Class - Osteichthyes (Bony Fishes)

• Habitat: Includes both marine and freshwater fish.
• Skeleton: Endoskeleton is bony.
• Mouth: Mostly terminal (at the front end).
• Gills: Four pairs of gills covered by an operculum on each side.
• Skin: Covered with cycloid or ctenoid scales.
• Air Bladder: Present, which helps regulate buoyancy.
• Heart: Two-chambered.
• Body Temperature: Cold-blooded.
• Reproduction: External fertilisation. Mostly oviparous (egg-laying) with direct development.
• Examples:
  • Marine: Exocoetus (Flying fish), Hippocampus (Sea horse).
  • Freshwater: Labeo (Rohu), Catla.
  • Aquarium: Betta (Fighting fish), Pterophyllum (Angel fish).

Class - Amphibia

• Name Meaning: "Amphi" means dual, "bios" means life, reflecting their ability to live in both aquatic and terrestrial habitats.
• Body: Divisible into head and trunk; a tail may be present. They have two pairs of limbs.
• Skin: Moist and without scales.
• Ear: A tympanum represents the ear.
• Cloaca: A common chamber where the alimentary, urinary, and reproductive tracts open.
• Respiration: By gills (in larvae), lungs, and through the skin.
• Heart: Three-chambered (two auricles, one ventricle).
• Body Temperature: Cold-blooded.
• Reproduction: External fertilisation. They are oviparous with indirect development (tadpole larva).
• Examples: Bufo (Toad), Rana (Frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).

Class - Reptilia

• Name Meaning: Refers to their creeping or crawling mode of locomotion.
• Habitat: Mostly terrestrial.
• Body: Covered by dry, cornified skin with epidermal scales or scutes.
• Ear: Tympanum represents the ear; no external ear openings.
• Heart: Usually three-chambered, but four-chambered in crocodiles.
• Body Temperature: Cold-blooded (poikilotherms).
• Reproduction: Internal fertilisation. They are oviparous with direct development.
• Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Crocodilus (Crocodile), Naja (Cobra), Vipera (Viper).

Class - Aves (Birds)

• Key Feature: Presence of feathers and the ability to fly (with exceptions like the Ostrich). They possess a beak.
• Limbs: Forelimbs are modified into wings. Hind limbs are adapted for walking, swimming, or clasping.
• Skeleton: Fully ossified (bony), and long bones are hollow with air cavities (pneumatic bones) to reduce weight for flight.
• Digestive Tract: Has additional chambers: the crop and gizzard.
• Heart: Completely four-chambered.
• Body Temperature: Warm-blooded (homoiothermous), meaning they can maintain a constant body temperature.
• Respiration: Lungs are supplemented by air sacs.
• Reproduction: Internal fertilisation. They are oviparous with direct development.
• Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock).

Class - Mammalia

• Habitat: Found in a wide variety of habitats, from polar ice caps to deserts.
• Key Features:
  • Presence of mammary glands, which produce milk to nourish young ones.
  • Skin is unique in possessing hair.
• Ears: External ears or pinnae are present.
• Teeth: Different types of teeth are present in the jaw.
• Heart: Four-chambered.
• Body Temperature: Warm-blooded (homoiothermous).
• Respiration: By lungs.
• Reproduction: Fertilisation is internal. They are viviparous (give birth to live young), with a few exceptions. Development is direct.
• Examples:
  • Oviparous (egg-laying): Ornithorhynchus (Platypus).
  • Viviparous: Macropus (Kangaroo), Pteropus (Flying fox), Canis (Dog), Felis (Cat), Elephas (Elephant), Balaenoptera (Blue whale), Panthera tigris (Tiger).

Salient Features of Different Phyla in the Animal Kingdom