NEET Biology Notes Animal Kingdom
Animals are believed to have been evolved from ancestral protists by a process of division of labour among cells. The classification of animals is based on organisation, of the body coelom, symmetry, habitat, germ layers, etc.
Coelom refers to a large fluid filled space lying between outer body wall and inner digestive tube.
The organisms may be acoelous (Porifera, Coelenterata and Platyhelminthes), pseudocoelous (Aschelminthes and Nematoda) or eucoelous (Annelida, Echinoderms and Chordata).
Classification of Animals
The aim of animal systematics is to arrange animals into groups that reflect evolutionary relationship. The group that originated from a single ancestral species and include all of its descendants, such a group is called a monophyletic group. In searching out monophyletic groups, taxonomists look for animal attributes called characters that indicate relatedness. A character is virtually based on those attributes that has a genetic basis and can be measured from an anatomical analysis of a sequence of nitrogenous bases in DNA or RNA.
Members of polyphyletic groups are originated from different ancestors. Since, each group should have a single ancestor, a polyphyletic group reflects insufficient knowledge of the group.
A paraphyletic group includes some, but not all, members of a lineage. Paraphyletic groups results when knowledge of the group is insufficient. Symmetry refers to distribution of body parts in accordance with body axis. It may be spherical (Volvox, Helizera and Radiolaria), radial (Porifera, Coelenterata and echinoderms), biradial (Ctenophora) and bilateral.
Animal kingdom is divided into two sub-kingdoms.
i.e. Protozoa and Metazoa.
(i) Protozoa are defined as single-celled eukaryotic organisms that feed heterotrophically and exhibit diverse motility mechanisms.
(ii) Metazoa are multicellular eukaryotes, which are further divided into three branches, i.e. Mesozoa, Parazoa and Eumetazoa.
