7 Animal Diversity: Origin and Classification

Dr R.K. Chaitanya

1.  Introduction

2. Evolutionary origin of animals

3. Animal Classification

3.1. Carolus Linnaeus classification

3.2. Five kingdom classification

3.3. Three domain classification: Tree of Life

3.4. Other animal classification systems 

4.      Description of animal phyla

5.      Salient features of different phyla in the animal kingdom

6.      Animal germ layers and their derivatives

7.      Larval forms of various animals

8.      State animals and birds

1.   Introduction

Among the 5 major kingdoms of biodiversity, the KindomAnimalia is one of the important one. All the members of this kingdom are multicellular, eukaryotes and heterotrophs. Most of the members can move independently and are regarded as motile. As per estimates, there are around 9 to 10 million species of animals and among these about 8 lakh species have been identified. India has only 2.4% of global land area but contains 7-8% of globally recorded species. Over 46,000 species of plants and 96,000 species of animals are recorded in India. Animals may lack symmetry or may have radial or bilateral symmetry. Bilaterally symmetrical animals have dorsal and ventral sides, as well as anterior and posterior ends. There are three major clades of bilaterian animals: Deuterostomia, Lophotrochozoa, and Ecdysozoa. With one exception, the phyla in these clades consist entirely of invertebrates, animals that lack a backbone; Chordata is the only phylum that includes vertebrates, animals with a backbone. They have been classified/subdivided into many categories on different basis as discussed in this module. The module discusses the evolutionary origin of animals, different animal classification systems, animal germ layers and their derivatives, State animals and birds etc.

2.   Evolutionary origin of animals

The history of animals spans more than half a billion years. Fossil biochemical evidence and molecular clock analyses indicate that animals arose over 700 million years ago. Genomic analyses suggest that key steps in the origin of animals involved new ways of using proteins that were encoded by genes found in choanoflagellates. This table provides an overview of how animals evolved from their distant common ancestor over four geologic eras.

Era	Period	Age (Millions of years ago)	Important Events
Neoproterozoic	Ediacaran	560-542	soft-bodied; radially symmetric invertebrate animals appear (fossils: Mawsonitesspriggi, Sprigginafloundersi, Cloudina Dickinsoniacostata)
Paleozoic	Cambrian	542-488	Sudden increase in diversity of many animal phyla (Cambrian explosion) (fossils: Pikaia(eel-like chordate, Marella (small arthropod, Anomalocaris(large animal with grasping limbs and a circular mouth), and Hallucigenia(animals with toothpick-like spikes
	Ordovician	488-444	Colonization of land by animals; invertebrates dominated
	Silurian	444-416	Radiation of crinoids; a continued proliferation and expansion of the brachiopods, and the oldest known fossils of coral reefs; also marks the wide and rapid spread of jawless fish
	Devonian	416-359	Diversification of bony fishes; first tetrapods and insects appear (Fossils: Coccosteuscuspidatus, Tiktaalik)
	Carbonifero us	359-299	Origin of reptiles; amphibians dominant
	Permian	299-251	Radiation of reptiles; origin of most present-day groups of insects; extinction of many marine and terrestrial organisms at end of period (Fossils: Dimetrodon, the largest known carnivore of its day)
Mesozoic	Triassic	251-199.6	Dinosaurs evolve and radiate; origin of mammals
	Jurassic	199.6-145.5	Dinosaurs abundant and diverse (Fossil: Rhomaleosaurus victor, a plesiosaur)
	Cretaceous	145.5-65.5	Dinosaurs become extinct at end of period
Cenozoic	Paleogene	65.5-23	Major radiation of mammals, birds, and pollinating insects; Origins of many primate groups
	Neogene	23-2.6	Continued radiation of mammals; Appearance of bipedal human ancestors
	Quaternary	2.6-0.01	Ice ages; origin of genus Homo

3.   Animal Classification

3.1.Carolus Linnaeus classification

All modern classification systems have their roots in the Carolus Linnaeus classification system. His two most important contributions to taxonomy were: i) A hierarchical classification system: species are grouped in increasingly broad taxa: Related genera are placed in the same family, families in orders, orders in classes, classes in phyla, phyla in kingdoms; ii) The system of binomial nomenclature (a 2-part naming method): Genus: The first part of the binomial to which the species belongs. The first letter of the genus is always capitalized, Specific epithet: The second part of binomial, is unique for each species within the genus, Entire binomial is italicized and scientific names are in Latin

3.2. Five kingdom classification

Many biologists including R.H. Whittaker recognized five kingdoms:Monera (prokaryotes), Protista (a diverse kingdom consisting (mostly of unicellular organisms), Plantae, Fungi, andAnimalia.However, phylogenies based on genetic data soon began toreveal a problem with this system: Some prokaryotes differ asmuch from each other as they do from eukaryotes. Such difficultieshave led biologists to adopt a three-domain system.

3.3.Three domain classification: Tree of life

The three-domain system is a biological classification introduced by Carl Woese et al. in 1977 that divides cellular life forms into archaea, bacteria, and eukaryote domains. Tree of life is based in part on sequence comparisons of rRNA genes or rDNA

3.4. Other animal classification systems

i)   Animal classification based on level of organization:

a) Cellular level of organisation: Animal kingdom is divided into two subkingdoms-protozoa and metazoa. Protozoa are defined as single-celled eukaryotic organisms and metazoa are multicellular eukaryotic organisms. The sub-kingdom metazoa is further sub-divided into two branches: parazoa and eumetazoa.

Ø Parazoa: Cells are arranged as loose cell aggregates, i.e., they exhibit cellular level of organisation; absence of tissue or organ system level of organisation. Eg. Phylum porifera

Ø Eumetazoa: Clade comprising all major animal phyla except porifera. Embryo gets layered during gastrulation. Exhibit either tissue or organ system level of organisation.

b) Tissue level of organisation: In phylum Cnidaria and Ctenophora, the arrangement of cells is more complex. Here the cells performing the same function are arranged into tissues.

c) Organ level of organization: Organ level is exhibited by members of Platyhelminthes and other higher phyla where tissues are grouped together to form organs, each specialised for a particular function. 

ii)      Animal classification based on symmetry

a) Symmetry: Animals can be categorised on the basis of their symmetry.

b) Asymmetry: Sponges are mostly asymmetrical, i.e., any plane that passes through the centre does not divide them into equal halves. 

c) Radial symmetry: When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry. Cnidarians, ctenophores and echinoderms have this kind of body plan. In phylum Echinodermata, larval stages are bilaterally symmetrical and adult stages are radially symmetric.

d) Bilateral symmetry: Animals like annelids, arthropods, etc., where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry

iii) Animal classification based on Number of germ layers: In all the animals except sponges, the embryo becomes layered during gastrulation. As development progresses, these layers, called germ layers, form the various tissues and organs of the body.

a) Diploblastic animals: Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals, e.g., Cnidarians and Ctenophores. An undifferentiated layer, mesoglea or mesohyl, is present in between the ectoderm and the endoderm.

b) Triploblastic animals: Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals (platyhelminthes to chordates).

iv) Animal classification based on body cavity

a) Body cavity: Presence or absence of a cavity between the body wall and the gut wall is very important in classification.

b) Coelomates: Most triploblastic animals have a body cavity, a fluid- or air-filled space located between the digestive tract and the outer body wall. This body cavity is also called a coelom. Animals possessing coelom are called coelomates, e.g., annelids, molluscs, arthropods, echinoderms, and chordates.

c) Pseudocoelomates: In some animals, the body cavity is not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates, e.g., aschelminthes

d) Acoelomates: The animals in which the body cavity is absent are called acoelomates, e.g., Platyhelminthes.

v) Animal classification based on presence or absence of notochord: Notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals

a) Non-chordates: Those animals which do not form notochord structure are called non-chordates, e.g., porifera to echinoderms.

b) Chordates: Four key characters of chordates: a notochord; a dorsal, hollow nerve cord; pharyngeal slits or clefts; and a muscular, post-anal tail.

vi) Animal classification based on embryonic development & segmentation

a) Embryonic development: Based on  certain  aspects  of  early development, many animals can be described as having one of two developmental modes: protostome development or deuterostome development. These modes can generally be distinguished by differences in cleavage, coelom formation, and fate of the blastopore.

b) Protostomes: Protostome development begins with spiral, determinate cleavage. The coelom forms from splits in the mesoderm and the mouth forms from the blastopore. Protostomes include the phyla Mollusca, Annelida and Arthropoda.

c) Deuterostomes:Deuterostome development is characterized by radial, indeterminate cleavage. Coelom forms from mesodermal outpocketings of the archenteron and the mouth forms from a secondary. Anus develops from blastopore. Deuterostomes include the phyla Echinodermata and Chordata.

4.   Description of animal phyla

Phylum	Description
Porifera (sponges)	Lack true tissues Body is supported by a skeleton made up of spicules or sponging fibres Sexes are not seperate (hermaphrodite) Reproduce asexually by fragmentation and sexually by formation of gametes Examples: Sycon(Scypha), Spongilla(Fresh water sponge) and Euspongia(Bath sponge)
Ctenophora (comb jellies)	Eight rows of ciliated “combs” or ctenes Bioluminescence is well-marked in ctenophores Bioluminescence is well-marked in ctenophores Examples: Pleurobrachiaand Ctenoplana
Cnidaria	Unique stinging structures (nematocysts) housed in specialized cells (cnidocytes) Corals have a skeleton composed of calcium carbonate. Cnidarians exhibit two basic body forms called polyp and medusa Those cnidarians which exist in both forms exhibit alternation ofgeneration (metagenesis) (e.g., Obelia). Examples: Physalia(Portuguese man-of-war), Adamsia(Sea anemone),Pennatula(Sea-pen), Gorgonia(Sea-fan) and Meandrina(Braincoral).
Platyhelminthes (flatworms)	A central nervous system that processes information from sensory structures Specialised cells called flame cells help in osmoregulation and excretion Development is throughmany larval stages Some members like Planariapossess highregeneration capacity Examples: Taenia(Tapeworm), Fasciola(Liver fluke)
Nematoda (roundworms)	Cylindrical pseudocoelomates with tapered ends Undergo ecdysis Alimentary canal is complete with a well developedmuscular pharynx An excretory tube removes body wastes from the body cavitythrough the excretory pore Examples: Ascaris(Round Worm), Wuchereria(Filaria worm), Ancylostoma(Hookworm)
Annelida (segmented worms)	Segmented body wall (metameres) and internal organs (except digestive tract, which is unsegmented) Aquatic annelids like Nereispossess lateral appendages, parapodia, which help in swimming Nephridia (sing. nephridium) help in osmoregulation and excretion Reproduction is sexual Examples: Nereis, Pheretima(Earthworm) and Hirudinaria(Blood sucking leech)
Mollusca	Second largest animal phylum Body parts include muscular foot, visceral mass, mantle Coelom reduced Most have hard shell made of calcium carbonate Mantle cavity in which feather like gills are present The 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(Seahare), Dentalium(Tusk shell) and Chaetopleura(Chiton)
Arthropoda	Largest phylum of Animalia Segmented body, jointed appendages, and exoskeleton made of protein and chitin The body consists of head, thorax and abdomen Respiratory organs are gills, book gills, book lungs or tracheal system Circulatory system is of open type (hemolymph) Sensory organs like antennae, eyes (compound and simple), statocysts or balance organs are present Excretion takes place through malpighian tubules Examples: Economically important insects – Apis(Honey bee), Bombyx(Silkworm), Laccifer(Lac insect); Vectors – Anopheles, Culexand Aedes(Mosquitoes); Gregarious pest – Locusta(Locust); Living fossil – Limulus (King crab)
Echinodermata (echinoderms)	The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical The most distinctive feature of echinoderms is the presence of water vascular system which helps inlocomotion, capture and transport of food and respiration An excretory system is absent Examples: Asterias(Star fish), Echinus (Sea urchin), Antedon(Sea lily), Cucumaria(Sea cucumber) and Ophiura(Brittlestar)
Chordata (chordates)	Characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits Urochordata and Cephalochordata Subphyla Urochordata and Cephalochordata are often referred to as protochordates In Urochordata, notochord is present only in larval tail, while inCephalochordata, it extends from head to tail region and is persistent throughout their life Examples: Urochordata – Ascidia, Salpa, Doliolum; Cephalochordata – Branchiostoma(Amphioxus or Lancelet) Vertebrata The members of subphylum vertebrata possess notochord during the embryonic period The notochord is replaced by a cartilaginous or bony vertebral column in the adult Vertebrates have a ventral muscular heart with two, three or fourchambers Kidneys for excretion and osmoregulation and paired appendages which may be fins or limbs Class Cyclostomata Elongated body bearing 6-15 pairs of gill slits for respiration Cyclostomes have a sucking and circular mouth without jaws Examples: Petromyzon(Lamprey) and Myxine(Hagfish) Pisces (Fish) Class – Chondrichthyes Notochord is persistent throughout life Gill slits are separate and without operculum (gill cover) The skin is tough, containing minute placoid scales Due to the absence 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) animals, i.e., they lackthe capacity to regulate their body temperature Many of them are viviparous Examples: Scoliodon(Dog fish), Pristis(Saw fish),Carcharodon(Great white shark), Trygon(Sting ray). Class – Osteichthyes It includes both marine and fresh water fishes with bony endoskeleton They have four pairs of gills which are covered by an operculum on each side Air bladder is present which regulates buoyancy They are mostly oviparous Examples: Marine – Exocoetus(Flying fish),Hippocampus (Sea horse); Freshwater – Labeo(Rohu),Catla(Katla), Clarias(Magur); Aquarium – Betta(Fighting fish), Pterophyllum(Angel fish) Tetrapoda Class amphibian Body is divisible into head and trunk The amphibian skin is moist (without scales) The eyes have eyelids A tympanum 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 The heart is three chambered (two auricles and one ventricle). These are cold-blooded animals  They are oviparous Examples: Bufo(Toad), Rana(Frog), Hyla(Tree frog),Salamandra(Salamander), Ichthyophis(Limblessamphibia). Class – Reptilia Body is covered by dry and cornified skin, epidermal scales or scutes They do not have external ear openings Tympanum represents ear Heart is usually three-chambered, but four-chambered in crocodiles Reptiles are poikilotherms Snakes and lizards shed their scales as skin cast They are oviparous Examples: Chelone(Turtle), Testudo(Tortoise), Chameleon (Tree lizard),Calotes(Garden lizard), Crocodilus(Crocodile), Alligator(Alligator), Hemidactylus(Wall lizard), Poisonous snakes – Naja(Cobra),Bangarus(Krait), Vipera(Viper) Class – Aves Presence of feathers and most of them can fly except flightless birds (e.g., Ostrich) The forelimbs are modified into wings. The 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). The digestive tract of birds has additional chambers, the crop and gizzard.  Heart is completely four-chambered. They are warm-blooded (homoiothermous) animals, i.e., they are able to maintain a constant body temperature. They are oviparous Examples: Corvus(Crow), Columba (Pigeon), Psittacula(Parrot), Struthio(Ostrich), Pavo(Peacock), Aptenodytes(Penguin), Neophron(Vulture) Class mammalia The most unique mammalian characteristic is the presenceof milk producing glands (mammary glands) by which the young ones are nourished The skin of mammals is unique in possessing hair External ears or pinnae are presentDifferent types of teeth are present in the jaw Heart is four-chambered They are homeothermous They are viviparous 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), Pantheratigris(Tiger), Pantheraleo(Lion)