ANIMAL KINGDOM

CLASSIFICATION AND FUNDAMENTAL FEATURES OF PHYLUM

FEATURES OF DIFFERENT PHYLUMS IN NON-CHORDATA

Have you ever wondered why we need to classify animals? So, the answer is there are over a million species of animals and they have completely different structures and forms. Their classification helps us to assign them a systematic position.

LEVEL OF ORGANISATION:

1. Cellular level

Description: The simplest level, where cells are arranged as loose aggregates and perform basic functions with minimal coordination.

Examples: Sponges (Phylum Porifera). 

2. Tissue level

Description: Cells with similar structure and function group together to form poorly defined tissues that carry out specific tasks.

Examples: Cnidarians (Phylum Cnidaria) like jellyfish and corals, and Ctenophores (Comb jellies).

3. Organ level

Description: Tissues organize into organs, each specialized for a particular function.

Examples: Flatworms (Phylum Platyhelminthes). 

4. Organ system level

Description: The most complex level, where multiple organs work together to form systems, each responsible for complex physiological functions.

Examples: Found in higher animals like Arthropods (e.g., insects, spiders), Mollusca (e.g., snails, clams), Annelida (e.g., earthworms), Echinodermata (e.g., starfish), Hemichordata (e.g., Balanoglossus), and Chordata (e.g., fish, birds, mammals). 

Important Note: The level of organization is a key basis for classifying animals, helping scientists understand the vast diversity and evolutionary relationships within the Animal Kingdom.

SYMMETRY

1. Asymmetry:

Animals with no discernible pattern or plane of symmetry are considered asymmetrical.

A plane cannot divide the animal into equal halves.

Example: Sponges (Porifera) are often asymmetrical. 

 

2. Radial Symmetry:

In radial symmetry, any plane passing through the central axis divides the animal into two identical halves. 

These animals typically have a top and bottom but no distinct front, back, or sides. 

Examples: Jellyfish (Cnidaria), starfish (Echinodermata), and sea anemones. 

 

Bilateral Symmetry:

Bilateral symmetry means the animal can be divided into two mirror-image halves (left and right) by a single plane. 

This type of symmetry is associated with cephalization, where sense organs and a head region are concentrated at the front. 

Examples: Most animals, including humans, insects, and other arthropods. 

 

DIPLOBLASTIC AND TRIPLOBLASTIC:

Diploblastic and triploblastic refer to the organisation of germ layers in animal embryos. Diploblastic animals have two germ layers (ectoderm and endoderm), while triploblastic animals have three (ectoderm, mesoderm, and endoderm). This difference in embryonic development reflects varying levels of complexity in body structure and organization. 

Diploblastic Organization:

Germ Layers:

Diploblastic animals develop from two primary germ layers: the outer ectoderm and the inner endoderm.

Mesoglea:

A non-cellular layer called mesoglea is present between the ectoderm and endoderm.

Examples:

Cnidarians (jellyfish, sea anemones) and Ctenophora (comb jellies) are examples of diploblastic animals. 

 

Triploblastic Organization:

Germ Layers:

Triploblastic animals have three germ layers: the outer ectoderm, middle mesoderm, and inner endoderm. 

Mesoderm:

The mesoderm gives rise to muscles, connective tissues, and other internal organs. 

Examples:

Flatworms, earthworms, insects, and humans are examples of triploblastic animals. 

Coelom:

In the animal kingdom, the coelom refers to a fluid-filled body cavity located between the digestive tract and the body wall. It's a key characteristic used in animal classification, with animals categorized based on the presence and formation of this cavity. Animals with a true coelom, completely lined by mesoderm, are called coelomates. Examples include annelids, molluscs, arthropods, echinoderms, hemichordates, and chordates. 

Here's a more detailed breakdown:

1. Coelomates: These animals possess a true coelom, a fluid-filled body cavity completely lined by mesoderm. This cavity houses and protects internal organs, allowing them to move and grow independently. 

2. Acoelomates: These animals lack a coelom, meaning there is no fluid-filled cavity between the body wall and the digestive tract. Examples include flatworms. 

3. Pseudocoelomates: In these animals, the body cavity is not completely lined by mesoderm. The cavity, called a pseudocoel, is located between the mesoderm and endoderm. Examples include roundworms. 

Significance of Coelom:

Hydrostatic Skeleton:

The coelom, filled with fluid, acts as a hydrostatic skeleton, providing support and enabling movement in soft-bodied animals. 

Organ Protection and Function:

The coelom protects internal organs and allows them to move and grow without being hampered by the body wall. 

Nutrient and Waste Transport:

The coelomic fluid aids in the transport of nutrients and waste products throughout the body. 

Immune System Support:

Coelomocytes (cells within the coelom) play a role in the immune response. 

Storage:

The coelom can also serve as a storage area for reproductive cells (sperm and eggs) and waste products. 

Segmentation:

Segmentation in the animal kingdom refers to the division of an animal's body into a series of repeating units called segments. These segments can be external, internal, or both, and often contain repeating organs. This division allows for specialization of segments, enabling different parts of the body to perform distinct functions, enhancing movement and overall adaptability. 

Key aspects of segmentation:

Metamerism:

This term specifically refers to segmentation where both external and internal structures are divided into repeating segments. 

Examples:

Annelids (like earthworms) and arthropods (like insects) are classic examples of segmented animals. 

Importance:

Segmentation allows for greater flexibility and efficiency in movement. It also enables the development of specialized body parts within different segments. 

Notochord:

The notochord is a flexible, rod-like structure found in all chordates t some point during their development. It plays a crucial role in providing structural support and influencing the development of the nervous system. In many vertebrates, the notochord is later replaced by the vertebral column, but it is a defining characteristic of the phylum Chordata. 

Phylum Porifera

This phylum includes all the sponges i.e., pore-bearing animals.

All sponges are generally marine.

They have a cellular level of organisation and are mostly asymmetrical.

They are diploblastic (derived from only two germ layers ectoderm and endoderm).

The body of sponges is porous and the pores are of two types- Ostia (minute pores) and Osculum.

Water enters into their body through ostia into the central cavity, spongocoel and goes out through the osculum.

They have a water transport or canal system. The function of canal system is food gathering, respiratory exchange and removal of waste.

Cells called as choanocytes or collar cells line up the spongocoel and the canals.

A skeleton is made up of spicules or spongin fibres.

Digestion is intracellular.

They are hermaphrodites (sexes are not separate. i.e. eggs and sperms are produced by the same individual).

Fertilisation is internal and development is indirect (larvae stage is present which is morphologically different from an adult).

Asexual reproduction occurs by fragmentation and budding and sexual reproduction is by the formation of gametes.

Examples: Sycon (Scypha), Spongilla (Freshwater sponge) and Euspongia (Bath sponge).

Important

Digestion is of two types: Intracellular and extracellular.

Intracellular is the digestion Inside the cell and is less efficient.

Example: Amoeba Extracellular is the digestion outside the cell and Is more efficient. Example: Man

 

Phylum Coelenterata (Cnidaria)

All the members of this phyla are aquatic, mostly marine.

They are sessile or free-swimming. They show radial symmetry and are diploblastic.

Cnidarians are the first multicellular animals that show tissue level of organisation.

They have a central gastro vascular cavity having a single opening, mouth on hypostome.

Digestion is extracellular and intracellular.

Cnidarians exhibit two basic body forms called polyp and medusa Polyp is a cylindrically shaped body and is sessile, e.g. Hydra, Adamsia, etc.

Medusa is umbrella shaped and is free-swimming like Aurelia or jellyfish.

Some members of the cnidaria show metagenesis.

Metagenesis is the alternation of asexual and sexual phases in their life cycle i.e., polyps reproduce medusa asexually and medusa form polyps sexually e.g. Obelia.

In corals, the skeleton is composed of calcium carbonate.

Examples: Physalia (Portuguese man of war). Adamsia (Sea anemone), Pennatula (Sea pen). Gorgonia (Sea fan) and Meandrina (Brain coral).

Important

The epidermis of coelenterates contains cnidoblasts (stinging cells).

Name of the phylum is cnidaria due to the presence of these stinging cells.

A cnidoblast has a nematocyst (stinging organ) which is present on the tentacles.

Cnidoblasts are used for anchorage defence and for the capturing of prey.

 

Phylum Ctenophora

They are also known as comb jellies or sea walnuts.

Members of ctenophores are exclusively marine.

They have radial symmetry, diploblastic and have tissue level of organisation.

Comb-like eight external rows of ciliary plates called comb plates are present on their body which help in swimming or locomotion.

That is why ctenophores are commonly called comb jellies.

They show bioluminescence (the property of living organisms to emit light).

Fertilisation is external and development is indirect.

They are hermaphrodites and only sexual reproduction is seen.

Examples: Pleurobrachia and Ctenoplana.

 

 

Phylum Platyhelminthes

They are called flatworms because they have a dorso-ventrally flattened body.

They show bilateral symmetry and triploblastic and have an organ level of organisation.

They are acoelomates (without coelom).

Flatworms are mostly endoparasites.

In parasitic forms, hooks and suckers are present and they do not have a digestive tract as they absorb the digested food directly from the host through their body surface.

For osmoregulation and excretion, flame cells are present.

Fertilisation is internal and development is indirect.

Planarians possess high regeneration capacity.

Examples: Taenia (Tapeworm) and Fasciola (Liver fluke).

 

Phylum Aschelminthes

The body is circular in cross-section, they are termed as roundworms.

Many roundworms live as parasites in plants and animals, and are also free living and generally aquatic.

They are bilaterally symmetrical, triploblastic with organ system level of organisation and are pseudocoelomates (false coelom-mesoderm is present as scattered pouches).

Alimentary canal is complete with a muscular pharynx.

Ammonia is the main excretory waste and is removed from the body through excretory pore.

They are dioecious, i.e. sexes are separate.

Females are longer than males and fertilisation in internal.

Development may be direct (the offspring resembles to the parent) or indirect.

Examples: Ascaris (Roundworm), Wuchereria (Filarial worm). Ancylostoma (Hookworm).

 

Phylum Annelida

They are also known as segmented animals.

They are bilaterally symmetrical, triploblastic, coelomate (true coeloms present) and have organ system level of organisation.

Blood vascular system is of a closed type.

The annelids are characterised by metameric segmentation. The segments are called metameres.

Because of the segments the phylum is named as Annelida (Latin, annulus: little ring).

They possess circular and longitudinal muscles which are smooth and help in locomotion.

Nereis, an aquatic annelid, possess lateral appendages known as parapodia, which help in swimming.

Osmoregulation and excretion are through a coiled tubular structure called nephridia.

Neural system consists of paired ganglia that are connected by lateral nerves to a double ventral cord.

Nereis is dioecious but earthworms and leeches are monoecious.

Reproduction is sexual.

Examples: Nereis, Pheretima (Earthworm) and Hirudinaria (Blood-sucking leech).

 

Phylum Arthropoda

Arthropoda is the largest phylum of Kingdom Animalia.

This phylum includes insects. Over two third of all named species on earth are arthropods.

They occur on land, in soil and water, and are parasites also.

They have an organ system level of body organisation.

They are generally triploblastic and their body is bilaterally symmetrical.

Externally their body is segmented. It consists of the head, thorax and abdomen.

The body is covered by thick, non-living chitinous cuticles which form the exoskeleton.

The characteristic feature of arthropods is jointed appendages.

Respiratory organs are gills, book gills, book lungs or tracheal system.

Sensory organs like antennae and eyes (simple or compound) are present and the circulatory system is of open type.

Malpighian tubules are present for excretion.

Balancing organs or statocysts are present Sexes are separate (dioecious).

Fertilisation is generally internal.

They are mostly oviparous.

Development may be direct or indirect.

Examples: Economically important species-

Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect).

Vectors-Anopheles, Culex and Aedes (Mosquitoes).

Gregarious pest- Locusta (Locust).

Living fossil- Limulus (King crab).

 

Phylum Mollusca

Phylum Mollusca is the second largest phylum after arthropods.

They are also known as soft-bodied animals.

They are terrestrial or aquatic.

They are bilaterally symmetrical, triploblastic, coelomate (true coeloms present) and have an organ system level of organisation.

The body of molluscs is unsegmented with a distinct head, muscular foot and visceral hump.

Body is covered by a calcareous shell.

Mantle is a thin, soft and spongy layer of skin which is formed over a visceral hump and the space between the mantle and hump is called the mantle cavity (containing feather-like gills helping in respiration and excretion).

Head region has sensory tentacles.

Mouth contains a rasping organ for feeding called radula.

They are dioecious and oviparous with indirect development.

Examples: Pila (apple snail). Pinctada (pearl oyster). Sepia (cuttlefish). Loligo (squid), Octopus (devil fish), Aplysia (sea-hare), Dentalium (tusk shell) and Chaetopleura (chiton)

 

Phylum Echinodermata

They are also known as spiny bodies animals.

Their endoskeleton is of calcareous ossicles.

All are marine with organ system level of organisation, triploblastic and coelomate animals.

The symmetry is bilateral in larvae but radial in adults.

Digestive system is complete and both mouth (ventral) and anus (dorsal) are present.

The main characteristic feature of echinoderms is the presence of the water vascular system.

It helps in locomotion, capture, transport of food and respiration.

Excretory system is absent and sexes are separate.

Fertilisation is external and development is indirect with free-swimming Larvae.

Examples: Asterias (starfish), Echinus (sea urchin). Antedon (sea lily), Cucumaria (sea cucumber) and Ophiura (brittle star).

 

 

HEMICHORDAТА

Due to similarities with chordates, earlier Hemichordata is considered as a subphylum under Phylum Chordata.

However, recently it is placed under a separate phylum under non-chordata (invertebrata).

The members of Phylum Hemichordata mostly live in burrows and are mostly marine.

Their body is soft, worm-like and can be divided into proboscis, collar and trunk.

They are bilaterally symmetrical with three germ layers i.e., triploblastic.

Hemichordates have organ system level of body organisation with complete digestive tract.

They have a stomochord which is a hollow outgrowth, arises from the roof of the buccal cavity, called "Buccal diverticulum" present in proboscis.

Respiration takes place through branchial position of pharynx-bearing gill silts.

The blood vascular system is open type and blood is colourless.

Excretion is done by single proboscis gland or glomerulus situated in the proboscis.

Nervous system is primitive, consisting mainly of an intraepidermal nerve plexus.

Sensory cells of the epidermis act as sense organs.

Reproduction is mostly sexual Sexes are separate or united.

Gonads are in one pair or in several pairs.

Fertilisation is external and development is mostly indirect through a free-swimming larva.

Examples: Balanoglossus and Saccoglossus.

Important

Hemichordata is commonly called as Acorn worms or tongue worms due to their appearance Proboscis and a collar together form the acorn which is used for burrowing in mud.

 

CHORDATA

Diagnostic Characters of Chordata

All the chordates possess some fundamental characters:

(1) Notochord: It is a solid unjointed, stiff but flexible rod-like structure situated on the dorsal side between the dorsal hollow of nerve cord and alimentary canal.

(2) Dorsal hollow nerve cord: The nerve cord of chordates is always hollow and lies dorsal to the notochord.

(3) Pharyngeal gill slits: All the chordates have at some stage of life, a series of paired narrow openings, the gill slits on the lateral sides of the pharynx.

(4) Post-anal tail: It is a post-anal part of the organism's body which is reduced or absent in many adult chordates.

 

Other Characters of Chordates

Chordates are bilaterally symmetrical and have three germinal layers, i.e. triploblastic, segmentation, coelomate, and possess organ system level of organisation.

They have special organs for respiration and excretion.

They possess a complete digestive tract and a closed circulatory system.

In chordates, the sexes are separate. Sexual reproduction is the common process of reproduction. Asexual reproduction is not seen.

 

Classification of Chordata

The Phylum Chordata is divided into three Subphyla Urochordata or Tunicata. Cephalochordata and Vertebrata.

The first two subphylums Urochordata and Cephalochordata are primitive chordates without a vertebral column and they are grouped together as Protochordates or acronic (without cranium, i.e. brainbox).

The vertebrates are higher chordates with vertebral columns.

 

Subphylum 1: Urochordata

The Urochordates are marine animals.

Remarkable character of Urochordata is that the notochord is found only in the tail of the larva and disappears in the adult form.

This subphylum is also called as Tunicata because the adult body is generally enclosed within a leathery test or tunic form by cellulose-like organic substance termed as tunicin.

Examples: Ascidia, Salpa, Dollolum, Herdmania.

 

Subphylum 2: Cephalochordata

The Cephalochordates are marine animals.

Remarkable character of cephalochordates is that the notochord extends up to the anterior end of the body Le., from head to tail and persists throughout their life.

Cephalochordata show progressive metamorphosis (changes from less developed larva to a well-developed adult).

Examples: Branchiostoma (Amphioxus or Lancelet).

 

 

 

Subphylum 3: Vertebrata

The Vertebrates are advanced chordates.

The notochord is only present in the embryonic stage, it is replaced by a cartilaginous or bony vertebral column in the adult, hence called Vertebrata.

There are some more basic features of vertebrates:

(1) The Subphylum Vertebrata is also known as Craniata because they have a cranium (brain box).

(2) The epidermis consists of many layers of cells. Epidermis may bear an exoskeleton of scales, feathers or hairs.

(3) Digestive tract Is complete.

(4) Endoskeleton is formed of cartilage or cartilage and bones.

(5) Heat is ventrally situated with two, three or four chambers

(6) There is a closed circulatory system consisting of blood vascular and lymphatic systems. RBCs are present.

(7) Respiratory organs may be gills, skin, buccopharyngeal cavity and lungs.

(8) A pair of kidney is present for excretion and osmoregulation.

(9) Nervous system consists of central nervous system (brain and spinal cord). peripheral and autonomic nervous system.

(10) Sense organs are eyes, ears, tongue, nasal chambers and skin.

(11) Endocrine glands are found in all vertebrates. The subphylum Vertebrata can be further divided into two-Agnatha and Gnathostomata.

Important

Mammals All vertebrates are chordates, but all chordates are not vertebrates.

 

Class 1: Cyclostomata

The distinguishing features of the class are as follows:

(1) Primitive marine vertebrates. All the members are ectoparasite on some fishes.

(2) Long. elongated body with cartilaginous endoskeleton.

(3) Ectoparasites having circular Jawless, mouths for sucking blood of host fishes.

(4) Respiratory system contains 6-15 pairs of gill slits.

(5) Paired fins and scales are absent.

(6) Circulatory system is a closed-type with two chambered heart.

(7) They migrate to fresh water for spawning and die shortly after spawning. Larvae undergo metamorphosis and then migrate back to sea.

Examples: Petromyzon (Lamprey) and Myxine (Hagfish).

 

Class 2: Chondrichthyes

The distinguishing features of this class are:

(1) Mostly marine, bearing cartilaginous exoskeleton.

(2) Dorsoventrally flattened streamlined body.

(3) Cold-blooded (poikilothermous) - body temperature is variable and they are unable to regulate their body temperature.

(4) Predacious- preying on other animals.

(5) Ventral mouth with powerful jaws and teeth modified as backwardly directed placoid scales.

(6) Tough skin with minute placoid scales (spiny, tooth-like projections).

(7) Respiratory system includes a pair of gill slits without operculum (gill cover).

(8) Absence of air bladder. Thus, buoyancy is absent. so, they must swim constantly to avoid sinking.

(9) Circulatory system consists of a two-chambered heart (one auricle and one ventricle).

(10) Some have special defence organs like electric organs in Torpedo and poison sting in Trygon.

(11) Sexes are distinct and fertilisation is internal Pelvic fins bear Claspers for copulation in males.

(12) Viviparous means to give birth to young ones and have placenta for nourishment inside the parent's body.

Examples: Scoliodon (Dogfish), Pristis (Sawfish). Carcharodon (great white shark) and Trygon (sting ray).

 

Class 3: Osteichthyes

The characteristic features of the class are as follows:  

(1) Includes both marine and freshwater fishes having bony endoskeleton.  

(2) Streamlined body to facilitate movement through water.

(3) Anterior mouth, mostly located at the terminal end.

(4) Respiratory system has four pairs of gills and gill slits covered by gill covers (operculum).

(5) Cycloid (smooth-edged scales), ctenoid scales. impermeable to water present on skin.

(6) Swim bladder present to regulate buoyancy and enable the fish to stay at a depth.

(7) Circulatory system with two-chambered heart (one auricle and one ventricle).

(8) Cold-blooded animals (poikilotherms). body temperature is variable and are unable to regulate body temperature.

(9) Sexes are separate with external fertilisation.

(10) Oviparous (egg-laying).

(11) Direct development and some fishes show parental care.

Examples: Exocoetus (Flying fish). Hippocampus (Sea horse), Labeo (Rohu), Catla (Catla), Clarias (Magur), Betta (Fighting fish) and Pterophyllum (Angelfish).

 

Class 4: Amphibia

The distinguishing features of Class Amphibia are as follows:

(1) First animals to survive on land but needs water for some activities, Le. amphibious in nature (can live on land as-well-as in water).

(2) Body divisible into head and trunk with two pairs of pentadactyl limbs, sometimes tail is present.

(3) Moist skin, presence of mucus glands with absence of scales.

(4) Sense organs- Eyes with moveable eyelids. tympanum acting as an ear.

(5) Cloaca-serving as a common opening for the alimentary canal, urinary tract and reproductive tract opening outside through cloacal aperture.

(6) Respiration through gills, lungs and skin.

(7) Circulatory system consists of a three chambered heart having two auricles and one ventricle.

(8) Cold-blooded with most of the forms, hibernating in winter and aestivating in summer.

(9) Sexes are distinct with external fertilisation.

(10) Oviparous (egg-laying).

Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog). Salamandra (Salamander) and Ichthyophis (limbless amphibian).

 

Class 5: Reptilia 

The characteristic features of Class reptilia are as follows:

(1) The name reptilia represents their creeping or crawling mode of locomotion, mostly terrestrial.

(2) Dry and rough skin with presence of scales or scutes and absence of glands.

(3) Shed off their scales periodically as skin cast. Example: snakes, lizards.

(4) Tympanum is present but no external ear openings.

(5) In a few reptiles, the limbs are absent, if present then there are the two pairs of pentadactyl limbs.

(6) Respiration occurs through lungs.

(7) Circulatory system with a three-chambered heart (two auricles and one ventricle), except Crocodiles which have a four-chambered heart.

(8) Cold-blooded (poikilotherms).

(9) Oviparous (egg-laying).

(10) Direct development, embryonic membranes are formed during development.

Examples: Chelone, Testudo, Chameleon, Calotes, Crocodilus, Alligator, Hemidactylus and Poisonous snakes- Naja. Bangarus and Vipera.

 

Class 6: Aves

The distinguishing features of Class Aves are as follows:

(1) Warm-blooded (homeotherms). body temperature is regulated, as they are able to maintain a constant body temperature.

(2) Unique features are presence of feathers, two pairs of limbs with forelimbs modified into wings for flight and hind limbs adapted for walking. perching and swimming.

(3) Upper jaw and lower jaw modified Into beaks adapted to various feeding modes.

(4) Dry skin, with absence of glands except the oil gland present on the tail.

(5) Delicate and light body with bony endoskeleton. bones are pneumatic i.e., contain air cavities to reduce weight which helps in flight.

(6) Lack teeth and swallow the unmasticated food. therefore, the digestive tract has additional chambers called crops and gizzard. Crop stores and softens the food and gizzard help in crushing the food.

(7)Respiration through lungs only with a thin walled air sac associated with lungs to supplement respiration.

(8) Circulatory system contains a four-chambered heart (two auricles and two ventricles), this facilitates in keeping the oxygenated and deoxygenated blood separate.

(9) Oviparous (egg-laying).

(10) Internal fertilisation, with direct development and embryonic membranes, are formed.

 Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Pigeon) and Neophron (Vulture).

 

Class 7: Mammalia

The characteristic features of Class Mammalia are as follows:

(1) Primarily terrestrial occur in all sorts of habitats and show great diversity in habits.

(2) Warm-blooded animals (homoiotherms). maintaining a constant body temperature.

(3) Certain unique features of mammals are:

(i) Presence of mammary glands (milk -secreting), thus named as mammals and they nourish their young ones with milk.

(ii) Presence of hair on the skin which helps in controlling the body temperature.

(iii) Possess external ears with large fleshy pinnae.

(iv) Each jaw of the mammal contains different types of teeth.

(4) Two pairs of pentadactyl limbs are present that are adapted for walking, running, climbing, and swimming.

(5) Respiration takes place by lungs.

(6) Circulatory system contains a four-chambered heart (two auricles and two ventricles), which keeps both the blood (oxygenated and deoxygenated) separate.

(7) Sexes are separate with internal fertilisation.

(8) Viviparous (give birth to young ones).

(9) Development is direct and takes place in the uterus of the female

Examples: Oviparous- Ornithorhynchus (Platypus).

Viviparous- Marcopus (Kangaroo), Pteropus (Flying Fox), Camelus (Camel), Macaca (Monkey). Rattus (Rat), Canis (Dog), Felis (Cat), Balaenoptera (Blue whale).