One of the ways life is classified is through the presence or absence of the vertebrate. Vertebrates and invertebrates evolved from a common ancestor that was speculated to have lived around 600 million years ago.
Evidence of true vertebrates began to appear 525 million years ago and ever since then, vertebrates have branched off into a long lineage that includes armoured fish and giant sauropods to woolly mammoths and modern man.
Characteristics of Vertebrates
A vertebrate is an animal that has all of the following characteristic features at some point in its life:
A stiff rod running through the length of the animal (it could either be the vertebral column and/or notochord)
Humans and all other vertebrates possess a notochord as an embryo and it eventually develops into the vertebral column.
A bundle of nerves run above the vertebral column (spinal cord) and the alimentary canal exists below it.
The mouth is present at the anterior portion of the animals or right below it.
The alimentary canal ends in the anus, which opens to the exterior. The tail extends after the anus.
5 Groups of Vertebrates
The five main groups of vertebrates are fish, amphibians, reptiles, birds, and mammals. There are actually more categories, since bony fish are separate from jawless fish (lampreys and hagfish), cartilaginous fish (sharks and rays), and lobe-fin fish (lungfish and coelacanths).
Fish
Collectively, fish include cartilaginous fish, like a shark, and bony fish, like a tuna or perch. But, most of the time, when people talk about five groups of vertebrates they only mean the bony fish. Bony fish share several distinctive characteristics:
Fish are ectotherms. In other words, they are cold-blooded. Their temperature depends on their environment.
They have gills. Fish live in water get oxygen from gases dissolved in it.
They have scales and fins.
Some lay eggs, while others give birth to live young in the water.
Amphibians
Examples of amphibians include frogs, toads, newts, salamanders, and axolotls. Like fish, they are cold-blooded. Unlike fish, they spend part of their lives breathing air. Here are major characteristics of amphibians:
Amphibians are cold-blooded.
They spend part of their life in the water, breathing with gills, and part of their life with lungs breathing air. Sometimes the adults still live in the water, while other species live on land.
Amphibians undergo a metamorphosis from their aquatic larval form into the adult form with lungs.
They have thin, moist skin that usually is smooth. They can breathe through their skin as well as their gills or lungs.
Most amphibians lay jelly-like masses of eggs in water.
Reptiles
Examples of reptiles include turtles, lizards, alligators, and snakes. Like fish and amphibians, they are cold-blooded. Like amphibians, they have four limbs or, in the case of snakes, evidence of limbs. But, amphibians need water, while reptiles are adapted for life on land.
Reptiles are cold-blooded.
They have lungs and breathe air.
Their young resemble small adults. There is no metamorphosis.
Reptiles have dry, scaly or plated skin.
Most reptiles lay leathery eggs, although some give birth to live young.
Birds
Examples of birds include eagles, sparrows, ostriches, and chickens. Birds are actually the living descendant of dinosaurs. So, all birds are dinosaurs, but not all dinosaurs were birds. Unlike fish, amphibians, and reptiles, birds are warm-blooded. Like reptiles, they hatch from eggs, breath air, and are adapted to life on land.
Birds are homeothermic or warm-blooded. Their metabolism regulates body temperature so that it remains fairly constant.
Birds have lungs and breathe air.
While baby birds look a bit different from their parents, there is no metamorphosis.
Birds have wings, beaks, and feathers.
They lay hard-shelled eggs.
Mammals
Examples of mammals include cats, dogs, squirrels, foxes, whales, and humans. Like birds, mammals have lungs and are warm-blooded.
Mammals are warm-blooded.
They have lungs.
Mammals give birth to their young (with a few exceptions) and feed their young milk.
Mammals have hair or fur.
Are Invertebrates Animals?
An animal is a living organism that feeds off organic matter (i.e. other animals or plants). Typically, they have sense organs that are capable of responding to inputs like light, sound or touch.
It is stated that 97% of all animals within the animal kingdom are invertebrates. Over 1.25 million species have been described, most of which are insects, there numerous more invertebrates that have not been discovered living on Earth today.
In numbers, the invertebrate species could be in the 5, 10 or 30 million marks this compared to only 60,000 vertebrates. A big reason why invertebrates are so successful is due to their reproduction rate. For example, insects like ants and bees lay eggs and these eggs can develop without fertilization.
Invertebrate animals are also among the oldest animals in existence. For example, jellyfish have been around for over 500 million years! Sponges and some crustaceans (animals like crabs and lobsters) also enjoy similar longevity. In fact, many of the world's oldest animals live underwater. That's because life on Earth started in the sea!
What are the 5 most common types of Invertebrates?
Within the vast number of Invertebrates, there are 5 main groups that many of them fall under. These include:
Protozoans -
Single-celled organisms such as paramecia, these invertebrates are free-living or can be parasitic. They feed on matter which can be other microorganisms, organic tissues or debris.
Annelids -
Also known as ringed worms or segmented worms. They are a large phylum, and over 22,000 species of phylum exist which includes earthworms and leeches. They are characterised by their body cavity, moveable bristles, and segmented body.
Echinoderms -
Marine invertebrates, including sea cucumbers, sea urchins and starfish. They are a very diverse group and have a really important role ecologically for near-shore environments in deep seas.
Molluscs -
Soft-bodied invertebrates that are usually enclosed in a calcium carbonate shell with a soft mantle covering their body. Organisms that are included within the molluscs type include Snails, octopuses, squid, clams, and scallops.
Arthropods -
Arthropods have 5 main characteristics, these types of invertebrate have Exoskeletons, where their bodies do not have internal bones. They have segmented bodies, internally and externally. Arthropods have jointed appendages, bilateral symmetry and finally, they also have an open circulatory system.