Invertebrates | Division of Invertebrates | Phylum


Invertebrates means that animal which doesn’t have vertebral column or backbone. Invertebrates means the animals without vertebral column. Invertebrates are divided into two types Protozoam and Metazoam. Protozoam means unicellular, which are microscopic the don’t consider as true animal. Metazoam, which are multicellular, we can see them with our naked eyes, and they are Eukaryotic.


Invertebrate is further divided into following nine (9) groups and Phylum.

  1. Phylum Protozoa
  2. Phylum Porifera
  3. Phylum Cnidaria
  4. Phylum Platyhelminthes
  5. Phylum Nematoda
  6. Phylum Annelida
  7. Phylum Arthropoda
  8. Phylum Mollusca
  9. Phylum Echinodermata

Also Read: Cell theory and Types of Cell
Also Read: Eukaryotic Cell, Membrane, Characteristics and Functions of Cell Membrane

Phylum Protozoa:

It consists of unicellular (single celled) microscopic organisms. These group of organisms are not considered as true animals. Protozoa are always found in marine water and have cilia for locomotion. They also have two big and small nucleus.

Phylum Porifera:

These are commonly pore bearing animals, Porifera are aquatic, mostly found in shallow water aggregate of cells. Lack of tissues

Many pores are there named ostia use for eating microscopic food from water. They are also called sponges. They are sessile (fixed at one place). The water goes out from body through osculum.

Phylum Cnidaria (Coelenterates)

They are multicellular and aquatic animals. They are diploblastic means having two derm ectoderm and endoderm. There is a fluid like substance between ectoderm and endoderm is mesoglea. These are sessile (coral) and motile (Hydra, Jelly Fish). They have cylindrical and umbrella like shape. Their prey is insects in water.

Phylum Platyhelminthes

They are commonly called flat worms, the body of these livings is flat, and these are triploblastic animals, which means animals having three derms ecto, meso and endo. the size of these animals is above 3cm. These are universal, mostly infects the cattle. Due to absent of mouth and anus they absorb food from the host.

Read in detail: Chordata | vertebrate

Phylum Nematoda

They are called round worms; the body of these animals is cylindrical. It is pointed at both ends. They are long and smooth, and they have straight digestive tube. These are free living mostly parasitic, found in body of animals and human.

Phylum Annelida

Animals have elongated segmental body. There are both segments in Annelida inner and outer, these animals can be found in water as well as on land. Most of these are free living, have circulatory system and these are blood suckers. e.g Earthworm and leech.

Phylum Arthropoda

This is the largest phylum in animal. They have jointed limbs and legs, also have exo-skeleton which protects their body. These are triploblastic can be found on water and land. There are two meta-morphosis in it, complete and incomplete meta-morphosis. Complete Meta-morphosis: In complete meta-morphosis first is larva than pupa to adult. Larva is completely change from adult, in larva stage there is lack of wings and reproductive organ. Incomplete Meta-morphosis: In incomplete meta-morphosis first stage is egg than nymph to adult, nymph is same as adult no change in it.

Phylum Mollusca

The body is soft and fleshy, a thin cover outside is mantle. Outside of it is a shell, made up from nonliving and CaCo3. A fleshy organ for movement is foot. Squid are the largest invertebrates. These are blood suckers mostly live in water.

Phylum Echinodermata

Echinodermata have rough spiny skin, and these only found in sea water. These are pentamerous means have a unique water vascular system, mostly found on rocks in sea. They don’t have head, eyes or even brain. e.g Starfish, Starfish simply have spiny star shape skin. Sea-Urchin has a ball like spiny body without arms.

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