Class 11 Biology Notes
The Animal Kingdom chapter classifies the animalia into major phyla based on fundamental body-plan features — levels of organisation, symmetry, germ layers, coelom, segmentation and presence or absence of a notochord. From the simplest sponges (Porifera) to the most complex chordates, each group has defining characteristics that make classification both logical and exam-relevant.
Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali
Animals are classified by levels of organisation (cellular → tissue → organ → organ-system), symmetry (radial vs bilateral), germ layers (diploblastic vs triploblastic), coelom (acoelomate/pseudocoelomate/coelomate), segmentation, and presence of a notochord.
Animals are classified based on several fundamental features of their body plan. These criteria form the foundation for placing organisms into the correct phylum.
Animals show four levels: Cellular (e.g. sponges — cells are loosely arranged with no tissues), Tissue (e.g. Cnidarians — cells form tissues like nerve and muscle), Organ (e.g. flatworms — tissues form organs), and Organ-system (e.g. vertebrates — organs form functional systems like digestive, circulatory, nervous).
Radial symmetry — the body can be divided into two equal halves by any plane through the central axis (e.g. Cnidarians, Echinoderms in adults). Bilateral symmetry — the body can be divided into two equal halves by only one plane (e.g. flatworms, arthropods, chordates). Asymmetry — no plane of symmetry (e.g. sponges).
Diploblastic animals have two germ layers — ectoderm and endoderm — with a non-cellular mesoglea between them (e.g. Cnidarians). Triploblastic animals have three germ layers — ectoderm, mesoderm and endoderm — the mesoderm gives rise to all body organs. All animals from Platyhelminthes onwards are triploblastic.
The coelom is a body cavity derived from mesoderm, lined by mesodermal cells on both sides. It acts as a shock absorber, provides space for organ development, and helps in body movement. Based on the coelom, triploblastic animals are: Acoelomate (no body cavity, e.g. Platyhelminthes), Pseudocoelomate (body cavity not completely lined by mesoderm, e.g. Aschelminthes/Nematoda), and Coelomate (true coelom, e.g. Annelida, Arthropoda, Mollusca, Echinodermata, Chordata).
Porifera means 'pore-bearing'. Sponges are the simplest multicellular animals with a cellular level of organisation. Their body is perforated by pores (ostia) through which water enters; water exits through the osculum. They have a water canal system (choanocytes/ collar cells line the canals and create water current). Sponges have a skeletal framework of spicules or spongin fibres. They are mostly hermaphrodite and reproduce asexually by fragmentation and sexually by gametes.
Cnidarians have a tissue level of organisation and radial symmetry. Their body wall has two layers — ectoderm and endoderm — separated by mesoglea (diploblastic). The body form is either polyp (sessile, e.g. Hydra, Adamsia) or medusa (free-swimming, e.g. Aurelia/jellyfish). Many show alternation of polyp and medusa (metagenesis). They have cnidoblasts (stinging cells) on tentacles for prey capture and defence. The digestive cavity is the gastrovascular cavity with a single opening (mouth/anus).
Ctenophores are diploblastic, radially symmetric animals with tissue level of organisation. They bear eight rows of ciliary comb plates for locomotion. They are marine and bioluminescent. Examples: Pleurobrachia, Ctenoplana. Their relationship to Cnidaria is debated — they lack cnidoblasts.
Platyhelminthes are triploblastic, bilaterally symmetric, acoelomate animals. Their body is dorso-ventrally flattened. They are either free-living (e.g. Planaria/Dugesia) or parasitic (e.g. Taenia/tapeworm, Fasciola/liver fluke). Parasitic forms show loss of sensory organs and digestive system. They reproduce sexually (monoecious/hermaphrodite) and have flame cells (protonephridia) for excretion and osmoregulation.
Aschelminthes are triploblastic, bilaterally symmetric, pseudocoelomate animals. Their body is cylindrical and unsegmented. They have a complete digestive system (mouth and anus). Examples: Ascaris (intestinal roundworm), Enterobius (pinworm), Wuchereria (filarial worm causing elephantiasis). females are larger than males (sexual dimorphism). Excretory organ is a renette cell or lateral excretory canal.
Annelids are triploblastic, bilaterally symmetric, coelomate animals with true segmentation (metameric body plan). They have a well-developed closed circulatory system and nephridia for excretion. Examples: earthworm (Pheretima — hermaphrodite), leech (Hirudinaria — blood-sucking ectoparasite, monoecious), Nereis (marine, dioecious, with parapodia for swimming).
Arthropoda is the largest phylum of animals — it includes insects, crustaceans, arachnids and myriapods. They are triploblastic, bilaterally symmetric, coelomate and segmented. Their body is divided into head, thorax and abdomen (insects) or cephalothorax and abdomen (arachnids). The exoskeleton is made of chitin. Locomotion is by jointed appendages. Examples: cockroach (Periplaneta — tri-jointed legs, compound eyes), butterfly, mosquito, prawn, scorpion, spider.
Mollusca is the second-largest animal phylum. They are triploblastic, bilaterally symmetric, coelomate with an organ-system level of organisation. The body has a head, muscular foot, visceral mass and a calcareous shell (in most). A rasping organ called the radula is used for feeding. They respire by gills (ctenidia). Examples: Pila (apple snail), Unio (freshwater mussel), Octopus (devil fish — shell is reduced), Pinctada (pearl oyster), Loligo (squid), Chaetopleura (chiton).
Echinoderms are triploblastic, coelomate, marine animals with radial symmetry in adults (pentameral — five-part) but bilateral symmetry in larvae. They have a unique water vascular system used for locomotion, feeding and gas exchange — tube feet (ampullae and podium) are part of this system. Endoskeleton is made of calcareous plates (ossicles). Examples: Asterias (starfish), Antedon (sea lily), Echinus (sea urchin), Holothuria (sea cucumber).
Hemichordates are worm-like marine animals with a partially developed notochord called a stomochord (in the collar region). They are deuterostomes, triploblastic, coelomate, and bilaterally symmetric. They have pharyngeal gill slits and a curved proboscis. Example: Balanoglossus (acorn worm), Saccoglossus. They were originally placed with Chordata but are now considered a separate phylum close to Echinodermata.
Chordates are defined by the presence of a notochord at some stage of life. They also have a dorsal hollow nerve cord, pharyngeal gill slits and a post-anal tail. Chordata is divided into three subphyla: Urochordata (notochord only in larval tail — e.g. Ascidia/Herdmania), Cephalochordata (notochord extends throughout life — e.g. Branchiostoma/Amphioxus), and Vertebrata (notochord replaced by vertebral column).
Remember
Chordates are characterised by four features: notochord, dorsal hollow nerve cord, pharyngeal gill slits and post-anal tail. All vertebrates are chordates, but not all chordates are vertebrates.
Example: An animal is triploblastic, acoelomate, bilaterally symmetric and parasitic. Identify its phylum and give two examples.
Solution: The animal belongs to Phylum Platyhelminthes (flatworms). Key identifiers: triploblastic + acoelomate + bilateral symmetry. Examples include Taenia solium (tapeworm, parasitic in human intestine) and Fasciola hepatica (liver fluke, parasitic in sheep liver).
Example: Why is Arthropoda the largest phylum of the animal kingdom? List four features that contribute to its success.
Solution: Arthropoda is the largest phylum (over 10 lakh described species) because of: (1) jointed appendages for diverse locomotion — walking, swimming, flying; (2) chitinous exoskeleton providing protection and preventing water loss; (3) segmental body plan allowing regional specialisation; and (4) ability to occupy almost every habitat — terrestrial, aquatic and aerial.
Revision
Memorise these before attempting numericals — most exam questions hinge on one of them.
Symmetry types
Germ layers
Coelom classification
Chordate features
Phylum progression
Body cavity types
Vertebrate classes
Exam tips
Where this topic appears in CBSE, JEE Main and NEET papers.
FAQ
Diploblastic animals have two germ layers — ectoderm and endoderm — with a non-cellular mesoglea between them (e.g. Cnidarians). Triploblastic animals have three germ layers — ectoderm, mesoderm and endoderm — and the mesoderm gives rise to all body organs. All triploblastic animals are from Platyhelminthes onwards.
The coelom is a fluid-filled body cavity derived from mesoderm and lined by mesodermal cells. It protects internal organs, allows independent organ movement, acts as a hydrostatic skeleton, and provides space for organ development. Animals with a true coelom are called coelomates (e.g. Annelida, Arthropoda, Chordata).
Arthropoda is the largest phylum, with over 10 lakh described species. It includes insects, crustaceans, arachnids and myriapods. Its success is attributed to jointed appendages, a chitinous exoskeleton, segmentation and ability to colonise almost every habitat.
All chordates possess four key features at some stage of their life: (1) a notochord, (2) a dorsal hollow nerve cord, (3) pharyngeal gill slits, and (4) a post-anal tail. In vertebrates, the notochord is replaced by the vertebral column during development.
Continue learning
Test yourself
Exam-style questions for this chapter — no login required. Submit to see your score instantly.
Check how much of this chapter you have actually locked in — exam-style questions with instant scoring.
Notes help, but doubts clear fastest in a live class. Narayan Gurukul Academy (ClassApna) runs small-batch CBSE, JEE and NEET coaching from our Mohali centre and online — with daily doubt support and mock tests.
One-on-one guidance available · Live online classes across India