Introduction
Over a million species of animals have been described, so classification is essential; it also helps place newly described species. Despite differences, animals share fundamental features — arrangement of cells, body symmetry, nature of coelom, and patterns of digestive, circulatory and reproductive systems — which form the basis of classification.
Key Concepts
4.1 Basis of Classification
4.1.1 Levels of organisation
Cellular — loose cell aggregates (sponges).Tissue — cells with the same function form tissues (coelenterates).Organ — tissues form organs (Platyhelminthes).Organ system — organs form functional systems (annelids, arthropods, molluscs, echinoderms, chordates).Digestive system: incomplete (single opening — Platyhelminthes) or complete (mouth and anus).Circulatory system: open (blood bathes tissues directly) or closed (blood in vessels — arteries, veins, capillaries).4.1.2 Symmetry
Asymmetrical — sponges mostly.Radial — any plane through the central axis gives two identical halves: coelenterates, ctenophores, echinoderms.Bilateral — identical left and right halves in only one plane: annelids, arthropods, etc.4.1.3 Diploblastic and triploblastic
Diploblastic — ectoderm and endoderm with undifferentiated mesoglea between (coelenterates).Triploblastic — third germ layer mesoderm (Platyhelminthes to chordates).4.1.4 Coelom — body cavity lined by mesoderm.
Coelomates — annelids, molluscs, arthropods, echinoderms, hemichordates, chordates.Pseudocoelomates — mesoderm in scattered pouches (aschelminthes).Acoelomates — no body cavity (Platyhelminthes).4.1.5 Segmentation — serial repetition of organs; metameric segmentation / metamerism in earthworm.
4.1.6 Notochord — mesodermally derived rod on the dorsal side during embryonic development; chordates have it, non-chordates (Porifera to Echinoderms) do not.
4.2 Classification of Animals — the phyla
4.2.1 Porifera (sponges) — mostly marine, asymmetrical, cellular level; water canal system: water enters by ostia → spongocoel → out through osculum (food, respiration, waste removal); choanocytes (collar cells); intracellular digestion; skeleton of spicules or spongin fibres; hermaphrodite; internal fertilisation; indirect development with larva. Examples: Sycon (Scypha), Spongilla (fresh water), Euspongia (bath sponge).4.2.2 Coelenterata (Cnidaria) — aquatic, mostly marine, radial; cnidoblasts/cnidocytes with nematocysts for anchorage, defence and prey capture; tissue level, diploblastic; gastro-vascular cavity with single opening (mouth on hypostome); extracellular and intracellular digestion; corals have CaCO₃ skeleton. Two body forms: polyp (sessile, cylindrical — Hydra, Adamsia) and medusa (umbrella-shaped, free-swimming — Aurelia). Metagenesis (alternation of generations) in Obelia. Examples: Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea-pen), Gorgonia (sea-fan), Meandrina (brain coral).4.2.3 Ctenophora (comb jellies, sea walnuts) — exclusively marine, radial, diploblastic, tissue level; eight rows of ciliated comb plates for locomotion; bioluminescence well marked; hermaphrodite; only sexual reproduction; external fertilisation; indirect development. Examples: Pleurobrachia, Ctenoplana.4.2.4 Platyhelminthes (flatworms) — dorso-ventrally flattened; mostly endoparasites; bilateral, triploblastic, acoelomate, organ level; hooks and suckers in parasites; flame cells for osmoregulation and excretion; hermaphrodite; internal fertilisation; Planaria has high regeneration. Examples: Taenia (tapeworm), Fasciola (liver fluke).4.2.5 Aschelminthes (roundworms) — circular in cross-section; organ-system level; bilateral, triploblastic, pseudocoelomate; complete alimentary canal with muscular pharynx; excretory tube and pore; dioecious — females often longer than males; internal fertilisation. Examples: Ascaris, Wuchereria (filaria worm), Ancylostoma (hookworm).4.2.6 Annelida — aquatic or terrestrial; organ-system level, bilateral, triploblastic, metamerically segmented, coelomate (Latin annulus = little ring); longitudinal and circular muscles; Nereis has parapodia; closed circulatory system; nephridia for osmoregulation and excretion; paired ganglia with double ventral nerve cord. Nereis is dioecious; earthworms and leeches monoecious. Examples: Nereis, Pheretima (earthworm), Hirudinaria (blood-sucking leech).4.2.7 Arthropoda — largest phylum; over two-thirds of named species; chitinous exoskeleton; body of head, thorax, abdomen; jointed appendages (arthros joint, poda appendages); respiration by gills, book gills, book lungs or tracheae; open circulation; compound and simple eyes, antennae, statocysts; excretion by malpighian tubules; mostly dioecious and oviparous. Examples: Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect); vectors Anopheles, Culex, Aedes; gregarious pest Locusta; living fossil Limulus (king crab).4.2.8 Mollusca — second largest phylum; unsegmented body with head, muscular foot and visceral hump; calcareous shell; mantle; mantle cavity with feather-like gills (respiratory and excretory); file-like radula for feeding; usually dioecious, oviparous, indirect development. Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea-hare), Dentalium (tusk shell), Chaetopleura (chiton).4.2.9 Echinodermata — endoskeleton of calcareous ossicles (spiny bodied); all marine; adults radial, larvae bilateral; mouth ventral, anus dorsal; water vascular system (locomotion, food capture, respiration); no excretory system; sexes separate; external fertilisation; free-swimming larva. Examples: Asterias (star fish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star).4.2.10 Hemichordata — earlier a sub-phylum of Chordata, now a separate non-chordate phylum; stomochord in collar region; worm-like marine; body of proboscis, collar, trunk; open circulation; gills; excretion by proboscis gland; external fertilisation. Examples: Balanoglossus, Saccoglossus.4.2.11 Chordata — notochord, dorsal hollow nerve cord, paired pharyngeal gill slits; post-anal tail; closed circulation.#### Chordates vs non-chordates (Table 4.1)
Notochord present vs absent.CNS dorsal, hollow, single vs ventral, solid, double.Pharynx with gill slits vs gill slits absent.Heart ventral vs dorsal (if present).Post-anal tail present vs absent.#### Subphyla of Chordata
Urochordata (Tunicata) — notochord only in larval tail: Ascidia, Salpa, Doliolum.Cephalochordata — notochord from head to tail, persistent: Branchiostoma (Amphioxus/lancelet).(Both are protochordates, exclusively marine.)Vertebrata — notochord replaced by vertebral column; ventral muscular heart with 2, 3 or 4 chambers; kidneys; paired fins or limbs. All vertebrates are chordates but all chordates are not vertebrates.Division Agnatha (no jaws) — class Cyclostomata. Division Gnathostomata (jaws) — super class Pisces (Chondrichthyes, Osteichthyes) and Tetrapoda (Amphibia, Reptilia, Aves, Mammals).#### Vertebrate classes
Cyclostomata — ectoparasites on fishes; 6–15 pairs of gill slits; circular sucking mouth without jaws; no scales or paired fins; cartilaginous skeleton; migrate to fresh water to spawn and die. Petromyzon (lamprey), Myxine (hagfish).Chondrichthyes — marine; cartilaginous endoskeleton; mouth ventral; notochord persistent; gill slits without operculum; placoid scales; teeth are modified placoid scales; no air bladder — must swim constantly; two-chambered heart; poikilothermous; males have claspers on pelvic fins; internal fertilisation; many viviparous. Electric organs (Torpedo), poison sting (Trygon). Scoliodon (dog fish), Pristis (saw fish), Carcharodon (great white shark), Trygon (sting ray).Osteichthyes — marine and fresh water; bony endoskeleton; mouth mostly terminal; four pairs of gills with operculum; cycloid/ctenoid scales; air bladder regulates buoyancy; two-chambered heart; cold-blooded; external fertilisation; mostly oviparous, direct development. Exocoetus (flying fish), Hippocampus (sea horse), Labeo (rohu), Catla, Clarias (magur), Betta (fighting fish), Pterophyllum (angel fish).Amphibia — dual life; two pairs of limbs; moist skin without scales; eyelids; tympanum; cloaca; respiration by gills, lungs and skin; three-chambered heart (two auricles, one ventricle); cold-blooded; external fertilisation; oviparous, indirect development. Bufo, Rana, Hyla, Salamandra, Ichthyophis (limbless).Reptilia — creeping; dry cornified skin with scales or scutes; no external ear opening, tympanum present; heart usually three-chambered, four-chambered in crocodiles; poikilotherms; snakes and lizards shed skin; internal fertilisation; oviparous. Chelone, Testudo, Chameleon, Calotes, Crocodilus, Alligator, Hemidactylus; poisonous snakes Naja, Bangarus, Vipera.Aves — feathers; beak; forelimbs modified into wings; oil gland at tail base; pneumatic hollow bones; crop and gizzard; four-chambered heart; warm-blooded (homoiothermous); lungs with air sacs; oviparous. Corvus, Columba, Psittacula, Struthio, Pavo, Aptenodytes, Neophron.Mammalia — mammary glands (most unique feature); hair; external ears (pinnae); different types of teeth; four-chambered heart; homoiothermous; viviparous with few exceptions (oviparous Ornithorhynchus). Macropus, Pteropus, Camelus, Macaca, Rattus, Canis, Felis, Elephas, Equus, Delphinus, Balaenoptera (blue whale), Panthera tigris, Panthera leo.Diagrams to Know
Figure 4.2 — diploblastic (ectoderm, mesoglea, endoderm) vs triploblastic (with mesoderm).Figure 4.3 — coelomate, pseudocoelomate, acoelomate cross-sections.Figure 4.16 — chordate features: notochord, nerve cord, gill slits, post-anal part.Figure 4.7 — cnidoblast.Key-Facts Sheet
Largest phylum: Arthropoda; second largest: Mollusca.Flame cells — Platyhelminthes; nephridia — Annelida; malpighian tubules — Arthropoda; proboscis gland — Hemichordata; no excretory system — Echinodermata.Crocodile — four-chambered heart among reptiles.Segmentation first seen in Annelida.Exam Focus
Q: "All vertebrates are chordates but all chordates are not vertebrates." Justify. Outline: all chordates have a notochord; in vertebrates it is replaced by a vertebral column in adults; urochordates and cephalochordates keep the notochord and have no vertebral column.Q: Modifications in birds for flight. Feathers, forelimbs as wings, hollow pneumatic bones, air sacs supplementing lungs, four-chambered heart, warm-bloodedness.Q: Importance of air bladder in Pisces. Regulates buoyancy in bony fishes; cartilaginous fishes lack it and must swim constantly to avoid sinking.Match (Exercise 14): operculum–Osteichthyes, parapodia–Annelida, scales–Reptilia, comb plates–Ctenophora, radula–Mollusca, hairs–Mammalia, choanocytes–Porifera, gill slits–Cyclostomata and Chondrichthyes.Key Terms
Symmetry (सममिति)Diploblastic (द्विकोरक)Triploblastic (त्रिकोरक)Coelom (देहगुहा)Pseudocoelom (कूटगुहा)Metamerism (विखंडता)Notochord (पृष्ठरज्जु)Choanocytes (कीप कोशिकाएँ)Cnidoblast (दंश कोशिका)Nephridia (वृक्कक)Mantle (प्रावार)Radula (रेतीजिह्वा)Water vascular system (जल संवहन तंत्र)Poikilotherm (असमतापी)Homoiotherm (समतापी)Common Mistakes
Calling echinoderms bilaterally symmetrical as adults — adults are radial, larvae bilateral.Placing Hemichordata inside Chordata — it is now a separate non-chordate phylum.Saying all reptiles have three-chambered hearts — crocodiles have four.Confusing operculum (bony fish) with its absence in cartilaginous fish.Saying sponges have separate sexes — they are hermaphrodite.