Study of External Features of Animals


To study some selected animals based on their external features. 



Classification of animals

                                                                                 Pic.1- Classification of Animals  



  • Kingdom: Protista 
  • Phylum: Amoebozoa 
  • Class: Tubulinea 
  • Order: Tubulinida 
  • Family: Amoebidae 
  • Genus: Amoeba 
  • Species: Proteus 

                                                     Pic.2 - Amoeba 

Nature of Organism: 

  • They are known as acellular or non-cellular organism.  
  • A protozoan body consists of only a protoplasm mass, called acellular or non-cellular animal. 


  • They are primarily aquatic, either free-living parasitic or commensal. 

Cellular Structure:  

  • A single cell performs all the vital activities; thus, the single cell acts like a whole body. 

Body Covering:  

  • The body of protozoa is either naked or covered by a pellicle. 


  • The locomotory organs are pseudopodia (false foot) or cilia or absent. 


  • The digestion is intracellular and occurs in food vacuoles.  



Kingdom  Animalia Animalia Animalia
Phylum Porifera Porifera Porifera
Class Calcarea Demospongia Demospongia
Order Leucosolenida Dictyoceratida Haplosclerida
Family Sycettidae Spongiidae Spongillidae
Genus Sycon Euspongia Spongilla
Species Sycon ciliatum Spongia officinalis Spongilla lacustris

       Pic.3 - Examples of Porifera 

Oldest Animal Species: 

  • Sponges are regarded as one of the oldest animal species. 
  • They are commonly known as Sponges. 


  • Despite being multicellular, they are among the simplest creatures without any tissues or organs. 


  • Sponges' close relationship with water requires an aquatic environment to survive. 

Dependency on Water: 

  • Water is crucial for nutrition, gas exchange, and excretion. 

Body Structure: 

  • The bodies of sponges have numerous ostia, also known as pores or holes. 
  • Sponge body structures enable water to flow through the body, filtering out food and absorbing dissolved oxygen while removing waste. 

Lack of Specialized Systems: 

  • This phylum of organisms lacks specialized neurological, circulatory, or digestive systems. 
  • Instead, they possess a water transportation or canal system that handles gas exchange, digestion, and excretion. 

Body Symmetry: 

  • The bodies of sponges lack symmetry and have a modified shape to maximize the efficiency of water flow through the inside central hollow. 



  • Kingdom: Animalia 
  • Phylum: Cnidaria 
  • Class: Hydrozoa 
  • Order: Anthoathecata 
  • Family: Hydridae 
  • Genus: Hydra 
  • Species: Hydra vulgaris 

                                         Pic.4 - Hydra 


  • Cnidaria, also known as Coelenterata, comprises a diverse group of aquatic organisms. 


  • They are characterized by specialized cells called cnidocytes, containing stinging structures known as nematocysts. 
  • Radially symmetrical aquatic organisms are primarily found in marine environments. 
  • They can be either sessile or free-swimming. 


  • Primarily marine, but some are freshwater. 
  • Sessile or free-swimming. 


  • Cnidoblasts or cnidocytes, found on tentacles and the body, give the phylum its name. 
  • Cnidocytes are used for prey capture, defence, and anchoring. 

Body Organization- Tissue-Level Organization 

  • Cnidarians are diploblastic and exhibit tissue-level organization. 

Gastro-Vascular Cavity: 

  • Single aperture, the mouth, on the hypostome in the centre of the gastro-vascular cavity. 

Representative Organisms: 

  • The phylum includes familiar organisms such as jellyfish, corals, and sea anemones. 

Life Cycle: 

  • Cnidarians exhibit two primary body forms during their life cycle: the polyp and the medusa. 
  • The polyp form is typically cylindrical, with a mouth surrounded by tentacles facing upward, and it is generally sessile, attaching to substrates like rocks or coral reefs. 
  • In contrast, the medusa form is bell-shaped and free-swimming, with tentacles hanging down. 
  • Cnidarians display a unique alternation of generations, where polyps asexually produce medusae, and medusae sexually produce polyps. 


  • Digestion occurs both within and outside of cells. 


  • Medusa form examples include Aurelia (jellyfish). 
  • Examples of polyp forms include Hydra, Adamsia, etc. 



  • Kingdom: Animalia 
  • Phylum: Platyhelminthes 
  • Class: Trematoda 
  • Order: Plagiorchiida 
  • Family: Fasciolidae 
  • Genus: Fasciola 
  • Species: Fasciola hepatica 

                     Pic.5 - Liver fluke 

Body Form: 

  • Flatworms are called so due to their Dorso-ventrally flattened bodies. 

Parasitic Nature: 

  • Most flatworms are endoparasites found in both humans and other animals. 

Body Characteristics: 

  • Flatworms are triploblastic, acoelomate, and bilaterally symmetrical organisms with organ-level organization. 
  • Parasitic forms often have hooks and suckers. 

Nutrient Absorption: 

  • Some flatworms absorb nutrients directly from the host through the surface of their bodies. 

Excretion and Osmoregulation: 

  • Specialized cells, known as flame cells, aid in excretion and osmoregulation. 

Reproductive Features: 

  • The sexes are not distinct in flatworms. 
  • Internal fertilization occurs. 
  • Numerous larval stages are involved in the development process. 

Regeneration Ability: 

  • Planaria, a member of flatworms, is known for its strong ability to regenerate. 



  • Kingdom: Animalia 
  • Phylum: Nematoda 
  • Class: Secernentea 
  • Order: Ascaridida 
  • Family: Ascarididae 
  • Genus: Ascaris 
  • Species: Ascaris lumbricoides 

                                                                                                         Pic.6 - Roundworm 

Body Shape: 

  • The body of Aschelminthes, known as roundworms, has a round cross-section. 

Ecological Diversity: 

  • Aschelminthes, or roundworms, can be either parasites on plants and animals or free-living in aquatic and terrestrial environments. 

Body Organization: 

  • Roundworms exhibit organ-system level body organization. 
  • They are triploblastic, pseudocoelomate, and bilaterally symmetrical creatures. 

Alimentary Canal: 

  • The pharynx in roundworms features a fully formed, muscular alimentary canal. 

Excretory System: 

  • An excretory tube transfers bodily waste from the body cavity through the excretory hole. 

Reproductive Features: 

  • Males and females are unique in roundworms as the sexes are separate (dioecious). 
  • Females are often larger than males. 



  • Kingdom: Animalia 
  • Phylum: Annelida 
  • Class: Clitellata 
  • Order: Haplotaxida 
  • Family: Megascolecidae 
  • Genus: Pheretima 
  • Species: Posthuma 

                                      Pic.7 - Earthworm 

Ecological Diversity: 

  • Annelids can be free-living, occasionally parasitic, aquatic (marine and freshwater), or terrestrial. 

Body Organization: 

  • They exhibit bilateral symmetry and organ-system-level body organization. 
  • Annelids are coelomate animals, triploblastic and metamerically segmented. 


  • The phylum name Annelida is derived from the fact that their body surface is divided into segments or metameres. 

Muscular System: 

  • Annelids have circular and longitudinal muscles that aid in movement. 
  • Aquatic annelids like Nereis have parapodia lateral appendages that aid in swimming. 

Circulatory System: 

  • Annelids have a closed circulation system. 

Excretion and Osmoregulation: 

  • Nephridia, also known as nephridium, aids in excretion and osmoregulation. 

Neural System: 

  • The neural system comprises two double ventral nerve cords and two paired ganglia. 

Reproductive Features: 

  • Earthworms and leeches are monoecious (having both male and female reproductive organs). 
  • Nereis, an aquatic variety, is dioecious (having separate sexes). 
  • Reproduction is sexual. 



Kingdom  Animalia Animalia
Phylum Arthropoda Arthropoda
Class Insecta Malacostraca
Order Hymenoptera Decapoda
Family Apidae Penaeidae
Genus Apis Penaeus
Species Mellifera Indicus

                           Pic.8 - Honey bee 

                                             Pic.9 - Prawn 

Phylum Characteristics: 

  • Arthropoda is the largest phylum in Animalia, including insects. 
  • Arthropods make up more than two-thirds of all identified species on Earth. 
  • They are organized at the organ-system level. 

Body Characteristics: 

  • Arthropods are segmented, coelomate, bilaterally symmetrical, and triploblastic creatures. 
  • They have a chitinous exoskeleton covering their bodies. 
  • The body is divided into the head, thorax, and abdomen, with joined appendages. 

Respiratory Organs: 

  • Respiratory organs can include gills, book gills, book lungs, or a tracheal system. 
  • They have an open circulatory system. 

Sensory Organs: 

  • Arthropods have sensory organs like antennae, compound eyes, simple eyes, statocysts, and balance organs. 


  • Malpighian tubules are used for excretion. 

Reproductive Features: 

  • Most arthropods are dioecious (having separate sexes). 
  • They reproduce through internal fertilization. 


  • Examples of economically significant insects include the honeybee and the silkworm. 



  • Kingdom: Animalia 
  • Phylum: Mollusca 
  • Class: Gastropoda 
  • Order: Architaenioglossa 
  • Family: Ampullariidae 
  • Genus: Pila 
  • Species: Globosa 


                                    Pic.10 - Apple snail 

Phylum Characteristics: 

  • Mollusca is the second-largest animal phylum. 
  • It has an organ-system level of organization and can be found in terrestrial or aquatic environments (marine or freshwater). 

Body Characteristics: 

  • Molluscs are coelomate, triploblastic, and bilaterally symmetrical creatures. 
  • The body is unsegmented and covered in a calcareous shell. 
  • It has a distinct head, a muscular foot, and a visceral hump. 

Visceral Hump and Mantle Cavity: 

  • A spongy skin covers the visceral hump. 
  • The mantle cavity, between the hump and the mantle, contains gills resembling feathers for respiration and excretion. 

Sensory Organs: 

  • Sensory tentacles are situated in the anterior head region. 

Feeding Organ: 

  • The mouth contains a file-like rasping organ called a radula for feeding. 



  • Kingdom: Animalia 
  • Phylum: Echinodermata 
  • Class: Asteroidea 
  • Order: Forcipulatida  
  • Family: Asteriidae  
  • Genus: Asterias  
  • Species: Asterias rubens 

                                               Pic.11 - Starfish 

Phylum Characteristics: 

  • Echinodermata is the phylum of animals with an endoskeleton made of calcareous ossicles. 

Habitat and Organization: 

  • Echinoderms are marine animals with an organ-system level of organization. 
  • Adult echinoderms are radially symmetrical, while larvae are bilaterally symmetrical. 

Body Characteristics: 

  • They are triploblastic and coelomate animals. 
  • The digestive system is complete, with the mouth on the lower side and the anus on the upper side. 

Water Vascular System: 

  • The most distinctive feature of echinoderms is the presence of a water vascular system or ambulacral system. 
  • This system aids in locomotion, respiration, capture, and transport of food. 

Excretory System and Reproduction: 

  • Echinoderms do not have an excretory system. 
  • The sexes in echinoderms are separate. 



Phylum Chordata Characteristics: 

  • Animals in the phylum Chordata are characterized by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits. 
  • They are bilaterally symmetrical, triploblastic, and coelomate with an organ-system level of organization. 
  • Chordates possess a post-anal tail and a closed circulatory system. 

Class Chondrichthyes: 

  • Kingdom: Animalia 
  • Phylum: Chordata 
  • Class: Chondrichthyes 
  • Order: Lamniformes 
  • Family: Lamnidae 
  • Genus: Carcharodon 
  • Species: Carcharodon carcharias 

                                                        Pic.12 - Shark 

  • Chondrichthyes are marine animals with a streamlined body and a cartilaginous endoskeleton. 
  • The mouth is located ventrally. 
  • The notochord is persistent throughout life. 
  • Gill slits are separate and lack an operculum. 
  • The skin is tough, containing minute placoid scales. 
  • Teeth are modified placoid scales that are backwardly directed. 
  • They have powerful jaws. 
  • Due to the absence of an air bladder, they must swim constantly to avoid sinking. 
  • The heart is two-chambered. 
  • Chondrichthyes are cold-blooded animals, lacking the capacity to regulate their body temperature. 
  • Sexes are separate, with males having pelvic fins bearing claspers. 
  • They undergo internal fertilization, and many of them are viviparous. 


Class Osteichthyes: 

  • Kingdom: Animalia 
  • Phylum: Chordata 
  • Class: Actinopterygii 
  • Order: Cypriniformes 
  • Family: Cyprinidae 
  • Genus: Labeo 
  • Species: Catla 


                                                  Pic.13 - Catla 

General Characteristics: 

  • Osteichthyes include both marine and freshwater fishes with a bony endoskeleton. 
  • Their body is streamlined for efficient swimming. 
  • The mouth is mostly terminal, located at the anterior end of the body. 

Gills and Operculum: 

  • They have four pairs of gills covered by an operculum on each side of their bodies for efficient oxygen extraction from water. 

Skin and Scales: 

  • The skin is covered with cycloid or ctenoid scales, providing protection. 

Air Bladder: 

  • Osteichthyes possess an air bladder, aiding in buoyancy control. 

Circulatory System: 

  • The heart is two-chambered, consisting of an atrium and a ventricle. 


  • Osteichthyes are cold-blooded animals, meaning they lack the ability to regulate their body temperature internally. 

Reproductive Features: 

  • Sexes are separate in Osteichthyes. 
  • Fertilization is usually external, occurring outside the body. 
  • They are mostly oviparous, with eggs hatching outside the parental body. 


Class Amphibia:  

  • Kingdom: Animalia 
  • Phylum: Chordata 
  • Class: Amphibia 
  • Order: Anura 
  • Family: Ranidae 
  • Genus: Rana 
  • Species: Rana temporaria 


                                                     Pic.14 - Frog 

Habitat and Limbs: 

  • Amphibians can live in both aquatic and terrestrial habitats. 
  • Most amphibians have two pairs of limbs. 

Body Structure: 

  • The body is divisible into a head and trunk. 
  • Some amphibians may have a tail. 

Skin Characteristics: 

  • Amphibian skin is moist and lacks scales. 

Sensory Organs: 

  • Eyes have eyelids. 
  • A tympanum represents the ear. 


  • The alimentary canal, urinary, and reproductive tracts open into a common chamber called the cloaca, which opens to the exterior. 


  • Respiration occurs through the gills, lungs, and the skin. 

Circulatory System: 

  • The heart is three-chambered, consisting of two auricles and one ventricle. 


  • Amphibians are cold-blooded animals. 

Reproductive Features: 

  • Sexes are separate. 
  • Fertilization is external, occurring outside the body. 


Class Reptilia:  

  • Kingdom: Animalia 
  • Phylum: Chordata 
  • Class: Reptilia 
  • Order: Squamata 
  • Family: Agamidae 
  • Genus: Calotes 
  • Species: Calotes versicolor  


                                                     Pic.15 - Garden lizard  

Locomotion and Terrestrial Habitat: 

  • The class name "Reptilia" refers to their creeping or crawling mode of locomotion. 
  • Reptiles are mostly terrestrial animals. 

Skin Characteristics: 

  • Dry and cornified skin, often with epidermal scales, covers their body. 

Ear and Hearing: 

  • Reptiles do not have external ear openings. 
  • The tympanum represents the ear. 

Limb Presence: 

  • Limbs are present in reptiles. 

Circulatory System: 

  • The heart is usually three-chambered, but in crocodiles, it is four-chambered. 


  • Reptiles are poikilothermic, meaning their body temperature varies with the environment. 


Class Aves: 

  • Kingdom: Animalia 
  • Phylum: Chordata 
  • Class: Aves 
  • Order: Columbiformes 
  • Family: Columbidae 
  • Genus: Columba 
  • Species: Columba livia  

                                         Pic.16 - Pigeon 

Feathers and Flight: 

  • Aves (birds) are characterized by the presence of feathers, and most can fly, except for flightless birds like the ostrich. 
  • They possess beaks, and their forelimbs are modified into wings. 

Hind Limbs and Scales: 

  • The hind limbs have scales modified for walking, swimming, or climbing tree branches. 

Skin and Endoskeleton: 

  • The skin is dry without glands, except for the oil gland at the base of the tail. 
  • The endoskeleton is fully bony, and the long bones are hollow with air cavities. 

Digestive System: 

  • The digestive tract of birds has additional chambers, including the crop and the gizzard. 

Circulatory System: 

  • The heart is completely four-chambered. 


  • Birds are warm-blooded animals, capable of maintaining a constant body temperature. 


  • Respiration is by the lungs, and air sacs connected to the lungs supplement respiration. 

Reproductive Features: 

  • Sexes are separate. 
  • Fertilization is internal. 
  • Birds are oviparous, and development is direct. 


Class Mammalia: 

  • Kingdom: Animalia 
  • Phylum: Chordata 
  • Class: Mammalia 
  • Order: Lagomorpha 
  • Family: Leporidae 
  • Genus: Oryctolagus  
  • Species: cuniculus  

                                                                                                    Pic.17 - Rabbit 

Habitat Diversity: 

  • Mammals are found in various habitats, including polar ice caps, deserts, mountains, forests, grasslands, and dark caves. 
  • Some mammals have adapted to fly or live in water. 

Mammalian Characteristic: 

  • The most unique mammalian characteristic is the presence of milk-producing glands (mammary glands) by which the young ones are nourished. 

Limb Adaptations: 

  • Mammals have two limbs adapted for walking, running, climbing, burrowing, swimming, or flying. 

Skin Characteristics: 

  • The skin of mammals is unique in possessing hair. 
  • External ears or pinnae are present. 

Dental Features: 

  • Different types of teeth are present in the jaw. 

Circulatory System: 

  • The heart is four-chambered. 


  • Respiration is by the lungs. 

Reproductive Features: 

  • Sexes are separate, and fertilization is internal. 
  • Mammals are viviparous with few exceptions, and development is direct. 


Learning Outcomes  

Students understand: 

  • The classification of the animal kingdom. 
  • The concept of external features of animals according to their phylum. 
  • The habitat of specific animals.