Study of External Features of Animals

Our Objective

To study some selected animals on the basis of their external features. 


The Theory

Classification of animals




They are known as acellular or non-cellular organism. A protozoan body consists of only mass of protoplasm, so they are called acellular or non-cellular animals. They are mostly aquatic, either free living or parasitic or commensal. Single cell performs all the vital activities thus the single cell acts like a whole body. Body of protozoa is either naked or covered by a pellicle.  

The locomotory organ are pseudopodia (false foot) or cilia or absent. The digestion is intracellular, occurs in food vacuoles. 


This animal species is regarded as belonging to the oldest animal species. They go by the name Sponges as well. These multicellular creatures are by far the simplest. Despite having several cells, they lack any tissues or organs. Since they must have a close relationship with water, sponges require an aquatic environment to survive.  

When it comes to nutrition, gas exchange, and excretion, water is crucial. The sponges' bodies have numerous ostia, often known as pores or holes. Sponge body structures are made to allow water to flow through the body, where it may filter out food and absorb dissolved oxygen while removing waste.  

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

Their bodies are not symmetrical, and their shape has been modified to maximize the efficiency of water flow via the inside central hollow.  


They are radially symmetrical, aquatic, primarily marine, sessile, or free-swimming organisms. The cnidoblasts or cnidocytes found on the tentacles and the body are where the name "cnidaria" comes from. Cnidoblasts are employed for prey capture, defense, and anchoring. Cnidarians are diploblastic and have tissue-level organization. They have a single aperture, the mouth on the hypostome, in the center of their gastro-vascular cavity.  

Digestion occurs both within and outside of cells. The latter is an umbrella-shaped, free-swimming organism like Aurelia or jelly fish, and the former is a sessile, cylindrical form like Hydra, Adamsia, etc.  


They are known as flatworms because of their Dorso-ventral flattened bodies. Most of them are endoparasites, which are present in both humans and other animals. Flatworms are triploblastic, acoelomate, bilaterally symmetrical organisms with organ-level organization. There are hooks and suckers in the parasitic forms. Some of them immediately absorb nutrients from the host through the surface of their bodies.  

Specialized cells known as flame cells help in excretion and osmoregulation. The sexes are not distinct. Internal fertilization occurs, and numerous larval stages are involved in development. Planaria is one of the members with a strong ability for regeneration. 


The aschelminthes body has a round cross section, they are known as "roundworms." They could be parasites on plants and animals or free-living, aquatic, and terrestrial organisms. The organ-system level of body organization is present in roundworms. They are triploblastic, pseudocoelomate, bilaterally symmetrical creatures. The pharynx has a fully formed, muscular alimentary canal. Through the excretory hole, an excretory tube transfers bodily wastes from the body cavity. Males and females are unique because the sexes are separate (dioecious). Often, women are taller than men. 


They can be free-living, occasionally parasitic, aquatic (marine and freshwater), or terrestrial. They exhibit bilateral symmetry and organ-system level body organization. They are coelomate animals that are triploblastic and metamerically segmented.  

Since their body surface is clearly divided into segments or metameres, the phylum name Annelida was given to them. They have circular and longitudinal muscles that aid in movement. Aquatic annelids like Nereis have parapodia, which are lateral appendages that aid in swimming. There is a closed circulation system in place.  

Nephridia, also known as nephridium, aid in excretion and osmoregulation. The neural system is made up of two double ventral nerve cords and two paired ganglia. Earthworms and leeches are monoecious, while Nereis, an aquatic variety, is dioecious. Sexual reproduction 


This is the largest phylum of Animalia which includes insects. Arthropods make up more than two-thirds of all identified species on earth. They are organised at the organ-system level. They are segmented, coelomate, bilaterally symmetrical, triploblastic, and creatures.  

Arthropods have a chitinous exoskeleton covering their bodies. The head, thorax, and abdomen make up the body. Their appendages are joined. Gills, book gills, book lungs, or tracheal system are examples of respiratory organs. An open circulatory system is used.  

These include sensory organs like antennae, compound eyes and simple eyes, statocysts, and balance organs. Malpighian tubules are used for excretion. Most of them are dioecious. They reproduce through internal fertilisation.  

Examples include economically significant insects such as the honeybee and the silkworm. 


The second-largest animal phylum is this one. Mollusca have an organ-system level of organisation and can be terrestrial or aquatic (marine or freshwater). They are coelomate, triploblastic, and bilaterally symmetrical creatures. Body is unsegmented and covered in a calcareous shell. It has a distinct head, a muscular foot, and a visceral hump.  

The visceral hump is covered by a spongy skin. The mantle cavity, which contains gills that resemble feathers, is the area between the hump and the mantle. They perform respiration and excretion.  

Sensory tentacles are situated in the anterior head region. The mouth contains a file-like rasping organ for feeding, called radula. 


These animals have an endoskeleton of calcareous ossicles so called as Echinodermata. They are marine with organ-system level of organisation. The adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical.  

They are triploblastic and coelomate animals. Digestive system is complete with mouth on the lower side and anus on the upper side. The most distinctive feature of echinoderms is the presence of water vascular system which helps in locomotion, capture and transport of food and respiration. An excretory system is absent. Sexes are separate.  


Animals belonging to phylum Chordata are characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. These are bilaterally symmetrical, triploblastic, coelomate with organ-system level of organisation. They possess a post anal tail and a closed circulatory system 

Class – Chondrichthyes  

They are marine animals with streamlined body and have cartilaginous endoskeleton. Mouth is located ventrally. Notochord is persistent throughout life. Gill slits are separate and without operculum. The skin is tough, containing minute placoid scales. Teeth are modified placoid scales which are backwardly directed. Jaws are very powerful. Due to the absence of air bladder, they must swim constantly to avoid sinking. Heart is two-chambered. They are cold-blooded animals, i.e., they lack the capacity to regulate their body temperature. Sexes are separate. Male pelvic fins bear claspers. They have internal fertilisation and many of them are viviparous. 

Class – Osteichthyes  

It includes both marine and fresh water fishes with bony endoskeleton. Their body is streamlined. Mouth is mostly terminal. They have four pairs of gills which are covered by an operculum on each side. Skin is covered with cycloid/ctenoid scales. Air bladder is present. Heart is two-chambered. They are cold-blooded animals. Sexes are separate. Fertilisation is usually external. They are mostly oviparous. 

Class – Amphibia  

Amphibians can live in aquatic as well as terrestrial habitats. Most of them have two pairs of limbs. Body is divisible into head and trunk. Tail may be present in some. 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. Sexes are separate. Fertilisation is external. 

Class – Reptilia  

The class name refers to their creeping or crawling mode of locomotion. They are mostly terrestrial animals and their body is covered by dry and cornified skin, epidermal scales. They do not have external ear openings. Tympanum represents ear. Limbs are present. Heart is usually three-chambered, but four-chambered in crocodiles. Reptiles are poikilotherms.  

Class – Aves  

The characteristic features of Aves (birds) are the presence of feathers and most of them can fly except flightless birds (e.g., Ostrich). They possess beak. The forelimbs are modified into wings. The hind limbs 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 bony and the long bones are hollow with air cavities. The digestive tract of birds has additional chambers, the crop and gizzard. Heart is completely four-chambered.  

They are warm-blooded animals, i.e., they can maintain a constant body temperature. Respiration is by lungs. Air sacs connected to lungs supplement respiration. Sexes are separate. Fertilisation is internal. They are oviparous and development is direct. 

Class – Mammalia  

They are found in a variety of habitats – polar ice caps, deserts, mountains, forests, grasslands, and dark caves. Some of them have adapted to fly or live in water. The most unique mammalian characteristic is the presence of milk producing glands (mammary glands) by which the young ones are nourished.  

They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming, or flying. The skin of mammals is unique in possessing hair. External ears or pinnae are present.  

Different types of teeth are present in the jaw. Heart is four-chambered. Respiration is by lungs. Sexes are separate and fertilisation is internal. They are viviparous with few exceptions and development is direct. 

Learning Outcomes  

  • Students understand about the classification of the animal kingdom.
  • Students understand the concept of external features of animals according to their phylum.
  • Students understand the habitat for the specific animals.