In this chapter: levels of organisation, symmetry, germ layers, coelom, segmentation and notochord; the eleven phyla from Porifera to Chordata; and the vertebrate classes.Basis of classification
| Basis | Types | Examples |
|---|---|---|
| Level of organisation | Cellular (loose cell aggregates); tissue; organ; organ system | Sponges; coelenterates; platyhelminthes; annelids to chordates |
| Digestive system | Incomplete (single opening) or complete (mouth and anus) | Incomplete: coelenterates, ctenophores, platyhelminthes; complete: aschelminthes onwards |
| Circulatory system | Open (blood in open spaces/sinuses) or closed (blood in vessels) | Arthropods, molluscs open; annelids, chordates closed |
| Symmetry | Asymmetrical; radial; bilateral | Sponges; coelenterates, ctenophores, adult echinoderms; annelids, arthropods etc. |
| Germ layers | Diploblastic (ectoderm, endoderm, with undifferentiated mesoglea between); triploblastic (with mesoderm) | Coelenterates, ctenophores; platyhelminthes to chordates |
| Coelom (body cavity lined by mesoderm) | Acoelomate; pseudocoelomate (mesoderm in scattered pouches); coelomate | Platyhelminthes; aschelminthes; annelids, molluscs, arthropods, echinoderms, hemichordates, chordates |
| Segmentation | Metameric segmentation (serial repetition of some organs) | Earthworm |
| Notochord | Mesodermal rod on the dorsal side during embryonic development | Chordates have it; non-chordates do not |
Non-chordate phyla
Porifera (sponges)
Mostly marine, asymmetrical, cellular level of organisation. Sponges have a water canal system: water enters through tiny pores (ostia) into the central cavity (spongocoel) and leaves through the osculum. It helps in food gathering, respiration and waste removal. Choanocytes (collar cells) line the spongocoel and canals. Digestion is intracellular. Skeleton of spicules or spongin fibres. Hermaphrodite; asexual reproduction by fragmentation, sexual by gametes; fertilisation internal; development indirect (larval stage). Examples: Sycon (Scypha), Spongilla (freshwater sponge), Euspongia (bath sponge).
Coelenterata (Cnidaria)
Aquatic, mostly marine, sessile or free-swimming, radially symmetrical. Named after cnidoblasts (cnidocytes), stinging cells with nematocysts on tentacles and body, used for anchorage, defence and capturing prey. Tissue level, diploblastic. A central gastro-vascular cavity opens through a single mouth on the hypostome; digestion is extracellular and intracellular. Corals have a skeleton of calcium carbonate. Two body forms: polyp (sessile, cylindrical: Hydra, Adamsia) and medusa (umbrella-shaped, free-swimming: Aurelia). In forms with both (Obelia), polyps produce medusae asexually and medusae form polyps sexually: this is metagenesis. Examples: Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea-pen), Gorgonia (sea-fan), Meandrina (brain coral).
Ctenophora (comb jellies, sea walnuts)
Exclusively marine, radial, diploblastic, tissue level. Eight external rows of ciliated comb plates for locomotion. Digestion extracellular and intracellular. Bioluminescence (emitting light) is well marked. Sexes not separate; external fertilisation; indirect development. Examples: Pleurobrachia, Ctenoplana.
Platyhelminthes (flatworms)
Dorso-ventrally flattened, mostly endoparasites in animals including humans. Bilateral, triploblastic, acoelomate, organ level. Parasites have hooks and suckers; some absorb nutrients directly through the body surface. Flame cells help in osmoregulation and excretion. Sexes not separate; internal fertilisation; many larval stages. Planaria has high regeneration capacity. Examples: Taenia (tapeworm), Fasciola (liver fluke).
Aschelminthes (roundworms)
Circular in cross-section. Free-living, aquatic, terrestrial or parasitic in plants and animals. Organ-system level, bilateral, triploblastic, pseudocoelomate. Complete alimentary canal with a well-developed muscular pharynx; an excretory tube removes wastes through the excretory pore. Sexes separate (dioecious); females often longer than males. Internal fertilisation; development direct or indirect. Examples: Ascaris (roundworm), Wuchereria (filaria worm), Ancylostoma (hookworm).
Annelida
Aquatic or terrestrial; free-living or parasitic. Organ-system level, bilateral, triploblastic, metamerically segmented (hence the name, from annulus, little ring), coelomate. Longitudinal and circular muscles help in locomotion. Aquatic Nereis has lateral appendages called parapodia for swimming. Closed circulatory system. Nephridia for osmoregulation and excretion. Nervous system of paired ganglia connected by lateral nerves to a double ventral nerve cord. Nereis is dioecious; earthworms and leeches are monoecious. Examples: Nereis, Pheretima (earthworm), Hirudinaria (blood-sucking leech).
Arthropoda
Arthropoda is the largest phylum and includes insects; over two-thirds of all named species on Earth are arthropods. Organ-system, bilateral, triploblastic, segmented, coelomate. Chitinous exoskeleton; body of head, thorax and abdomen; jointed appendages (arthros, joint; poda, appendages). Respiration by gills, book gills, book lungs or a tracheal system. Open circulatory system. Sense organs: antennae, compound or simple eyes, statocysts (balance). Excretion by Malpighian tubules. Mostly dioecious, internal fertilisation, mostly oviparous; development direct or indirect.
- Economically important: Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect)
- Vectors: Anopheles, Culex and Aedes (mosquitoes)
- Gregarious pest: Locusta (locust); living fossil: Limulus (king crab)
Mollusca
The second largest phylum. Terrestrial or aquatic; organ-system, bilateral, triploblastic, coelomate. Body unsegmented, with a distinct head, muscular foot and visceral hump; covered by a calcareous shell. A soft, spongy mantle covers the visceral hump; the mantle cavity contains feather-like gills for respiration and excretion. The head has sensory tentacles; the mouth has a file-like rasping organ, the radula. Usually dioecious and oviparous, with indirect development. Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea-hare), Dentalium (tusk shell), Chaetopleura (chiton).
Echinodermata
The name means "spiny-bodied", from their endoskeleton of calcareous ossicles. All marine; organ-system level. Adults are radially symmetrical; larvae are bilateral. Triploblastic, coelomate. Complete digestive system with the mouth on the lower (ventral) side and anus on the upper (dorsal) side. The distinctive water vascular system helps in locomotion, capture and transport of food, and respiration. No excretory system. Sexes separate; external fertilisation; indirect development with free-swimming larvae. Examples: Asterias (starfish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star).
Hemichordata
Once a sub-phylum of chordates, now a separate phylum under non-chordates. Small, worm-like, marine. Organ-system, bilateral, triploblastic, coelomate. Body of proboscis, collar and trunk. A rudimentary structure in the collar region, the stomochord, is similar to a notochord. Open circulation; respiration by gills; excretion by the proboscis gland. Sexes separate; external fertilisation; indirect development. Examples: Balanoglossus, Saccoglossus.
Phylum Chordata
Chordates have three fundamental features: a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. Also bilateral, triploblastic, coelomate, organ-system level, with a post-anal tail and closed circulation.
| Chordates | Non-chordates |
|---|---|
| Notochord present | Notochord absent |
| Central nervous system dorsal, hollow, single | Central nervous system ventral, solid, double |
| Pharynx perforated by gill slits | Gill slits absent |
| Heart ventral | Heart dorsal (if present) |
| Post-anal tail present | Post-anal tail absent |
- Urochordata (Tunicata): notochord only in the larval tail. Ascidia, Salpa, Doliolum.
- Cephalochordata: notochord from head to tail, throughout life. Branchiostoma (Amphioxus, lancelet).
- Vertebrata: notochord in the embryo, replaced by a cartilaginous or bony vertebral column in the adult. So all vertebrates are chordates, but not all chordates are vertebrates. Vertebrates also have a ventral muscular heart with two, three or four chambers, kidneys, and paired fins or limbs.
Vertebrata → Agnatha (no jaws: Cyclostomata) and Gnathostomata (jaws) → Pisces (Chondrichthyes, Osteichthyes) and Tetrapoda (Amphibia, Reptilia, Aves, Mammalia)
The vertebrate classes
| Class | Key features | Examples |
|---|---|---|
| Cyclostomata | Ectoparasites on some fishes. Elongated body, 6 to 15 pairs of gill slits, sucking circular mouth without jaws, no scales or paired fins, cartilaginous cranium and vertebral column, closed circulation. Marine, but migrate to fresh water for spawning and die a few days later; larvae return to the ocean after metamorphosis. | Petromyzon (lamprey), Myxine (hagfish) |
| Chondrichthyes | Marine, streamlined, cartilaginous endoskeleton, ventral mouth, notochord persistent throughout life. Gill slits separate, no operculum. Skin tough, with minute placoid scales; teeth are modified placoid scales pointing backwards. No air bladder, so they must swim constantly to avoid sinking. Two-chambered heart; cold-blooded. Some have electric organs (Torpedo) or a poison sting (Trygon). Males have claspers on pelvic fins; internal fertilisation; many viviparous. | Scoliodon (dogfish), Pristis (sawfish), Carcharodon (great white shark), Trygon (sting ray) |
| Osteichthyes | Marine and freshwater, bony endoskeleton, mostly terminal mouth. Four pairs of gills covered by an operculum. Cycloid or ctenoid scales. Air bladder regulates buoyancy. Two-chambered heart; cold-blooded. Sexes separate; usually external fertilisation; mostly oviparous; direct development. | Marine: Exocoetus (flying fish), Hippocampus (sea horse). Freshwater: Labeo (rohu), Catla, Clarias (magur). Aquarium: Betta (fighting fish), Pterophyllum (angel fish) |
| Amphibia | Live in water and on land. Two pairs of limbs; moist skin without scales; eyelids; a tympanum represents the ear. Alimentary, urinary and reproductive tracts open into a common cloaca. Respiration by gills, lungs and skin. Three-chambered heart (two auricles, one ventricle); cold-blooded. External fertilisation; oviparous; indirect development. | Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander), Ichthyophis (limbless amphibian) |
| Reptilia | Mostly terrestrial; dry, cornified skin with epidermal scales or scutes; no external ear opening (tympanum represents the ear). Heart usually three-chambered, four-chambered in crocodiles. Cold-blooded. Snakes and lizards shed their scales as skin casts. Internal fertilisation; oviparous; direct development. | Chelone (turtle), Testudo (tortoise), Chameleon, Calotes (garden lizard), Crocodilus, Alligator, Hemidactylus (wall lizard). Poisonous snakes: Naja (cobra), Bungarus (krait; NCERT prints it as Bangarus), Vipera (viper) |
| Aves | Feathers; most can fly (except flightless birds like the ostrich). Beak; forelimbs modified into wings; hind limbs with scales. Skin dry, without glands except the oil gland at the base of the tail. Fully ossified endoskeleton; long bones hollow with air cavities (pneumatic). Digestive tract has a crop and gizzard. Four-chambered heart; warm-blooded. Lungs with air sacs. Internal fertilisation; oviparous; direct development. | Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin), Neophron (vulture) |
| Mammalia | Mammary glands; hair on skin; external ears (pinnae); different types of teeth in the jaw. Four-chambered heart; warm-blooded; lungs. Internal fertilisation; viviparous with a few exceptions; direct development. | Oviparous: Ornithorhynchus (platypus). Viviparous: Macropus (kangaroo), Pteropus (flying fox), Camelus, Macaca (monkey), Rattus, Canis, Felis, Elephas, Equus, Delphinus (dolphin), Balaenoptera (blue whale), Panthera tigris, Panthera leo |
Common traps: (1) Crocodiles are reptiles with a four-chambered heart. (2) The platypus is an egg-laying mammal. (3) Cartilaginous fish have no air bladder; bony fish do. (4) Echinoderm larvae are bilateral even though adults are radial. (5) Hemichordates are non-chordates.NEET focus
- Basis of classification applied to each phylum (symmetry, germ layers, coelom).
- Unique features: choanocytes, cnidoblasts, comb plates, flame cells, parapodia, Malpighian tubules, radula, water vascular system.
- Examples with their common names, which is where most of the marks in this chapter come from.
- Heart chambers, cold vs warm blooded, and fertilisation type in vertebrate classes.
Practice questions
Choanocytes are characteristic of:
- Coelenterates
- Sponges
- Ctenophores
- Flatworms
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Which animal shows metagenesis?
- Hydra
- Aurelia
- Obelia
- Adamsia
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Pseudocoelom is found in:
- Fasciola
- Ascaris
- Pheretima
- Pila
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The water vascular system is a feature of:
- Mollusca
- Echinodermata
- Hemichordata
- Porifera
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In which of these is the notochord present only in the larval tail?
- Branchiostoma
- Ascidia
- Balanoglossus
- Petromyzon
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Which reptile has a four-chambered heart?
- Calotes
- Chelone
- Crocodilus
- Naja
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Placoid scales and claspers are found in:
- Labeo
- Scoliodon
- Hippocampus
- Exocoetus
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The living fossil among arthropods is:
- Locusta
- Laccifer
- Limulus
- Bombyx