All animals are eukaryotic and multicellular organisms that are characterized by their nutritional habits. The animal kingdom is usually subdivided into groups designed to reflect the evolutionary relationships of major lineages. According to the system of classification of R.H. Whittaker (1961) there are 26 animal phyla. As per animal diversity each phylum is composed of animals with a similar overall body plan and apparently a common predecessor.
Sponges (phylum Porifera) are generally placed in a separate subkingdom, known as Parazoa, while all other animals, evidently derived from a separate protistan modification, form the subkingdom Eumetazoa. Parazoa lack tissue organization, and their cells are the primary units of structure and function.
Above the level of sponges there are two lines of animal diversity (Radiata and Bilateria) on the basis of body symmetry, whether radial or bilateral. Radiata display a tissue level of organization and consists of two phyla— Cnidaria and Ctenophora. The remaining members of the animal diversity are included in Bilateria. All bilateral forms are triploblastic, possessing a third primary germ layer known as mesoderm.
The division Bilateria has three sections : Acoelomata, Pseudocoelomata and Eucoelomata. Flatworms (Platyhelminthes) and proboscis worms (Rhynchocoela) belong to the section Acoelomata. Platyhelminthes are the simplest of the triploblastic animals, but they exemplify an organ level of complexity compared with the tissue level shown by cnidarians and Ctenophores.
The remaining members of the animal kingdom are included in Bilateria. All bilateral forms are triploblastic, possessing a third primary germ layer known as mesoderm. The division Bilateria has three sections : Acoelomata (without coelom), Pseudocoelomata (with a pseudocoelom) and Eucoelomata (with a true coelom). Flatworms (Platyhelminthes) and proboscis worms (Rhynchocoela) belong to the section Acoelomata.
Rotifera and Nematoda are included in the section Pseudocoelomata of animal diversity. Both of these phyla possess a pseudocoelom. Nematodes possess characteristic tapered ends and are the most numerous of all phyla in terms of actual numbers of individuals. Rotifers are tiny and, despite their multicellularity, they may be smaller than some large amoebas. Their name (wheel carrier) comes from the ridge of beating cilia (corona) which surrounds the mouth. Other phyla of tiny pseudocoelomates include hermaphroditic Gastrotricha and spiny Kinorhyncha, which are virtually trapped within the external layer of cuticle.
The coelomate Bilateria have two main branches—Protostomia and Deuterustomia—based on variances in the developing embryo. In protostomes, the blastopore forms the mouth, and the anus is formed secondarily, while in deuterostomes, the blastopore forms the anus, and the mouth is formed secondarily.
Moreover, protostomes demonstrate spiral, determinate cleavage, whereas deuterostomes tend toward radial, indeterminate cleavage. Protostomes and deuterostomes also differ in the origin of mesoderm and in the formation of coelom cavity. Annelida, Mollusca, and Arthropoda are the three major protostome phyla which probably share a common evolutionary animal diversity. Arthropods and most of the molluscs have an open circulatory system in which large spaces (haemocoels) stand between arteries and veins. Annelids possess a closed circulatory system in which enclosed vessels (arteries, capillaries, and veins) ramify throughout the transport system. Echinodermata and Chordata are the two major phyla of the deuterostome group. Chordata consists of three subphyla: (i) Urochordata (tunicates), which occur in a benthic habitat (ii)Ephalochordata, and (iii) Vertebrata.
The subphylum Vertebrata is typically divided into two groups—Agnatha and Gnathostomata Agnatha are jawless fish that were once numerous but now survived only by hagfish and lampreys Gnathostomata (with true jaws) are further divided into two super classes—Pisces and Tetrapoda. The superclass Pisces includes two living classes of fish—Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish).
The superclass Tetrapoda (with two pairs of limbs) includes four classes of animal diversity—Amphibia, Reptilia, Aves and Mammalia of vertebrates. Amphibia (frogs, toads, and salamanders) are only partially successful as terrestrial inhabitants. Reptilia (lizards and snakes, turtles and tortoises, alligators and crocodiles) have scales on a rough waterproof integument that shields against drying out. Other modifications of reptiles for successful terrestrial life include functional lungs, an enclosed egg in which the embryo can absorb yolk well protected by a leathery shell; internal fertilization; and a variety of behavioral patterns that enable them to survive extremes of temperature and shortages of food.
The class Mammalia shares with insects an extreme mastery of the terrestrial habitat. Mammals have hair (fur), which afford an efficient mechanism for insulation. The noticeable characteristic of all mammals is the ability of the mother to produce nourishing milk for the young in mammary glands. The very primitive mammals known as monotremes do not have nipples on breasts, but they do produce milk, which runs down the ventral hairy surface and is lapped up by the young. The class Mammalia is usually divided into three subclasses— Protheria, Metatheria, and Eutheria.
The subclass of animal diversity Protheria are rather avian in character. They lay eggs, lack teeth, and possess a beak. Their young hatch from eggs at a comparatively early stage and cling to the hair of the mother. The mammalian subclass Metatheria consists of marsupials or pouched animals. They are mammalian in their possession of teeth rather than a beak, and they give birth to live young. But the young are borne at an early stage in their development and complete their fetal life within a pouch on the underside of the female body. The members of the subclass Eutheria are 'true' or placental mammals. In this case a placenta is formed within the uterus, which is a major metabolic depot for exchange of food, oxygen, and waste between the mother and fetus.