Invertebrates
The other day, I was preparing for a class on 'Invertebrates'. I introduced the lesson by asking students to quickly write down (in 30 seconds) as many animals they could think of! After the stipulated time, I displayed the names of common animals - goat, sheep, cow, lion, tiger, elephant, monkey, donkey etc. And I asked the students to cancel out these names from their list. After this, I found out which student(s) had the highest number of animals still listed. I asked these students to call out the names they had listed. And then I pointed out that very few of us think of invertebrates when we think of animals. When I asked the students the reason, they pointed out that it was perhaps because we do not see them!! Then I put up a statistic that I found interesting as well - that of all the animals species in the world, only 5% are vertebrates while 95% are invertebrates!! And yet, we can name so few of those 95%!! I told the students then that the lesson would be dedicated to those neglected 95% of animals...! I would like to dedicate this page to the huge(ly neglected) group of animals...the Invertebrates.
The Invertebrates are the animals that do not have a backbone - or a 'vertebra'. Their body structure is therefore relatively simple. The entire 'Class' of Invertebrates is divided into several phyla (singular - phylum), starting from the simplest group - the Protozoans to the most complex group - the Echinoderms. The characteristics of the animals belonging to this group as well as the names of a few are listed below:
The Invertebrates are the animals that do not have a backbone - or a 'vertebra'. Their body structure is therefore relatively simple. The entire 'Class' of Invertebrates is divided into several phyla (singular - phylum), starting from the simplest group - the Protozoans to the most complex group - the Echinoderms. The characteristics of the animals belonging to this group as well as the names of a few are listed below:
Protozoa
This includes the microscopic, unicellular organisms that have a well-defined nucleus (they are eukaryotes!). They live in water and move with the help of specialised structures such as spontaneously developed "false feet" or pseudopodia (in Amoeba), tiny hair like 'cilia' (in Paramecium) and a whip-like 'flagella' (in Euglena).
The figures alongside are those of Amoeba (the pink one!), Paramecium (slipper shaped) and the Euglena (green one!). In fact, Euglena is green and is capable of preparing its own food, like plants, while it also has other animal-like behaviour!
The figures alongside are those of Amoeba (the pink one!), Paramecium (slipper shaped) and the Euglena (green one!). In fact, Euglena is green and is capable of preparing its own food, like plants, while it also has other animal-like behaviour!
Porifera
This includes all the sponges. They are found deep in the ocean, fixed to some base or substratum. They have tiny pores on their body called 'ostia' and a relatively bigger opening at the top called the 'osculum'. Water enters and exits through these pores, and along with it, food also enters in the form of plankton etc.
Cnidaria
The animals in this group have a cylindrical hollow body called a 'coelenteron'. And for this reason, the group was referred to as 'Coelenterata'. Today, however, biologists refer to the phylum as Cnidaria, because of the characteristic 'stinging cells' (called cnidocytes/cnidoblasts) present within the coelenteron that helps these animals to capture its prey. To this group belong animals such as the Hydra, the sea anemone (Nemo would help you pronounce that one!!), jellyfish, coral etc.
When talking of coral, we picture the colourful, hard, stone like structures found in the Great Barrier Reef etc. These are actually the skeleton made of Calcium Carbonate that is produced by the coral polyps themselves. When the polyps die, the next generation builds up on the skeleton of the previous ones. A coral reef is home to a lot of fish and other marine organisms. Of late, the corals are seen to be dying out, because of the level of pollution in the ocean waters. This is a major area of concern. When we think of corals, we imagine only the ones that build the colourful rock like structures. These are actually the 'Hard Corals'. There are also 'Soft Corals', such as the beautiful sea fan etc, which live individually, attached to a substrate. These are exquisite to look at. Go through this website for more information on corals.
When talking of coral, we picture the colourful, hard, stone like structures found in the Great Barrier Reef etc. These are actually the skeleton made of Calcium Carbonate that is produced by the coral polyps themselves. When the polyps die, the next generation builds up on the skeleton of the previous ones. A coral reef is home to a lot of fish and other marine organisms. Of late, the corals are seen to be dying out, because of the level of pollution in the ocean waters. This is a major area of concern. When we think of corals, we imagine only the ones that build the colourful rock like structures. These are actually the 'Hard Corals'. There are also 'Soft Corals', such as the beautiful sea fan etc, which live individually, attached to a substrate. These are exquisite to look at. Go through this website for more information on corals.
Platyhelminthes
The 'helminths' are worms. To this group belong the flat, ribbon-shaped worms. The most common example considered here is that of the parasitic Taenia solium (commonly known as the tape worm). This lives in the human intestine and lives on digested food. A lot of platyhelminths are present in the ocean as well.
Nemathelminthes
Like the previous phylum, this is also a group of worms - the 'round' worms. Some of these are parasitic as well, as seen in the case of Ascaris (commonly called the round worm). Their body does not have segments, and is smooth and tapering at both ends.
Annelida
The word 'annulus' means a ring. This group of animals has a body that is made up of several ring-like segments. As the organism grows, rings are added to the body. Examples of annelids are earthworm, the leech and Nereis.
The earthworm has a very important role to play in maintaining soil fertility. In fact, Charles Darwin referred to these animals as perhaps having made the most significant contribution to the fertility of soil. As the earthworm burrows through the soil, it eats up the soil, and egests it out through its anus. In so doing, it brings to surface several minerals that were perhaps lying in the lower layers and therefore inaccessible to crops. Also, it aerates the soil, ensuring that spaces are present in the soil where oxygen can be trapped (roots need this oxygen for respiring).
The leech is an ectoparasite (an external parasite, unlike the tapeworm which is an endoparasite, living inside the body). The leech sucks blood from humans, cows, horses etc, and then falls off once it has had its fill of blood.
The earthworm has a very important role to play in maintaining soil fertility. In fact, Charles Darwin referred to these animals as perhaps having made the most significant contribution to the fertility of soil. As the earthworm burrows through the soil, it eats up the soil, and egests it out through its anus. In so doing, it brings to surface several minerals that were perhaps lying in the lower layers and therefore inaccessible to crops. Also, it aerates the soil, ensuring that spaces are present in the soil where oxygen can be trapped (roots need this oxygen for respiring).
The leech is an ectoparasite (an external parasite, unlike the tapeworm which is an endoparasite, living inside the body). The leech sucks blood from humans, cows, horses etc, and then falls off once it has had its fill of blood.
Arthropoda
This is the largest phylum, as a maximum number of animal species fit into this phylum. The arthropods (arthro=jointed; poda=feet) or organisms with jointed feet, include all the insects, scorpions, crabs etc. These animals possess an exoskeleton (outer skeleton! Unlike our internal skeleton or endoskeleton made of bones!!) made of chitin (pronounced kite-in, and meaning a type of carbohydrate). Being a huge group, it is further divided into four subphyla:
- CRUSTACEA - comprising of animals such as crabs (observe the joints in their feet!)
- MYRIAPODA - consisting of animals such as centipedes and millipedes, which have several feet per segment.
- ARACHNIDA - consisting of animals such as the spider and scorpion.
- INSECTA - Most animals in the world belong to this group - otherwise called the insects! This includes bees, butterflies, bugs, grasshoppers, flies, mosquitoes etc.
Mollusca
This group includes most animals that have a calcerous shell (made of Calcium carbonate), such as snails, oysters, clams etc. The animals move with the help of a muscular foot that emerges out of the shell and grips on to the floor, thereby helping it to move. Other examples of this group are octopus, which has an internal skeleton instead of an external shell.
Echinodermata
The echinoderms are the spiny skinned animals (echino=spiny, dermis=skin) present in the ocean. They move with the help of tiny structures called 'tube feet'. Examples of this group are starfish, sea cucumbers, sea urchins.