Animal Kingdom Notes- Circulatory System, Digestive System

In this world, there are millions of living organisms, and there are even a few that have yet to be discovered. Animals, plants, microbes, and other living entities that are known to man are included in this category. They’re all really diverse. However, there are some qualities that are shared by groups of creatures, and this is what connects them.

Animal classification allows us to better grasp their qualities and distinctions from other organisms. Animals are the most well-known organisms. They are classified as part of the Kingdom Animalia, sometimes known as the Animal Kingdom, in scientific terms. Despite differences in structure and shape across animals, there are essential similarities among them in terms of cell organization, body symmetry, coelom type, and digestive, circulatory, and reproductive system patterns. These characteristics are used to categorize animals. Some of them are as follows :

Basis Of Classification

According To Symmetry 

Radial Symmetry – 

Radial symmetry occurs when any plane traveling through the body’s central axis divides the organism into two identical halves.

Echinoderms, coelenterates, and ctenophores

Bilateral Symmetry – 

Bilateral symmetry is found in animals such as annelids, arthropods, and others whose bodies can be divided into identical left and right halves in just one plane.

Asymmetrical – 

Sponges are generally asymmetrical, meaning that any plane passing through the center does not split them in half evenly.

According To Circulatory System: 

There are two types of circulatory systems: open and closed.

Open circulatory system – In this type of system, the blood is pushed out of the heart in an open type circulatory system. for example, Mollusca and arthropods.

Closed Circulatory System: Blood flows via a series of vessels called capillaries, arteries, and veins in this type of circulatory system.

According To The Digestive System

The digestive system is divided into two categories. There are two types of digestive systems: complete and incomplete.

Complete Digestive System: There are two entrances to the outside of the body in this sort of digestive system, a back-end, and a mouth, for example. Chordates and arthropods.

Incomplete Digestive System: There is only one opening to the outside of the body, which functions as both the back-end and the mouth. Platyhelminthes, for example.

According To Levels Of Organization

Organ level -Animal tissues of similar capacity are categorized into shaped organs at the organ level of organization. Every organ is designated for a specific function. Platyhelminthes, for example.

Tissue level – Animal cells demonstrate active division among themselves at the tissue level of organization.

Tissues are formed when cells that execute the same purpose work together.

Organ framework level  – The organ framework level of organization is seen in organisms where organs establish the shape of functional frameworks, and each framework has its physiological potential.

Cellular level – The cellular level of organization is made up of animals with cells lumped together as free cells.


Oliveboard Mobile App
  • Video Lessons, Textual Lessons & Notes
  • Topic Tests covering all topics with detailed solutions
  • Sectional Tests for QA, DI, EL, LR
  • All India Mock Tests for performance analysis and all India percentile
  • General Knowledge (GK) Tests

Free videos, free mock tests, and free GK tests to evaluate course content before signing up!

According To Layers

Animals in which the cells are arranged in two embryonic layers, external ectoderm, and internal endoderm, are called diploblastic animals, e.g., coelenterates.

 Animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals, e.g., Platyhelminthes to chordates,

According To The Body Cavity

Animals in which the body cavity is lined by mesoderm is called coelom. Animals possessing coelom are called coelomates, e.g., annelids, molluscs, arthropods, echinoderms, hemichordates, and chordates.

In some animals, mesoderm does not line the body cavity of some animals; instead, it appears as scattered pouches in between the ectoderm and endoderm. Such a bodily cavity is called Pseudocoelom, and animals with them are known as pseudocoelomates, such as aschelminthes. The animals in which the body cavity is absent are called acoelomates, e.g., Platyhelminthes.

Classification Of Animals

Animals are classified according to some basic features like

Phylum Porifera.

These are the most basic multicellular animals, mostly found in marine environments. Pores can be found all over the body of these species. They have a canal system that allows water, food particles, and oxygen to circulate. Tissue differentiation and division are modest in the body design. Spongilla, Sycon, and other forms are commonly referred to as Sponges.

Phylum Platyhelminthes

Flatworms are the popular name for these creatures. Their bodies are dorsoventrally flattened. With three germ layers, they are the first triploblastic creatures. The body is also bilaterally symmetrical, with the same design on both the left and right halves. Flatworms can be parasitic or non-parasitic. Planaria, Liver Fluke, and Tapeworm are just a few examples.

Phylum Coelenterata 

These organisms have a more differentiated body. They are aquatic creatures. A sac-like compartment exists within the body, with a single opening for ingesting and egestion. These creatures are known as diploblastic because they contain two germ layers. These creatures can be found living alone or in colonies. Jellyfish, Sea Anemone, and Hydra are some examples.

Phylum Annelida 

Annelids can be found in a variety of environments, including land, freshwater, and even the ocean. They have three germ layers and a body that is bilaterally symmetrical (Triploblastic). They have a real body cavity, which is a unique trait. The body is segmented as well, with organ distinction visible. Earthworms and leeches are two examples.

Phylum Nematoda 

These animals maintain bilateral symmetry and triploblastic nature. On the other hand, the body is more cylindrical and has not been flattened. The coelom in the bodily cavity is not the same as the coelom in the coelom in the coelom. As a result, it’s known as a pseudo coelom. Organs are missing, yet tissues are present. These species have a completely straight alimentary canal. The majority of the species in this phylum are disease-causing parasitic worms. Ascaris and Wucheria are two examples.

Phylum Arthropoda 

They are the most numerous species in the animal kingdom, and this phylum contains the majority of insects. The term “Arthropoda” refers to animals with jointed legs. These creatures’ bodies are separated into three sections: head, thorax, and belly. They have a pair of compound eyes in addition to the jointed legs. The presence of an open circulatory system is another distinctive trait of these creatures. Butterfly houseflies, spiders, mosquitoes, crabs, and other insects are examples.

Phylum Echinodermata 

 Animals with prickly skin are known as echinoderms. They only exist in a maritime environment. They are free-roaming creatures. Adults have radial symmetry, whilst larvae have bilateral symmetry. These creatures have a coelomic cavity and are triploblastic.  Starfish, sea cucumber, and sea urchin are some examples.

Phylum Mollusca 

 These creatures can be found in water environments. They might be freshwater or marine species. The body has little segmentation, and the coelomic cavity has shrunk. The anterior head, ventral muscular foot, and dorsal visceral mass are the most common divisions of the body. The animal’s mobility is aided by the foot. Snails, mussels, and octopuses are examples.

Phylum Vertebrata 

These are the most advanced creatures, with advanced traits such as a proper digestive system, circulatory system, and so on. Body tissues and organs are differentiated in a sophisticated way, and these animals have an internal skeleton and a real vertebral column.

Phylum Protochordata 

The protochordate is triploblastic and bilaterally symmetrical. They’ve got a coelom. The presence of a notochord at some point in their life cycle is a new body feature seen in these creatures. They are known as chordates because of the presence of a notochord. It is, however, sometimes rudimentary. They are only found in the sea. Herdmania and Balanoglossus are two examples.

Frequently asked questions

Which is the biggest class of the Animal Kingdom?

Insects make up more than half of the living organisms, and thus they are the biggest class of the Animal Kingdom.

Which is the second largest phylum of the Animal Kingdom?

Mollusca is the second largest phylum of the Animal Kingdom.

How many species are there in Animal Kingdom?

At present, 1.7 million living organisms are known.