Invertebrates – Evolution, Diversity, and Classification Study Guide


The study of diverse forms of life, their characteristics, and how they evolve is known as evolution. It is a scientific concept used by biologists to study population genetic variation. There are various theories on how life on our planet originated. The theory of evolution is based on the idea that all species are interconnected and evolve.

According to Darwin’s theory of evolution, every organism reproduces and multiplies massively, and some characteristics are continuously handed down from one generation to the next. Every living species has a different rate of reproduction, and some species reproduce at a higher rate than others.

Invertebrates account for 97 percent of all creatures found on earth. Invertebrates get their name from the Latin word vertebra, which refers to a joint and (in some cases) a joint from the spinal column.

Evolution of invertebrates

The first microscopic creature appeared in the ocean 3.5 billion years ago. The first invertebrates were born in the water. Cambrians, for example, were soft-bodied animals with a carapace or shell.


There are a lot of key traits in the evolution of invertebrates. The characteristics are multicellularity, tissues and organs, radial and bilateral symmetry, cephalization, mesoderm, complete digestive system, coelom, a segmented body, and notochord.Symmetry, specialization, and cephalization are three major traits in invertebrates’ evolution.Let’s look at each of these to see how invertebrates developed over time to become the wonderful animals they are today.

  • Symmetry: Most invertebrates have bilateral symmetry (two halves mirror images of each other), whereas some have radial symmetry (all sides are the same). The term is derived from the symmetry that ‘radiates’ from the center, comparable to slices of pizza. This form of symmetry may be seen in sea stars and sea anemones. Some invertebrates, such as sponges, have no symmetry at all. They are also the most primitive of all animals, having no nerves, muscles, or organs.

  • Tissue Specialization: Essentially, as animals get larger and more complex, their tissues specialize in producing various body parts. Nerve cells differ from muscle cells in that they perform different – or specialized – tasks, similar to how kidney cells differ from lung cells.

  • Cephalization: Cephalization is the development of an organism’s forward (front) end into a distinct head. Some creatures exhibit complete cephalization, yet since their bodies are not separated into distinct trunks and heads, they cannot have a distinct physical head.

Invertebrate classification

The evolution of invertebrates has established many fundamental characteristics of higher organisms, from the earliest sponge species to the more recent echinoderms.

Classification of invertebrates

There are more than 30 phyla of invertebrates. Except for one, invertebrates make up the entirety of the animal phyla. Chordata is divided into three subphyla, two of which are invertebrate-dominated. Vertebrata is the third subphylum, which includes all vertebrate organisms. Moreover, many invertebrate species have been extinct, which is only confirmed by the fossil record. The major classification of invertebrates that are still alive today are as follows:

  • Porifera: The sponges are members of the Porifera phylum, also comprising the earliest invertebrates. Sponges aren’t made of real tissues, and sponge tissues are replaced by specialized cells that function like digestion and reproduction.

  • Cnidaria: Cnidaria includes jellyfish, hydrozoans, and corals. They are radially symmetrical and have actual tissues. Many cnidarian species play a vital role in the massive coral reefs present in tropical waters.

  • Platyhelminthes: Flatworms are the most well-known members of the phylum Platyhelminthes. They have bilateral symmetry and are produced from three germ layers of embryonic cells.

  • Nematoda: Flatworms and roundworms are among the parasitic species found in the phylum Nematoda, some of which are infectious to humans or livestock. Unlike flatworms, roundworms have a complete digestive tract and a partially filled body cavity.

  • Annelida: Annelida is a group of segmented worms that includes leeches and ordinary earthworms. They have a specialized digestive system, a closed circulatory system, and a primitive brain system. The closed circulatory system of the worm pumps blood throughout the whole body. Annelids have a well-developed body cavity, excretory, and nervous system, including a rudimentary brain.

  • Arthropoda: Arthropoda is a vast phylum that encompasses about 80% of all living species in the world. They have segmented bodies with jointed limbs and an open circulatory system with many hearts. They have a thick exoskeleton made of chitin, a complex polymer, and some include gills for gas exchange. They have a nervous system, an excretory system, and a primitive brain.

  • Mollusca: Mollusca is the phylum of clams, octopi, and squids. Mollusks have a muscular foot called a mantle, which may be used for mobility and are distinguishing characteristics. The exoskeleton of many mollusks is made of solid calcium carbonate. Another distinctive feature is the radula, a specialized feeding mechanism present within the mouths of mollusks. Chitin teeth are used to consume or scrape food by the radula.

  • Echinodermata: The phylum Echinodermata is the most closely related to Chordata. Echinoderms, like chordates, have an embryonic development in which the anus opening is formed first, then the mouth opening. They have an internal skeleton made of calcium spines. Echinoderms are particularly unique in that, as juveniles, they are bilaterally symmetrical, but as adults, they become radially symmetrical.

What is biodiversity, and why is it important?

Biodiversity refers to the abundance and diversity of life on the planet, and it is our planet’s most complicated and vital characteristic. Life could not exist without biodiversity.

Biodiversity classification

Diversity is classified into three categories:

  • Genetic Biodiversity
  • Species Biodiversity
  • Ecological Biodiversity

Species diversity: A species’ diversity is the number of different species found in a given area. That is the essence of biodiversity. The term includes a range of species, from plants to microorganisms.

Genetic Biodiversity: It describes the genetic resources of different species. Every individual of a species has a unique genetic composition, and as a result, each individual has a distinct look. Rice, wheat, maize, barley, and other grains have different variants.

Ecological Biodiversity: An ecosystem comprises both living and nonliving creatures that interact with one another. Ecological biodiversity refers to the diversity of plant and animal species that live in close proximity and are linked by food chains and food webs. Ecological diversity can be found in different ecosystems such as deserts, rainforests, and mangroves.


  • For a multitude of reasons, invertebrates are essential to the natural world.
  • Invertebrates are extraordinarily diverse, accounting for the vast majority of species on the earth while still harboring many species that are yet to be discovered.
  • Pollination, decomposition, and nutrient release are just a few of the ecological functions they perform in food webs.


1. What are the characteristics of invertebrates?

The characteristics of invertebrates are:

  • They don’t have a spine
  • They have many cells
  • They have no cell walls
  • They reproduce by combining two reproductive cells called gametes

2. How are invertebrates classified?

Protozoa, Porifera, Coelenterata, Platyhelminthes, Nematoda, Annelida, Echinodermata, Mollusca, and Arthropoda, are among the most familiar invertebrates. Arthropods are insects, crustaceans, and arachnids.

3. What is the main group of invertebrates?

Invertebrates are divided into four groups, which are given below by phylum:

  • Mollusca is a phylum of animals
  • Annelida is a phylum of plants
  • Arthropods are a phylum of insects
  • Coelenterata is a phylum of bacteria

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