Animal Kingdom — Class 11 Biology

This chapter explains how the animal kingdom is classified using fundamental features shared across diverse forms, then surveys the major phyla. The basis of classification includes levels of organisation (cellular, tissue, organ and organ-system), body symmetry (asymmetrical, radial or bilateral), the number of germ layers (diploblastic or triploblastic), the nature of the body cavity or coelom (coelomate, pseudocoelomate or acoelomate), segmentation and the presence of a notochord. Using these, the chapter describes Porifera (sponges with choanocytes), Coelenterata or Cnidaria (with cnidoblasts and polyp and medusa forms), Ctenophora (comb jellies), Platyhelminthes (flatworms), Aschelminthes (roundworms), Annelida (segmented worms), Arthropoda (the largest phylum, with jointed appendages), Mollusca (soft-bodied with a calcareous shell), Echinodermata (spiny-skinned with a water vascular system) and Hemichordata. Phylum Chordata is defined by a notochord, dorsal hollow nerve cord and pharyngeal gill slits; it includes protochordates and the vertebrates, which are further divided into Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia. The chapter notes which classes are cold-blooded and which are warm-blooded, and contrasts oviparous and viviparous reproduction. A key point is that all vertebrates are chordates but not all chordates are vertebrates.