Chapter-4: Animal Kingdom

Basis of Classification

Although animals share various common characteristics, they differ in the structure of body organs and other systems. These patterns are used as the basis of animal classification. Understanding these fundamental features helps in placing organisms into proper taxonomic categories.

Levels of Organization

Even though all animals are multicellular, they do not exhibit the same pattern of organization of cells.

• Cellular level of organization: Cells are arranged as loose cell aggregates. Division of labour is rudimentary among cells. Example: Porifera.
• Tissue level of organization: Cells performing the same function are arranged into tissues. Example: Coelenterates.
• Organ level of organization: Tissues are grouped together to form organs, each specialized for a particular function. Example: Platyhelminthes.
• Organ system level of organization: Organs have associated to form functional systems, where each system concerns a specific physiological function. Example: Annelids, Arthropods, Molluscs, Echinoderms, and Chordates.

Exam-Oriented Note: Note the complexity jump from cellular aggregates in sponges to organ system levels in higher phyla.

Symmetry

Animals can be categorized on the basis of their symmetry.

• Asymmetrical: Any plane that passes through the centre does not divide the animals into equal halves. Example: Sponges.
• Radial symmetry: Any plane passing through the central axis of the body divides the organism into two identical halves. Example: Coelenterates, ctenophores, and adult echinoderms.
• Bilateral symmetry: The body can be divided into identical left and right halves in only one plane. Example: Annelids, arthropods, etc.

Diploblastic and Triploblastic Organisation

Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals. An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm (Example: Porifera, Coelenterates).

Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals (Example: Platyhelminthes to Chordates).

Coelom

The presence or absence of a cavity between the body wall and gut wall is very important in classification. The body cavity, which is lined by mesoderm, is called coelom.

• Coelomates: Animals possessing coelom. Example: Annelids, molluscs, arthropods, echinoderms, hemichordates, and chordates.
• Pseudocoelomates: In these animals, the mesoderm is present as scattered pouches in between the ectoderm and endoderm, so the body cavity is not lined by mesoderm. Example: Aschelminthes.
• Acoelomates: Animals in which the body cavity is absent. Example: Platyhelminthes.

Segmentation

In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs. For example, in earthworms, the body shows metameric segmentation, and the phenomenon is known as metamerism.

Notochord

Notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals. Animals with notochord are called chordates and those animals which do not form this structure are called non-chordates (e.g., Porifera to Echinoderms).

Classification of Animals

The broad classification of Animal Kingdom is based on common fundamental features like notochord level, levels of organization, symmetry, coelom, and segmentation. The major groups include non-chordates and chordates.

Non-Chordates (Upto Phyla Level)

Non-chordates are animals without a notochord. They are classified into several phyla ranging from primitive sponges to advanced echinoderms.

Phylum - Porifera (Sponges)

Generally known as sponges. They are mostly marine and asymmetrical animals. They have a cellular level of organization and a water transport or canal system. Water enters through minute pores (ostia) in the body wall into a central cavity, spongocoel, from where it goes out through the osculum.

• Salient Features: Water transport system helps in food gathering, respiratory exchange, and removal of waste. Digestion is intracellular. Body supported by a skeleton made up of spicules or spongin fibres. Sexes are not separate (hermaphrodite).
• Examples: Sycon (Scypha), Spongilla (Freshwater sponge), Euspongia (Bath sponge).

Phylum - Coelenterata (Cnidaria)

They are aquatic, mostly marine, radially symmetrical, sessile or free-swimming animals. The name cnidaria is derived from the stinging cells or cnidoblasts present on the tentacles and the body.

• Salient Features: Cnidoblasts are used for anchorage, defense, and for capturing prey. They exhibit tissue level of organization and are diploblastic. Some cnidarians exist in two forms: polyp (sessile and cylindrical like Hydra) and medusa (umbrella-shaped and free-swimming like Aurelia). Alternation of generation between forms is called metagenesis.
• Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), Meandrina (Brain coral).

Phylum - Ctenophora (Comb Jellies)

Exclusively marine, radially symmetrical, diploblastic organisms with tissue level of organization. The body bears eight external rows of ciliated comb plates which help in locomotion.

• Salient Features: Digestion is both extracellular and intracellular. Bioluminescence is well-marked in ctenophores. Sexes are not separate. Reproduction takes place only by sexual means. Fertilization is external with indirect development.
• Examples: Pleurobrachia and Ctenoplana.

Phylum - Platyhelminthes (Flatworms)

They have dorso-ventrally flattened body, hence are called flatworms. Mostly endoparasites found in animals including human beings.

• Salient Features: Bilaterally symmetrical, triploblastic and acoelomate animals with organ level of organization. Hooks and suckers are present in the parasitic forms. Flame cells help in osmoregulation and excretion. Sexes are not separate. Fertilization is internal and development is through many larval stages.
• Examples: Taenia (Tapeworm), Fasciola (Liver fluke), Planaria (high regeneration capacity).

Phylum - Aschelminthes (Roundworms)

The body of the aschelminthes is circular in cross-section, hence the common name roundworms. They may be free-living, aquatic and terrestrial or parasitic in plants and animals.

• Salient Features: Bilaterally symmetrical, triploblastic, and pseudocoelomate animals. Alimentary canal is complete with a well-developed muscular pharynx. An excretory tube removes body wastes through the excretory pore. Sexes are separate (dioecious). Females are longer than males. Fertilization is internal.
  • Examples: Ascaris (Roundworm), Wuchereria (Filarial worm), Ancylostoma (Hookworm).

  Phylum - Annelida (Segmented Worms)

  They may be aquatic (marine and fresh water) or terrestrial; free-living and sometimes parasitic. They exhibit organ-system level of body organization and bilateral symmetry.

  • Salient Features: Triploblastic, metamerically segmented and coelomate animals. Their body surface is distinctly marked out into segments or metameres. Aquatic forms like Nereis possess lateral appendages, parapodia, which help in swimming. Closed circulatory system is present. Nephridia help in osmoregulation and excretion. Neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
  • Examples: Nereis, Pheretima (Earthworm), Hirudinaria (Blood sucking leech).

  Phylum - Arthropoda

  This is the largest phylum of Animal Kingdom which includes insects. Over two-thirds of all named species on earth are arthropods.

  • Salient Features: They have organ-system level of organization. Bilaterally symmetrical, triploblastic, segmented and coelomate animals. Body is covered by chitinous exoskeleton. Body consists of head, thorax and abdomen. Have jointed appendages. Respiratory organs are gills, book gills, book lungs or tracheal system. Circulatory system is of open type. Excretory organs are Malpighian tubules. Sense organs like antennae, eyes (compound and simple), statocysts or balancing organs are present.
  • Examples: Economically important insects (Apis, Bombyx, Laccifer), Vectors (Anopheles, Culex, Aedes), Living fossil (Limulus).

  Phylum - Mollusca

  Mollusca is the second largest animal phylum. Molluscs are terrestrial or aquatic (marine or fresh water) having an organ-system level of organization.

  • Salient Features: Bilaterally symmetrical, triploblastic and coelomate animals. Unsegmented body with a distinct head, muscular foot and visceral hump. A soft and spongy layer of skin forms a mantle over the visceral hump. Space between the hump and the mantle is called the mantle cavity in which feather-like gills are present. Anterior head region has sensory tentacles. Mouth contains a file-like rasping organ for feeding, called radula.
  • Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Sea hare), Dentalium (Tusk shell), Chaetopleura (Chiton).

  Phylum - Echinodermata (Spiny-skinned Animals)

  These animals have an endoskeleton of calcareous ossicles and, hence, the name Echinodermata. All are marine with organ-system level of organization.

  • Salient Features: Adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical. Triploblastic and coelomate animals. Digestive system is complete with mouth on the lower (ventral) side and anus on the upper (dorsal) side. Most distinctive feature is the presence of water vascular system which helps in locomotion, capture and transport of food and respiration. An excretory system is absent.
  • Examples: Asterias (Starfish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber), Ophiura (Brittle star).

  Phylum - Hemichordata

  Hemichordata was earlier considered as a sub-phylum under phylum Chordata. But now it is placed as a separate phylum under non-chordates.

  • Salient Features: Small group of worm-like marine animals with organ-system level of organization. Bilaterally symmetrical, triploblastic and coelomate animals. Body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk. Circulatory system is of open type. Respiration takes place through gills. Excretory organ is proboscis gland.
  • Examples: Balanoglossus and Saccoglossus.

  Chordates (Upto Class Level)

  Animals belonging to phylum Chordata are fundamentally characterized by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits.

  Comparison: Chordates vs Non-Chordates

  Chordates	Non-Chordates
  Notochord present	Notochord absent
  Central nervous system is dorsal, hollow and single	Central nervous system is ventral, solid and double
  Pharynx perforated by gill slits	Gill slits are absent
  Heart is ventral	Heart is dorsal (if present)
  Post-anal tail present	Post-anal tail absent

  Subphyla of Chordata

  Phylum Chordata is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata.

  • Urochordata (Tunicata): Notochord is present only in larval tail. Examples: Ascidia, Salpa, Doliolum.
  • Cephalochordata: Notochord extends from head to tail region and is persistent throughout life. Example: Branchiostoma (Amphioxus or Lancelet).
  • Vertebrata: Possess notochord during the embryonic period. The notochord is replaced by a cartilaginous or bony vertebral column in the adult. Thus all vertebrates are chordates but all chordates are not vertebrates.

  Classes of Subphylum Vertebrata

  Subphylum Vertebrata is further divided into two divisions: Agnathans (lacks jaw) and Gnathostomata (bears jaws).

  Class - Cyclostomata

  All living members of the class Cyclostomata are ectoparasites on some fishes.

  • Salient Features: Have an elongated body bearing 6-15 pairs of gill slits for respiration. Sucking and circular mouth without jaws. Cranium and vertebral column are cartilaginous. Circulation is closed type. Cyclostomes are marine but migrate for spawning to fresh water. After spawning, within a few days, they die. Their larvae, after metamorphosis, return to the ocean.
  • Examples: Petromyzon (Lamprey) and Myxine (Hagfish).

  Class - Chondrichthyes (Cartilaginous Fishes)

  They are marine animals with streamlined body and have cartilaginous endoskeleton.

  • Salient Features: Mouth is located ventrally. Notochord is persistent throughout life. Gill slits are separate and without operculum (gill cover). Skin is tough, containing minute placoid scales. Teeth are modified placoid scales which are backwardly directed. Jaws are very powerful. Due to lack of air bladder, they have to swim constantly to avoid sinking. Heart is two-chambered (one auricle and one ventricle). Some of them have electric organs (e.g., Torpedo) and some possess poison sting (e.g., Trygon). They are cold-blooded (poikilothermous). Sexes are separate. In males pelvic fins bear claspers. Fertilization is internal. Many of them are viviparous.
  • Examples: Scoliodon (Dogfish), Pristis (Sawfish), Carcharodon (Great white shark), Trygon (Sting ray).

  Class - Osteichthyes (Bony Fishes)

  It includes both marine and fresh water fishes with bony endoskeleton.

  • Salient Features: Body is streamlined. Mouth is mostly terminal. Four pairs of gill slits are covered by an operculum on each side. Skin is covered with cycloid/ctenoid scales. Air bladder is present which regulates buoyancy. Heart is two-chambered (one auricle and one ventricle). Cold-blooded animals. Sexes are separate. Fertilization is usually external. Mostly oviparous and development is direct.
  • Examples: Marine - Exocoetus (Flying fish), Hippocampus (Sea horse); Freshwater - Labeo (Rohu), Catla (Katla), Clarias (Magur); Aquarium - Betta (Fighting fish), Pterophyllum (Angel fish).

  Class - Amphibia

  As the name indicates (Amphi: dual, bios: life), amphibians can live in aquatic as well as terrestrial habitats.

  • Salient Features: Most of them have two pairs of limbs. Body is divisible into head and trunk. Tail may be present in some. Amphibian skin is moist (without scales). Eyes have eyelids. Timpanum represents the ear. Alimentary canal, urinary and reproductive tracts open into a common chamber called cloaca which opens to the exterior. Respiration is by gills, lungs and through skin. Heart is three-chambered (two auricles and one ventricle). Cold-blooded animals. Sexes are separate. Fertilization is external. Oviparous and development is indirect.
  • Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog), Salamandra (Salamander), Ichthyophis (Limbless amphibia).

  Class - Reptilia

  Class name refers to their creeping or crawling habit (Latin, repere or reptum, to creep or crawl).

  • Salient Features: Mostly terrestrial animals and their body is covered by dry and horny skin, epidermal scales or scutes. They do not have external ear openings. Tympanum represents ear. Limbs, when present, are two pairs. Heart is usually 3-chambered, but 4-chambered in crocodiles. Reptiles are poikilotherms. Snakes and lizards shed their scales as skin cast. Sexes are separate. Fertilization is internal. Oviparous and development is direct.
  • Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator), Hemidactylus (Wall lizard), Poisonous snakes - Naja (Cobra), Bungarus (Krait), Vipera (Viper).

  Class - Aves

  The salient features of Aves (birds) are the presence of feathers and most of them can fly except flightless birds (e.g., Ostrich).

  • Salient Features: Possess beak. Forelimbs are modified into wings. Hind limbs generally have scales and are modified for walking, swimming or clasping the tree branches. Skin is dry without glands except the oil gland at the base of the tail. Endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic). Digestive tract of birds has additional chambers, the crop and gizzard. Heart is completely four-chambered. Warm-blooded (homoiothermous) animals, i.e., they are able to maintain a constant body temperature. Respiration is by lungs. Air sacs connected to lungs supplement respiration. Sexes are separate. Fertilization is internal. Oviparous and development is direct.
  • Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

  Class - Mammalia

  They are found in a variety of habitats - polar ice caps, deserts, mountains, forests, grasslands and dark caves. Some of them have adapted to fly or live in water.

  • Salient Features: Most unique mammalian characteristic is the presence of milk producing glands (mammary glands) by which the young ones are nourished. They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying. Skin of mammals is unique in possessing hair. External ears or pinnae are present. Different types of teeth are present in the jaw. Heart is four-chambered. Homoiothermous. Respiration is by lungs. Sexes are separate and fertilization is internal. Viviparous with few exceptions and development is direct.
  • Examples: Oviparous - Ornithorhynchus (Platypus); Viviparous - Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel), Macaca (Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant), Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale), Panthera tigris (Tiger), Panthera leo (Lion).