As the name suggests, invertebrates lack vertebral column, also known as spine or backbone. Insects, earthworms, spiders, octopuses, leeches, prawns, snails, oysters, squids and jellyfish are all examples of invertebrates. 97 percent of all animals found on earth are invertebrates.

Evolution of invertebrates

Charles Darwin propounded the theory of evolution by natural selection in 1859. In his book ‘The origin of species’ Darwin described how creatures evolve over generations through inheritance of behavioural and physical traits. According to the theory individuals with more adaptive traits will have better chance of survival and have more offspring compared to individuals with less adaptive traits. Over time descendants of individuals with more adaptive traits will become more prevalent in the population because of the adaptive traits they have inherited from their adaptive ancestors and thus evolution through natural selection happens. Whole lot of traits contribute in the evolution of invertebrates. The characteristics are multicellularity, cephalization, tissues and organs, mesoderm, complete digestive system, coelom, radial and bilateral symmetry, a segmented body, and notochord. Symmetry, cephalization and specialization are three key traits in invertebrates’ evolution. Let’s take a look at each of these traits to understand how invertebrates developed over time to become the wonderful animals they are today.

• Symmetry: Most but not all invertebrates possess bilateral symmetry (two halves almost absolute mirror images of each other), some have radial symmetry (all sides are the same). The symmetry that ‘radiates’ from the center is radial symmetry, symmetry of slices of pizza is example of such symmetry. Such symmetry is seen in sea anemones and sea stars. There are invertebrates that lack any symmetry, for example sponges.
• Tissue specialization: Essentially, as animals evolve and get larger and more complex, their tissues gain specialization in producing various body parts. Muscle cells differ from nerve cells as they have to perform specialized jobs different from each other. In the same way kidney cells differ from lung cells.
• Cephalization: Cephalization is nothing but development of an organism’s front end into a distinct head. Some creatures display complete cephalization, yet since their bodies are not divided into distinct trunks and heads, they cannot possess a distinct physical head.

Invertebrate classification

The evolution of invertebrates has caused diversification. There are over 30 phyla of invertebrates. Except for one, invertebrates compose the entirety of the animal phyla. Of the three subphyla of Chordata two are invertebrate-dominated and the third subphylum is Vertebrata. Vertebrata includes all vertebrate organisms. Fossil record provides confirmation to the well accepted fact that many invertebrate species have been extinct. The major classification of invertebrates that have avoided extinction are as follows:

• Porifera: Key members of the Porifera phylum are sponges. These earliest invertebrates are lowest multicellular animals. Sponges lack real tissues and have specialized cells instead.
• Cnidaria: Cnidaria are radially symmetrical and possess actual tissues. Jellyfish, hydrozoans and corals are included in Cnidaria. Many cnidarian species play a key role in the enormous coral reefs existing in tropical waters.
• Platyhelminthes: Flatworms are the most eminent members of the phylum Platyhelminthes. They possess bilateral symmetry and are formed from three germ layers of embryonic cells.
• Nematoda: Roundworms and flatworms are amongst the parasitic species found in the phylum Nematoda. Some of these are infectious to humans or livestock. Unlike flatworms, roundworms possess a complete digestive tract and a partly filled body cavity.
•  Annelida: Annelida is a collection of segmented worms that includes ordinary earthworms and leeches. They possess a specialized digestive system, a primitive brain system and a closed circulatory system. Annelids possess a very well-developed body cavity, excretory system, nervous system and a rudimentary brain.
• Arthropoda: Arthropoda encompasses about 80% of all living species in the world. They possess segmented bodies with jointed limbs and an open circulatory system with multiple hearts. They possess a thick exoskeleton made of chitin (a complex polymer). They have an excretory system, a nervous system and a primitive brain.
• Mollusca: Mollusca phylum is of clams, squids and octopi. Mollusks possess a muscular foot called a mantle, which may be utilised for mobility. The exoskeleton of several mollusks is made of solid calcium carbonate. Another distinguishing feature is the radula, a specialized feeding mechanism present inside the mouths of mollusks. Chitin teeth are utilised to consume or scrape food by the radula.
• Echinodermata: The phylum most closely related to Chordata is Echinodermata. Echinoderms possess an embryonic development in which the anus opening is formed first, then the mouth opening, just like chordates. Unique thing about Echinoderms is that, as juveniles, they are bilaterally symmetrical, but as adults, their symmetry becomes radial.