Invertebrates: Types, Characteristics, Classification

Invertebrates are those organisms that lack backbone.vertebra in their bodies. Over 90% of animal species are invertebrates. Invertebrates play a significant role in the ecosystem. 

Some invertebrates help in cleaning the unneeded bacteria and fungi from the environment and turning them into fertilizers to nutrify the soil. Also, invertebrates help to grow crops through pollination and maintain their quality.

What are Invertebrates?

The term ‘Vertebrate’ is acquired from the Latin word "vertebra" which basically means a joint. In contrast to this, an invertebrate refers to animals without joints because the prefix "in" in invertebrate means "not" or "without".

In simple words, "invertebrates'' are creatures which lack a spinal column in their body. These animals basically don't have a backbone and never in their lifetime does it evolve.This is the major difference between vertebrates and invertebrates. They are soft-bodied animals from inside but have a tough and rigid outer skeleton for protecting themselves.

About 98% of the animal species or genera are invertebrates. They include sea stars, sea urchins, insects, crabs, spiders, snails, squid, jellyfish, sponges, octopus and many more. Invertebrates are ectothermic (cold-blooded) animals i.e., their body temperature keeps on changing with any change in the environment and therefore they can survive both on land and in water.

Also, invertebrates are the unrecognized heroes of the ecosystem as they provide numerous services such as pollination, decomposition, nutrient release and many more which are crucial for the survival of nature and human beings. Invertebrates are of different sizes which vary from 50um rotifers to 56 ft colossal squid.

Characteristics of Invertebrates

Invertebrates exhibit a number of characteristics unique to them: 

Symmetry

All invertebrates have some sort of symmetry that can be radial, bilateral, biradial, spherical or even asymmetrical. Microscopic invertebrates and Macroinvertebrates are mostly bilateral, radial and biradial. But some invertebrates can be asymmetrical (having no symmetry) too, such as phylum Porifera (sponges), gastropods (sea snails, snails). Asymmetry can also be seen in fiddler and hermit crabs.

Size

All invertebrates are of different sizes ranging from microscopic Nematodes to 50 ft Platyhelminthes.

Shape 

Invertebrates vary from shape to shape. Some invertebrates are plant-like such as Porifera and Cnidaria; some are vermiform such as Nematodes and Annelids; some are star-like such as starfish and some are leaf and ribbon-like such as flatworms.

Reproduction

Most of the invertebrates partly reproduce by sexual reproduction like vertebrates. Some of them reproduce through asexual mode and some reproduce through both the modes.

Classification of Invertebrates

Invertebrates are classified and divided into 2 main categories— Microscopic invertebrates and Macroinvertebrates, having 9 phyla which are provided in the table given below with examples: 

Microscopic Invertebrates

Phylum Nematoda

Also called roundworms, Nematodes have bilaterally symmetrical, cylindrical and unsegmented bodies. They are mostly found in water bodies and soil. These organisms are mostly microscopic but some species can be up to 2 inches. Though having a minute size, they are capable of causing diseases like, filariasis, trichinosis.

Macroinvertebrates

Phylum Porifera

As the name "Porifera" suggests, these are multicellular organisms that have ostia (pores) all over their body. They are spongy in appearance and hence called sponges. They do not move as they are attached to the substratum. They can absorb and retain fluids. Initially, they were considered as plants because of their green colour but after observing their feeding habits and life cycle, they are referred to as animals. They are a few centimetres in size but can be as tall as 6.6 ft.

Phylum Ctenophora

Also called comb jellies or sea walnuts, ctenophores have jelly-like, transparent and biradially symmetrical bodies having comb-like ciliary plates for movement. They are mostly seen in shallow water bodies. They lack organs such as respiratory, circulatory, skeletal and excretory. Their size ranges from 0.04 inches to 5 ft.

Phylum Cnidaria

Also called coelenterata, Cnidarians have two cell layered, radially symmetrical bodies. Their size ranges from Hydra (less than a half mm) to the Lion's mane jellyfish (20 mm) which is the largest Cnidarian.

Phylum Echinodermata

These are colourful, unsegmented, multicellular marine organisms found in deep seas having calcium carbonate skeleton and ability to regenerate. Their size ranges up to 6.6 ft.

Phylum Platyhelminthes

These are bilaterally symmetrical organisms having an unsegmented body except in Cestoda class. They do not have endo and exoskeleton and therefore have a soft body. They have a complicated reproductive system. They have feeding habits of carnivores or scavengers. Trematodes' size are generally between 0.04 to 0.4 inches but few species have sizes even greater than 50 ft.

Phylum Annelida

These organisms have bilaterally symmetrical, segmented bodies which vary in size (below 1mm to 3m). They have a good digestive and circulatory system. Their blood contains haemoglobin which makes it red coloured.

Phylum Arthropoda

This phylum is the largest one in which organisms have bilaterally symmetrical bodies. Their body is divided into three parts namely, head, thorax & abdomen. They have a good digestive system and an open circulatory system. Their size ranges from 1mm to 4m.

Phylum Mollusca

These are amphibians having a bilaterally symmetrical body protected by a shell. Their body is divided into three parts namely, head, a visceral mass, ventral foot. Their size can vary from 1mm to 20m with few microscopic organisms too.

Things to Remember

• About 98% of the whole animal species are invertebrates.
• As mentioned earlier, invertebrates have a soft body as they do not possess a spinal column but octopus is the only invertebrate which has a hard body part, its hooked beak.
• Invertebrates are ectotherms— cold-blooded organisms. Most of them live most of their lives in water and if outside, they absorb heat from their surroundings. 
• Most invertebrates go through a process called metamorphosis i.e., their body changes their form as they develop.