CHAPTER 4 ANIMAL KINGDOM
Basis of classification
The classification of the animal kingdom is based on the different essential characteristics such as level of organization, habitat, symmetry.
Levels of Organization
• Cellular Level of Organization
• Tissue Level of Organization
• Organ Level of Organization
• Organ system Level of Organization
Patterns of organ systems
• Digestive System (Incomplete Digestive System and Complete Digestive System)
• Circulatory System
• Open Type
• Closed Type
Body symmetry
• Bilateral Symmetry
• Radial Symmetry
• Asymmetrical
Levels of Organisation
All members of Animalia are multicellular, heterotrophic eukaryotes. But, all of them do not exhibit the same pattern of organization of cells. The cells in their body are of several types. These are organized into several functional units of progressively increasing complexity.
Cellular Level
In this level, the body shows some division of labour among cells. They are remarkably independent and can change their form and function. It is found in sponges. The body consists of many cells arranged as loose cell aggregates but, the cells do not form tissues.
Tissue Level
Here, in coelenterates, the arrangement of cells is more complex. The cells performing the same function are arranged into tissues, hence is called tissue level of organization.
Organ Level
In Platyhelminthes and other higher phyla, tissues are grouped together to form organs, each specialized for a particular function, i.e., organ level organization is present.
Organ System Level
In animals like annelids, arthropods, molluscs, echinoderms and chordates, organs have associated to form functional systems, each system concerned with a specific physiological function. This is called organ system level of organisation. Organ systems in different groups of animals exhibit various patterns of complexities.
Like the digestive system in Platyhelminthes has only a single opening to the outside of the body that serves as both mouth and anus and is thus, called incomplete.
A complete digestive system has two openings, i.e., mouth and anus.
Symmetry
The symmetry refers to the arrangement of parts on the opposite sides of the body of a three dimensional animal.
On the basis of symmetry, animals can be of following types:
Asymmetrical:
Animals in which, any plane passes through the center does not divide them into equal halves such animals are called asymmetrical, e.g., Sponges.
Symmetrical:
The body of some animals can be divided into two similar equal halves by one or more planes. Such animals are called symmetrical
The symmetry can be further divided as:
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, e.g., Coelenterates, ctenophores and echinoderms.
Bilateral Symmetry:
In some animals, body can be divided into identical left and right halves in only one plane. This is called bilateral symmetry, e.g., Annelids, arthropods, etc.
Diploblastic and Triploblastic Organisation
Germ layers are group of cells behaving as a unit during early stages of embryonic development. It differentiate to give rise to all the tissues/ organs of the fully formed individuals.
On the basis of germ layers animals are classified as follows:
Diploblastic
Animals in which the cells are arranged in two embryonic layers an external ectoderm and an internal endoderm, are called diploblastic animals. In addition, an undifferentiated layer, mesoglea is present in between the ectoderm and the endoderm. e.g., Coelenterates.
Triploblastic
The animals in which the developing embryo has a third germinal layer mesoderm, in between the ectoderm and endoderm are called triploblastic animals, e.g., All animals from phylum-Platyhelminthes to phylum-Chordata.
Coelom
The body cavity (between the body wall and gut wall) which is lined by mesoderm is called coelom. The presence or absence of coelom is very important in classification. On the basis of coelom, animals can be classified in three different groups.
Acoelomates
The animals in which the body cavity is absent are called acoelomates, e.g., Poriferans, platyhelminthes, coelenterates, ctenophors and flatworms.
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.
Coelomates
The animals having true coelom are called coelomates. A true coelom arises within mesoderm and is therefore, lined by mesodermal tissues, i.e., externally by parietal peritoneum and internally by visceral peritoneum.
Body Plan
Animals have three types of body plans. These are:
Cell aggregate plan: The body consists of a cluster or aggregation of cells which have rudimentary differentiation but are not organized into tissues or organs. The cells are specialized, organised into tissues and show division of labour. It is found in coelenterates and flatworms.
Blind sac body plan
A blind sac body plan is characterized by a digestive cavity that has a single aperture, which functions as mouth and anus. The cells are specialized and they have a division of labour. Digestion is both intracellular and extracellular.
Coelenterates are diploblastic animals and possess a blind sac body plan. The body has a single aperture that functions as mouth and anus. The opening is guarded by tentacles which helps to catch the prey. The digestion takes place in the gastrovascular cavity.
Tube Within a Tube Body Plan
Overview of Tube Within A Tube Body Plan. Embryological development: Most of the metazoans possess tubes within the tube body plan and it mainly relies on the development of germ layers and the central cavity coelom.
Segmentation
In some animals, the body is externally and internally divided into segments or somites with a serial repetition of at least some organs.
There are two types of segmentation:
Metameric Segmentation:
A segmentation that simultaneously divides body both externally and internally is called metamerism or metameric segmentation. This kind of segmentation is found in annelids, arthropods and chordates.
Pseudometamerism:
It is found in tapeworm, the body is divisible into parts or segments called proglottides. They develop from the neck but are not embryonic in or T n r a repetition which appears due to repeated budding as known as false segmentation or pseudometamerism.
Notochord
It 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.
Cephalisation
It is the differentiation of head in anterior part of the body. It involves the concentration of nervous tissue and sense organs in the head.
Appendages
The projecting structures of the body that perform specific functions like locomotion, capturing of food, sensation, etc., are called appendages, eg., Wings, fins, limbs, tentacles, parapodia, setae, etc.
Digestive System
Digestive tract is the passage through which food is taken for digestion, absorption and egestion. The digestive tract that has a single opening for both ingestion and egestion is called incomplete digestive tract, eg., in flatwprms and coelenterates.
The digestive tract with two external openings, one for ingestion and other for egestion is called complete digestive tract. It is present in aschelminthes and higher animals.
Respiratory System
Respiration occurs in different ways in different animals:
• The tiny aquatic animals like Amoeba, Hydra, etc., respire through the body surface. This is called body surface respiration.
• Larger aquatic animals have special organs called gills for respiration. This is called branchial respiration, e.g., In prawns, fishes and mussels.
• The land animals respire through lungs. This is called pulmonary respiration. It occurs in frogs, snails, lizards, birds, and mammals.
• Insects have tracheal respiration, which occurs through trachea i.e., an intercommunicating tube through which gas exchange occurs.
• In animals like earthworm, leech, frogs etc., moist skin acts as respiratory surface. This is called cutaneous respiration.
• Scorpions have book lungs and king crabs have book gills for respiration.
• In frog, gas exchange also occurs through the lining of buccopharyngeal cavity. Hence, called buccopharyngeal respiration.
• Frogs have three modes of respiration, i.e., Cutaneous, buccopharyngeal and pulmonary.
Excretory System
1. Excretory system is involved in the removal of nitrogenous waste products from the body of an organism with the help of excretory organs.
2. The excretion occurs in different ways in different organisms. Such as
3. The excretory organs are absent in those organisms where organization level is below the tissue level. Here, each individual cell takes part in excretion.
4. In animals like sponges, coelenterates, all the cells are in contact with water. Excretion occurs by general body surface.
5. In vertebrates, kidneys are the excretory organs.
Based on excretory products animals can be classified into four categories as given below:
• Aminotelic, excretory product is amino acids, e.g., Starfish, Unio, etc.
• Ammonotelic, excretory product is ammonia, e.g., most invertebrates and some molluscs.
• Ureotelic, excretory product is urea, e.g., Cartilaginous fishes, snail, prawn, mammals and aquatic reptiles.
• Uricotelic, excretory product is uric acid, e.g., Insects, terrestrial crustaceans, lizards, snakes, birds etc.
Nervous System
The nervous system is the aggregation of nerve cells that help in coordinating and controlling various activities of the body.
Endocrine System
The endocrine glands are also called ductless glands. These secrete hormones. Endocrine glands occur in all vertebrates and in some invertebrates (like insects).
Sensory System
This system consists of specialized cells, tissues and organs which can pick up a stimulus and transmit the same to the nervous system.
Sensory system consists of different structures in different organisms, e.g., Antennal (tactile and smell), tentacles (tactile), skin (tactile), statocyst (balancing), ear (hearing), olfactory epithelium (smell), taste buds (taste), eyes (vision), lateral line organs (current receptors), etc.
Skeletal System
Skeletal system is a hard, internal or external framework that provides support and shape to the body. Some animals which are devoid of a skeleton have soft body, e.g., Platyhelminthes, aschelminthes, annelida.
Skeleton system can be of following types:
Exoskeleton:
It is the hard supporting and protective framework present on exterior of the body. It is made of non-living matter, e.g., External shells of molluscs, cuticle of arthropods, scales of fishes and reptiles, feathers of birds, hair, hoofs, nails, horns and claws of mammals.
Endoskeleton:
It is a hard supporting framework present in the interior of the body. In invertebrates such as sponges, it is made up of calcareous or siliceous spicules. In vertebrates, it is composed of hard living tissues called cartilages and bones. Endoskeleton supports whole body of an organism.
Sex
Animals generally have sex organs to produce sexual reproduction. When both male and female sex organs are found in some individual, it is called hermaphrodite or bisexual or monoecious, e.g., Liver fluke, tapeworm, earthworm, leech, etc.
The animals with either female or male sex organ is known as unisexual or dioecious, e.g., Frog, lizards, birds, dog, etc. When male and female can be distinguished on the basis of external features, the condition is called sexual dimorphism, e.g., Lion and lioness, man and women, peacock and peahen, etc.
Reproduction
Reproduction in organisms can be either asexual or sexual.
Asexual Reproduction:
This kind of reproduction does not involve fusion of gametes. It is found in lower animals like sponges, coelenterates, annelids, platyhelminthes. The common methods are budding, fission, fragmentation and regeneration.
Sexual Reproduction:
It involves formation and fusion of gametes. The male gametes called sperms are motile while, the female gametes called ova are generally non-motile.
Fertilization
External Fertilization:
Animals such as many invertebrates, some marine fishes and most amphibians, shed both eggs and sperms into water, where fertilization and development occur. This is called external fertilization.
Internal Fertilization:
In land animals and some aquatic animals, the sperms are introduced by the male into reproductive tract of female during copulation. This is called internal fertilization.
Fertilization occurs in the genital organs of the female.
Oviparous Animals
Egg-laying animals are called oviparous animals. In these animals’ fertilization is internal, but embryonic development is external. In external embryonic development, the embryo develops outside the female body. In these animals, the females lay eggs, in which embryonic development takes place. The developing embryo gets nutrition from the reserve food. This condition is called oviparity. The young ones hatch out of eggs after full development.
Viviparous Animals
Animals that give birth to young ones are called viviparous animals. In these animals both fertilization and embryonic development are internal. In internal embryonic development, the embryo develops inside the female body. The developing embryos get total nutrition and oxygen from the mother. This condition is called matrotrophy. In these animals, the young ones are delivered out of the female womb after full development.
Examples of viviparous animals are a few snakes and mammals except duckbill platypus and echidna.
Non chordate
• They are cylindrical, triploblastic, coelomate, or pseudocoelomate animals.
• Respiration in these animals takes place through gills, trachea or body surface.
• Most of the times, sexes cannot be distinguished among the members.
• Modes of reproduction involve sexual and asexual Fertilization is external, though internal fertilization also occurs in.
• The body of non-chordates generally includes an open type of circulatory system.
Characteristic Features of Phylum Porifera:
1. They are generally marine aquatic organisms, with a few freshwater species.
2. Their bodies are asymmetrical.
3. Body shape can be cylindrical, vase-like, rounded or sac-like.
4. They are diploblastic animals with two layers, the outer dermal layer and the inner gastral layer. There is a gelatinous, non-cellular mesoglea, in between these two layers.
5. The body has many pores called the ostia and a single large opening called osculum at the top.
Class Common Name of Some Sponges
Biological Name Common Name
| Biological Name | Common Name |
| Sycon | Crown Sponge |
| Spongilla | Freshwater Sponge |
| Euplectella | Venus flower basket |
| Euspongia | Bath Sponge |
| Cliona | Boring Sponge |
| Pheronema | Bowl Sponge |
| Haliclona | Finger Sponge |
Phylum Coetentcrata (Cnidana)
There are about 9000 species of cnidarians. The name Cnidaria (Knide nettle or sting cells) is derived from the stinging cell or cnidoblasts present on the ectoderm of tentacles and body of these animals.
Advancement Over Sponges
The cnidarians or coelenterates exhibit advancement over sponges, as they possess a tissue level of organization of the body with well defined layers of cells and a digestive cavity.
Phylum Ctenophora
Phylum-Ctenophora (Ketene comb; pores bearing) or comb jellies or sea walnuts are exclusively marine forms. The term ‘Ctenophora’ was coined by Georges Cuvier. It includes about 50 species.
General Features important general features ‘phylum Ctenophora are below:
• Habitat and Habit These are of exclusively marine forms. They are found solitary; pelagic or free swimming.
• Body Organization They are diploblastic, acoelomate with tissue grade of organization. Body is soft, delicate, transparent and gelatinous, like jelly fishes without segmentation.
• Body Symmetry They are biradially symmetrical. The arrangement of comb plates gives the appearance of radial symmetry, the tentacles and branching of gastrovascular canals show bilateral symmetry.
• Digestive System Digestion is both extra cellular and intracellular. Skeletal, circulatory, respiratory and excretory systems are absent.
Phylum Platyhelminthes
Phylum Platyhelminthes (Platy flat; helminth worm) include flatworms. The group include the first simplest triploblastic group of animals. Gegenbaur coined the term ‘Platyhelminthes’. It includes about 12,000 species of animals. They have leaf like or ribbon like body.
General Features:
Some important general features of phylum-Platyhelminthes are discussed below:
• Habit and Habitat Majority of forms are parasitic (tapeworms, liver flukes, blood flukes), etc., and free-living forms (planarians). Hooks and suckers are present in parasitic forms.
• Symmetry The body is bilaterally symmetrical with definite orientation like anterior and posterior end.
• Cephalization Primitive cephalization is present in free-living flatworms.
• Germ Layers They are triploblastic.
• Body Cavity They are acoelomate. The space between the body wall and body organ is filled by.
Phylum Aschelminthes
Phylum Aschelminthes or Nemathelminthes or Nematoda (Nema thread; helminth worms) includes roundworms. They are commonly called as nematodes.
General Feature of Phylum Aschelminthes:
Some important general features of phylum- aschelminthes are discussed below:
• Habitat and Habit They are mostly free living and may occur in water or within the soil. There are several parasitic species which live within the body of animals or plants, e.g., Guinea worms, whipworms, eyeworms, etc.
• Symmetry They show bilateral symmetry and have organ system level of organization.
• Germ Layers They are triploblastic animals and have tube within tube body plan.
• Body Walt The body wall contains an outer cuticle, syncytial epidermis and a muscle layer. Circular muscles are absent.
Advancement Over Flatworms
Aschelminthes show advancement over flatworms as they contain complete alimentary canal and sexes are separate.
Disease Caused by Aschelminthes:
• Ascaris lumbricoides or giant intestinal roundworm is an endoparasite of the small intestine of humans. It causes ascariasis.
• Wuchereria (filaria) or filarial worm is an endoparasite in the lymphatic vessels and lymph nodes of humans. It causes elephantiasis in the legs, arms, scrotum, etc.
• Ancyclostoma duodenale or hookworm is an endoparasite in the small intestine of humans. It causes ancylostomiasis disease.
• Loa loa the eyeworm lives in subdermal connective tissue of man. It causes loiasis disease characterised by subcutaneous smelling mosdy around the eyes.
Phylum Annelida
Phylum—Annelida (Annulus — ring; lidos — form) includes segmented worms. The & term ‘Annelida’ was first coined by Lamarck (1809). It includes about 12,000 species of animals.
General Features of Phylum Annelida:
• Habit and Habitat They may be aquatic, terrestrial and free-living or parasitic.
• Body Wall The outermost covering of body is thin and moist cuticle secreted by the epidermis.
• Metamerism The body is divided into segments or metameres by ring like grooves-the annuli. It is called metameric segmentation. The segmentation is external as well as internal.
• Symmetry Annelids are bilaterally symmetrical.
Phylum Arthropoda
Phylum-Arthropoda (Arthron – jointed; podos — foot) include the first and simplest segmented animals. These are commonly called as jointed legged animals. It is the largest group of animals that include about 1,000,000 insect species, 1,02,248 spiders and scorpion species, 1,03,248 arachnoid species and 47,000 crustacean species representing about 80% of total known animal species.
Some important general features of phylum Arthropoda are discussed below:
• Habitat and Habit They may be aquatic or terrestrial. They may occur as free-living or parasitic forms e.g., Bed bugs, ticks, mosquitoes, etc.
• Body Parts Body is segmented externally. It has distinct head, thorax and abdomen. Head bears many fused segments and sense organs.
• Symmetry and Body Organization Arthropods are bilaterally symmetrical. They are triploblastic with organ system level of organization.
• Appendages They have jointed, paired appendages, which are present in some or all somite’s or segments. These perform various functions like walking, clinging, jumping, feeding, etc.
Phylum Mollusca
The phylum—Mollusca (Molluscs — soft bodied) includes the soft bodied, unsegmented, acoelomate animals. These are called molluscs or shelled animals. Johnston (1650) coined. the term ‘Mollusca’. Mollusca is the second largest animal phylum and includes about 85,000 species. The study of molluscs is called ‘Malacology’.
Some important general features of phylum mollusca are discussed below:
• Habit and Habitat Molluscs are mostly of marine forms (Sepia, Octopus, Chiton, etc.) Some are freshwater (e.g., Unio and Pila) and some are also terrestrial forms (e.g., Land snails). Few molluscs are parasites also, e.g., Glochidium larva, etc.
• Symmetry These are generally bilaterally symmetrical and some are asymmetrical due to torsion or twisting during growth.
• Germ Layers and Organization They are triploblastic and possess organ system level of organization.
• Body Form They have unsegmented, soft body covered by a calcareous shell, which is differentiated into head, muscular foot and visceral hump.
Phylum Echinodermata
Phylum—Echinodermata (Echinos—spines; derma—skin) includes the spiny skinned animals, which are exclusively marine. Jacob Klein (1734) coined the term ‘Echinodermata’. It includes about 6,000 species.
Some important general features of phylum—Echinodermata are discussed below:
• Habit and Habitat These are marine forms and are bottom dwellers.
• Symmetry The adults have radial (pentamerous) symmetry but, the larval forms have bilateral symmetry.
• Germ Layers and Organization They are triploblastic and exhibit organ system grade of organization.
• Head It is absent in echinoderm and body also lacks segmentation.
Phylum Hemichordate
Hemichordate (Hemi – half; chordate- notochord) was earlier placed as a sub-phylum under Phylum-Chordata. But, now it is considered as a separate phylum under Non-chordate. These are also called half chordates. This phylum consists of a small group of worm-like animal.
Some important general features of phylum—hemichordate are discussed below:
• Habit and Habitat They are exclusive marine and mostly live in burrows.
• Symmetry and Body Organization They are bilaterally symmetrical and triploblastic. They have organ system level of organization.
• Body Form They are soft-bodied, cylindrical and unsegmented body is divisible into proboscis, collar and trunk. The body cavity is true coelom. A true notochord is absent.
• Digestive Systems Digestive system is complete.
The subphylum Vertebrata is further divided as follows:
Class Cyclostomata
• Cyclostomata are ectoparasites on some fishes.
• Elongated body & The presence of 6-15 pairs gills sites.
• Cyclostomes have a sucking and circular mouth without jaws.
• Scales on body and paired fins.
• Circulation is of closed type.
• They are marine but migrate for spawning to fresh water.
Examples: Petromyzon (Lamprey) and Myxine (Hagfish).
Class – Chondrichthyes
• Marine and streamlined body endoskeleton is cartilaginous.
• The skin is tough ventrally mouth gills are separated.
• Heart is two-chambered one auricle and one ventricle.
• They are Cold blooded, Sexes are separate & having internal fertilization many of them are viviparous.
Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).
Class – Osteichthyes
• Both marine and fresh water fishes with bony endoskeleton.
• Body is streamlined, terminal mouth, four pair of gills.
• Skin is covered with cycloid scales.
• Heart is two-chambered one auricle and one ventricle.
• cold-blooded animals, Sexes are separate, having external fertilization & 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
• They can live in aquatic and terrestrial habitats, two pair of limbs.
• Body divide into head and trunk, tail may be present in some, skin is moist without scales, A tympanum represents the ear.
• Alimentary canal, urinary and reproductive tracts open into a common chamber called cloaca.
• Respiration by gills, lungs and through skin, heart is three chambered two auricles and one ventricle.
• Cold blooded, 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
• The mode of locomotion is creeping or crawling terrestrial animals body is covered by dry skin, epidermal scales or scutes.
• Do not have external ear openings Tympanum represents ear, Limbs when present is two paired, heart is three chambered but four chambered in crocodiles.
• Snakes and lizard’s scales on skin, 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), Bangarus (Krait), Vipera (Viper).
Class – Aves
• Aves (birds) are the presence of feathers, They possess beak forelimbs are modified into wings, hind limbs have scales and are modified for walking, swimming or clasping.
• Endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic). digestive tract of birds has additional chambers crop and gizzard.
• Heart is four chambered, warm-blooded respiration by lungs Sexes are separate. Fertilization is internal. They are oviparous and development is direct.
Examples: Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).
Class – Mammalia
• Presence of mammary glands milk producing glands by which young ones is nourished, two pairs of limbs, Skin is hairy, External ears or pinnae are present, Heart is four chambered, They are homoiothermous.
• Respiration is by lungs, Sexes are separate and fertilization is internal. They are 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).