Animal Diversity

5.1 Levels of Organisation, Symmetry, Coelom & Germ Layers

Animals are multicellular, heterotrophic eukaryotes that lack a cell wall. Their extraordinary diversity can be organised through four fundamental architectural criteria that cut across all phyla: level of organisation, body symmetry, number of germ layers, and nature of the body cavity.

Aristotle ~350 BCE — first systematic animal classification · Linnaeus 1758 — Systema Naturae 10th ed., starting point of zoological nomenclature · Lamarck 1809 — coined invertebrate; Philosophie Zoologique · Cuvier 1817 — four embranchements; comparative anatomy · Whittaker 1969 — five-kingdom system · Margulis 1981 — five-kingdom revised (endosymbiosis)

5.1.1 Levels of Organisation

The most primitive animals have loosely associated cells with no tissue coordination. Moving up the scale:

• Cellular level — cells carry out all functions more or less independently; division of labour is at the cell level only. Example: Porifera (sponges).
• Cell-tissue level — similar cells are aggregated into tissues performing a common function, but no distinct organs. Example: Cnidaria.
• Tissue-organ level — tissues associate into organs; organs are not grouped into systems. Example: Platyhelminthes.
• Organ-system level — organs coordinate into systems. From Aschelminthes (Nematoda) upwards. This is the dominant plan in all higher invertebrates and all vertebrates.

5.1.2 Body Symmetry

Symmetry describes how an animal's body parts are arranged around a central point or axis.

• Asymmetry — no plane of symmetry. Most adult sponges.
• Radial symmetry — any plane through the central axis divides the body into mirror-image halves. Suited to sedentary or slow drifting lifestyles. Cnidaria (polyp and medusa), adult Echinodermata (pentaradial).
• Bilateral symmetry — only one plane (sagittal) divides into equal halves. Associated with active movement, cephalisation (head formation). Platyhelminthes and all higher phyla. Note: echinoderm larvae are bilaterally symmetrical; adults become pentaradial.

5.1.3 Germ Layers

Embryonic development lays down germ layers that give rise to all adult tissues. The number of layers is phylogenetically significant:

• Diploblastic — two germ layers: outer ectoderm and inner endoderm, with a gelatinous mesoglea between them. Porifera and Cnidaria.
• Triploblastic — three layers: ectoderm, mesoderm, endoderm. All animals from Platyhelminthes upward. Mesoderm gives rise to muscles, circulatory system, gonads, excretory organs.

5.1.4 Body Cavity (Coelom)

A coelom is a fluid-filled body cavity that develops within the mesoderm. Its presence, absence, or partial formation is one of the most tested characters in competitive examinations.

• Acoelomate — no body cavity; mesoderm is solid (parenchyma fills the space). Example: Platyhelminthes.
• Pseudocoelomate — a body cavity is present but it is not lined by mesoderm on both sides (derived from blastocoel). Example: Aschelminthes (Nematoda). The cavity is between mesoderm and endoderm.
• Eucoelomate (true coelomate) — body cavity fully lined by mesoderm (peritoneum). Annelida, Arthropoda, Mollusca, Echinodermata, Chordata and all higher phyla.

Eucoelomates split further into Protostomia (mouth from blastopore — Annelida, Arthropoda, Mollusca) and Deuterostomia (anus from blastopore, mouth secondary — Echinodermata, Hemichordata, Chordata).

*Porifera are sometimes considered parazoa — they lack true tissues; the diploblastic assignment is debated.

5.2 Protozoa to Porifera — the Simplest Animals

The single-celled and near-single-celled organisms studied in traditional zoology syllabi — protozoans and sponges — illustrate the transition from unicellular to multicellular body plans and from intracellular to extracellular digestion.

5.2.1 Protozoa (Protista — animal-like)

Protozoa are now classified under Kingdom Protista but remain in the zoology syllabus. They are unicellular, eukaryotic, and heterotrophic (or mixotrophic). Locomotion is by pseudopodia, flagella, or cilia.

Euglena is a borderline case: it is photosynthetic (has chloroplasts) but animal-like in lacking a cell wall and being motile. It is the classic example of why the plant/animal divide breaks down at the unicellular level.

5.2.2 Porifera (Sponges)

Porifera means "pore-bearer." Sponges represent the most primitive metazoa — they have a cellular grade of organisation (not a tissue grade) and are called parazoa. Key features:

• Body wall: outer pinacocytes (flat, epithelium-like cells) and inner choanocytes (collar cells with flagella that beat to drive water current).
• Water enters through numerous pores (ostia), flows through a central cavity (spongocoel), and exits via the large osculum.
• Skeleton of spicules: calcareous (CaCO₃) in Calcarea; siliceous (SiO₂) in Hexactinellida (glass sponges); organic spongin fibres in Demospongiae (Euspongia — bath sponge).
• Digestion is intracellular (inside choanocytes and amoebocytes).
• No nervous system. Totipotent archaeocytes (amoebocytes) can regenerate whole sponges — basis for asexual reproduction by gemmules and fragmentation.
• Spongilla is a freshwater sponge found in Indian rivers. Sycon is a marine calcareous sponge.

5.2.3 Canal Systems of Porifera

The complexity of the water-current canal system increases progressively:

5.3 Cnidaria, Ctenophora & Platyhelminthes

5.3.1 Cnidaria (Coelenterata)

Cnidaria (from Greek knide = nettle) are diploblastic, radially symmetrical, tissue-grade animals with a gastrovascular cavity (coelenteron) and the unique cnidocytes — stinging cells containing nematocysts used for prey capture and defence. The name Coelenterata (older term) is now mostly replaced by Cnidaria.

• Two body forms: sedentary polyp (cylindrical, e.g., Hydra, coral polyp) and free-floating medusa (umbrella-shaped, e.g., jellyfish). Many species show alternation of generation (metagenesis) — polyp (asexual) ↔ medusa (sexual).
• Gastrovascular cavity has a single opening (mouth = anus) — no through gut.
• Digestion is both extracellular (cavity) and intracellular (gastrodermal cells).
• Nervous system is a primitive nerve net — no brain.

Coral reefs are formed by hermatypic (reef-building) corals of the order Madreporaria. Coral polyps secrete calcareous skeletons; over thousands of years these accumulate into reefs. Reef bleaching occurs when symbiotic Symbiodinium (zooxanthellae) are expelled due to thermal stress.

5.3.2 Ctenophora

Ctenophora (comb jellies) are marine, biradially symmetric, diploblastic-like (though some workers consider them triploblastic). Key features:

• Eight rows of comb-plates (ctenes) — fused cilia used for locomotion.
• Bioluminescent — produce light by luciferase reaction.
• No cnidocytes — distinguish from Cnidaria. Instead, sticky colloblasts capture prey.
• Digestion partly extracellular in the gastrovascular canal.
• Example: Pleurobrachia (sea gooseberry), Ctenoplana.

5.3.3 Platyhelminthes (Flatworms)

Platyhelminthes (flat = platy, worm = helmis) are the first triploblastic, bilaterally symmetric, acoelomate animals. Being dorsoventrally flattened maximises surface area for gas exchange (no circulatory system). Key features:

• Excretion by flame cells (protonephridia) — the beating cilia create a "flame."
• Digestive system: absent in Cestoda; branched gastrovascular cavity in Turbellaria and Trematoda.
• Hermaphroditic (mostly); sexual reproduction complex.

5.4 Aschelminthes (Nemathelminthes) & Annelida

5.4.1 Aschelminthes / Nematoda (Roundworms)

Aschelminthes (also called Nemathelminthes) are triploblastic, bilaterally symmetric, pseudocoelomate animals with an elongate, cylindrical, unsegmented body tapering at both ends. The cuticle is shed periodically (ecdysis), placing them in the ecdysozoan clade. Musculature consists of longitudinal muscle fibres only (no circular muscles — hence the characteristic thrashing movement). The body cavity (pseudocoelom) acts as a hydrostatic skeleton.

• Digestive system: complete gut (mouth → oesophagus → intestine → anus) — unlike flatworms.
• Excretion: renette cells (glandular) or H-shaped excretory canals; no flame cells.
• Dioecious (separate sexes); marked sexual dimorphism — females usually larger.
• No circulatory system; oxygen diffuses through body wall.

5.4.2 Annelida (Segmented Worms)

Annelida (Latin annellus = little ring) are triploblastic, bilaterally symmetric, metamerically segmented, eucoelomate animals. Metamerism (serial repetition of body segments) allows specialisation of body regions and is the basis for complex locomotion. The coelom is the first true coelom in the animal kingdom.

• Closed circulatory system (in Oligochaeta and Polychaeta) — blood stays within vessels. Haemoglobin dissolved in plasma (not in cells).
• Excretion by nephridia — one pair per segment (metanephridia in earthworm).
• Nervous system: pair of cerebral ganglia (brain) + double ventral nerve cord with segmental ganglia.
• Locomotion: longitudinal + circular muscles acting on hydrostatic skeleton of coelom; setae (chaetae) for grip.

Pheretima posthuma (earthworm) is called the "farmer's friend" as it aerates and enriches soil. It is hermaphroditic but cross-fertilises. The clitellum secretes cocoon for eggs. Hirudinaria secrets hirudin, used medicinally to prevent blood clotting (leeching in microsurgery).

5.5 Arthropoda — the Most Diverse Phylum

Arthropoda (Greek arthron = joint + pous = foot) is the largest phylum in the animal kingdom, comprising approximately 80% of all known animal species (~1.1 million described species). They are triploblastic, bilaterally symmetric, eucoelomate, metamerically segmented animals with jointed appendages.

5.5.1 Defining Characters

• Exoskeleton of chitin — provides protection, prevents desiccation; must be shed during growth (moulting / ecdysis).
• Jointed appendages — serially homologous, variously modified for walking, swimming, feeding, sensing.
• Open circulatory system — blood (haemolymph) bathes organs in haemocoelic spaces; dorsal heart pumps.
• Body divided into distinct tagmata (functional regions): head + thorax + abdomen (in insects); cephalothorax + abdomen (in arachnids, crustaceans).
• Respiration: gills (aquatic), tracheae (insects), book lungs (spiders), book gills (Limulus).
• Excretion: Malpighian tubules (insects/arachnids); green glands/antennal glands (crustaceans).
• Compound eyes; antennae for mechanoreception and chemoreception.

5.5.2 Major Arthropod Classes

5.5.3 Insect Orders (Diagnostic Features)

5.6 Mollusca, Echinodermata & Hemichordata

5.6.1 Mollusca

Mollusca (Latin molluscus = soft) is the second largest phylum by species number (~100,000 species). They are triploblastic, bilaterally symmetric, eucoelomate, unsegmented animals with a characteristic soft body covered by a fleshy mantle that usually secretes a calcareous shell. The underside bears a muscular foot used for locomotion. A rasping tongue-like radula is present (absent in bivalves).

• Open circulatory system (haemocoel) — except Cephalopoda (closed).
• Respiration by ctenidia (gills); some gastropods have a lung (Pulmonata).
• Excretion by kidneys (Bowman's glands / Keber's organ in bivalves).

Pinctada margaritifera and P. fucata are the pearl oysters; a pearl forms when an irritant (sand grain) is coated with nacre (calcium carbonate + conchiolin). Cephalopods have the most developed nervous system among invertebrates; octopus can learn and solve puzzles.

5.6.2 Echinodermata

Echinodermata (Greek echinos = spiny + derma = skin) are exclusively marine, deuterostome, eucoelomate animals. Adults show pentaradial symmetry but larvae are bilaterally symmetric — a key evolutionary clue linking them to chordates. Their most diagnostic feature is the water vascular system (WVS) — a hydraulic system of canals and tube feet used for locomotion, food capture, and gas exchange.

• Endoskeleton of calcareous plates (ossicles) covered by thin epidermis; spines project outward (hence "spiny skin").
• No excretory or circulatory organs; gas exchange by skin gills (dermal branchiae) and tube feet.
• Remarkable power of autotomy (shedding arms when threatened) and regeneration (starfish can regenerate a whole arm).
• Separate sexes; fertilisation external; free-swimming larvae (dipleurula, bipinnaria, echinopluteus).

5.6.3 Hemichordata

Hemichordata ("half-chordate") occupy a pivotal phylogenetic position — they share features with both Echinodermata (deuterostome larva) and Chordata (pharyngeal gill slits). They are triploblastic, bilaterally symmetric, eucoelomate, deuterostome animals. Key features:

• Body divided into proboscis (protosome), collar (mesosome), and trunk (metasome).
• A short, rigid stomochord in the proboscis — resembles notochord structurally but is NOT a true notochord (does not arise from same embryological origin).
• Pharyngeal gill slits — shared with Chordata; used for filter feeding and gas exchange.
• Open circulatory system; excretion by glomerulus (in proboscis).
• Example: Balanoglossus (acorn worm / tongue worm) — marine, burrowing.

5.7 Chordata — Common Features & Sub-phyla

Chordata is the phylum to which humans belong. All chordates share four hallmark features present at some stage of their life (may be embryonic or larval only):

1. Notochord — a flexible, rod-like, mesodermal structure providing axial support. In vertebrates, replaced by the vertebral column in adults.
2. Dorsal hollow nerve cord — develops from ectoderm (neural tube); hollow (with central canal). Contrast with the solid, ventral nerve cords of invertebrates.
3. Pharyngeal gill slits — lateral openings in the pharynx region; used for filter feeding and/or gas exchange. In terrestrial vertebrates, present only in embryo, modified to form other structures (e.g., Eustachian tubes, tonsils, thymus, parathyroid).
4. Post-anal tail — muscular tail extending posterior to the anus; may be reduced/vestigial in adults.

5.7.1 Sub-phyla of Chordata

Vertebrata is the third sub-phylum and the most advanced — notochord replaced by vertebral column; paired appendages; well-developed brain in cranium; closed circulatory system with chambered heart.

Aristotle ~350 BCE — first divided animals into Enhaima (blooded = vertebrates) and Anhaima (bloodless = invertebrates) · Cuvier 1817 — four embranchements; separated vertebrates and invertebrates anatomically · Lamarck 1809 — coined "invertebrate" in Philosophie Zoologique · Linnaeus 1758 — Systema Naturae 10th ed. — starting point of zoological nomenclature; established binomial system for animals · Whittaker 1969 — five-kingdom system; separated Monera, Protista, Fungi, Plantae, Animalia

5.8 Vertebrate Classes — Pisces, Amphibia, Reptilia, Aves, Mammalia

Vertebrata is the most diverse and familiar sub-phylum of Chordata. The vertebral column (backbone) replaces the notochord and forms the axial skeleton. A well-developed brain in a cranium, paired appendages, a closed circulatory system with a chambered heart, and differentiated sense organs are defining vertebrate features.

5.8.1 Pisces (Fishes)

Pisces are cold-blooded (ectothermic), aquatic, gill-breathing vertebrates with a 2-chambered heart (1 auricle + 1 ventricle). They are divided into two groups:

5.8.2 Amphibia

Amphibia (Greek amphi = both + bios = life) lead a dual life — larvae aquatic with gills, adults semi-terrestrial with lungs. They are cold-blooded, 3-chambered heart (2 auricles + 1 ventricle), pulmocutaneous respiration (lungs + moist glandular skin). Fertilisation mostly external (in water). Amniotic egg absent (anamniotes).

5.8.3 Reptilia

Reptilia are cold-blooded (ectothermic), amniotic (amniotic egg with membranes — amnion, allantois, chorion, yolk sac allowing terrestrial laying). Key features:

• Skin: dry, covered with keratinous epidermal scales — prevents desiccation.
• Heart: generally 3-chambered (2 auricles + 1 partially divided ventricle); Crocodilia have 4 chambers (Foramen of Panizza connects the two aortae).
• Excretion: uricotelic — uric acid conserves water; suited to terrestrial life.
• Respiration: lungs only.

Examples: Calotes versicolor (garden lizard, "girgit"), Varanus (monitor lizard), Chamaeleon, Naja naja (Indian cobra), Bungarus (krait), Crocodylus (crocodile), Testudo (tortoise), Chelone (sea turtle), Draco (flying lizard).

5.8.4 Aves (Birds)

Aves are warm-blooded (endothermic), oviparous vertebrates. They evolved from theropod dinosaurs (Maniraptora clade); Archaeopteryx (Jurassic) is the transitional fossil. Key features:

• Feathers (modified scales) — for flight, insulation, display.
• 4-chambered heart; right systemic aortic arch retained.
• Hollow (pneumatic) bones — reduce weight for flight.
• Air sacs connected to lungs — unidirectional airflow; highly efficient gas exchange.
• Uricotelic (uric acid), same as reptiles.
• Forelimbs modified into wings; hindlimbs clawed feet.

5.8.5 Mammalia

Mammalia are warm-blooded, viviparous (mostly), hairy vertebrates that nurse young with milk from mammary glands. They have a 4-chambered heart with left systemic aortic arch retained (opposite to birds), a muscular diaphragm for breathing, and differentiated teeth (heterodonty).

5.9 Parasitic & Locomotor Adaptations; Metamerism

5.9.1 Parasitic Adaptations

Endoparasites show a suite of adaptations for living inside a host:

• Loss of locomotory organs — tapeworms (Taenia) have no legs, cilia, or flagella; they live bathed in host-digested nutrients.
• Loss of digestive system — cestodes (tapeworms) absorb food through the tegument (body surface).
• Attachment organs — suckers, hooks (scolex of tapeworm), holdfast organs.
• Protective cuticle — resistant to host enzymes and immune response.
• High reproductive output — Taenia proglottids each contain both male and female organs; up to 1,000 proglottids, each with ~50,000 eggs.
• Complex life cycles — multiple hosts and larval stages (Fasciola: sheep/cattle primary host, snail intermediate host).
• Anaerobic metabolism — intestinal parasites use glycolysis in low-O₂ environment.

5.9.2 Locomotor Adaptations

Different groups have evolved distinct locomotion mechanisms matched to their environments:

• Pseudopodial — cytoplasmic streaming forms temporary projections (Amoeba).
• Flagellar — whip-like flagella rotate or undulate (Euglena, sperm cells).
• Ciliary — coordinated metachronal beating of thousands of cilia (Paramecium).
• Muscular hydrostatic — earthworm uses longitudinal + circular muscles acting on coelomic fluid; setae provide grip.
• Jointed appendages — arthropods use lever-action of jointed exoskeletal limbs driven by antagonistic muscle pairs.
• Water vascular system — tube feet in echinoderms work by hydraulic extension and muscular contraction.
• Jet propulsion — cephalopods (Octopus, squid) expel water forcefully from the mantle cavity for rapid movement.
• Undulation of fins/body — fish locomotion; lateral undulation by axial muscles; median fins stabilise.

5.9.3 Metamerism (Metameric Segmentation)

Metamerism is the serial repetition of similar body segments (metameres) along the antero-posterior axis. It is a fundamental body-plan feature in Annelida and Arthropoda and the embryonic basis of vertebrate segmentation (somites).

• In Annelida: each segment contains a set of excretory (nephridia), circulatory (lateral vessels), nervous (ganglion), and muscular structures. True metamerism — internal organs repeat.
• In Arthropoda: segments are grouped into tagmata (specialised functional regions); some ancestral segments fuse. Not as "pure" as annelid metamerism.
• In Vertebrates: embryonic somites (blocks of mesoderm) give rise to vertebrae, ribs, and segmental muscles. Internal organs are not serially repeated.
• Advantage of metamerism: allows regional specialisation, redundancy, and controlled flexibility of body.

5.10 Quick-Reference Tables

*Prototheria (platypus, echidna) are egg-laying mammals; Metatheria (marsupials) have brief gestation.

End of Chapter 5 · Animal Diversity. HPRCA-pat. indicates HPRCA / state-TGT pattern questions; literal past-paper items will be flagged with year when official papers are sourced.