Learning Objectives
- Understand the basis of animal classification
- Learn the distinguishing features of each animal phylum
- Know the classification from Porifera to Chordata
- Differentiate between vertebrate classes
Key Concepts
Basis of Classification
Levels of Organization: Cellular (Porifera), Tissue (Coelenterata), Organ (Platyhelminthes), Organ-system (Aschelminthes onwards). Symmetry: Asymmetrical (sponges), Radial (Coelenterata, Ctenophora), Bilateral (most animals). Germ Layers: Diploblastic (two layers โ ectoderm, endoderm; e.g., Coelenterata) or Triploblastic (three layers โ ectoderm, mesoderm, endoderm; Platyhelminthes onwards).
Coelom: Body cavity lined by mesoderm. Coelomates (Annelida onwards), Pseudocoelomates (body cavity not lined by mesoderm; Aschelminthes), Acoelomates (no body cavity; Platyhelminthes).
Notochord: Chordates possess it; Non-chordates lack it. Segmentation: Body divided into metameres (Annelida, Arthropoda).
Non-Chordate Phyla
Porifera (Sponges): Cellular organization, canal system (water transport), choanocytes (collar cells), spicules/spongin for skeleton. Examples: Sycon, Euspongia (bath sponge). Hermaphrodite, high regeneration capacity.
Coelenterata (Cnidaria): Tissue level, radial symmetry, diploblastic, cnidoblasts (stinging cells). Polyp (sessile, e.g., Hydra) and Medusa (free-swimming, e.g., jellyfish) forms. Examples: Hydra, Obelia, Aurelia, corals.
Platyhelminthes (Flatworms): Organ level, bilateral, triploblastic, acoelomate. Flame cells for excretion. Examples: Taenia (tapeworm), Fasciola (liver fluke), Planaria.
Aschelminthes (Roundworms): Pseudocoelomate, complete digestive tract. Examples: Ascaris, Wuchereria (filariasis), Ancylostoma (hookworm).
Annelida (Segmented worms): Coelomate, metameric segmentation, closed circulatory system. Examples: Nereis, Pheretima (earthworm), Hirudinaria (leech).
Arthropoda (Largest phylum): Jointed appendages, chitinous exoskeleton, open circulatory system, Malpighian tubules for excretion. Examples: Apis (bee), Bombyx (silkworm), Limulus (king crab โ living fossil).
Mollusca (Second largest): Soft-bodied, mantle, radula (rasping organ), open circulation (except cephalopods). Examples: Pila (snail), Octopus, Loligo (squid), Pinctada (pearl oyster).
Echinodermata: Spiny skin, water vascular system, tube feet for locomotion, exclusively marine. Examples: Asterias (starfish), Echinus (sea urchin), Cucumaria (sea cucumber).
Hemichordata: Worm-like marine organisms with stomochord (not true notochord). Example: Balanoglossus, Saccoglossus.
Phylum Chordata
Characters: Notochord, dorsal hollow nerve cord, paired pharyngeal gill slits at some stage. Divided into Urochordata (Ascidia, Salpa), Cephalochordata (Branchiostoma/Amphioxus), and Vertebrata (notochord replaced by vertebral column).
Vertebrate Classes: Cyclostomata (jawless, parasitic โ lamprey, hagfish), Chondrichthyes (cartilaginous fish โ shark, ray, torpedo), Osteichthyes (bony fish โ rohu, catla, seahorse), Amphibia (dual life โ frog, toad, salamander), Reptilia (creeping, three-chambered heart except crocodile which has four โ lizard, snake, turtle, crocodile), Aves (feathers, flight, four-chambered heart, warm-blooded โ crow, pigeon, ostrich), Mammalia (hair, mammary glands, four-chambered heart โ human, whale, bat, platypus โ egg-laying mammal).
Summary
Animals are classified based on body symmetry, germ layers, coelom, segmentation, and notochord. Non-chordates range from Porifera to Hemichordata. Chordates possess a notochord, dorsal nerve cord, and gill slits. Vertebrates are classified into Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia.
Important Terms
- Coelom: Body cavity lined by mesoderm
- Notochord: Flexible rod-like structure for support in chordates
- Metameric segmentation: Body divided into repeated segments (metameres)
- Cnidoblasts: Stinging cells unique to Cnidaria
- Water vascular system: Unique to echinoderms; used for locomotion
- Choanocytes: Collar cells in sponges for water circulation
- Radula: Rasping organ in molluscs for feeding
- Malpighian tubules: Excretory organs in arthropods
Quick Revision
- Arthropoda = largest phylum; Mollusca = second largest
- Porifera: cellular level; Coelenterata: tissue level
- Acoelomate: Platyhelminthes; Pseudocoelomate: Aschelminthes; Coelomate: Annelida onwards
- Diploblastic: Coelenterata; Triploblastic: Platyhelminthes onwards
- Chordate features: notochord + dorsal nerve cord + gill slits
- Crocodile = only reptile with 4-chambered heart
- Platypus and Echidna = egg-laying mammals (Prototheria)
- Limulus = living fossil (king crab)