The animal kingdom is represented by some of the most well-developed and complex species. Animals are multicellular heterotrophic organisms that develop from embryonic layers. Some 8.7 million species of animals are estimated to exist! Of these, scientists have successfully identified and classified about 2 million species.
Considering the vast diversity and huge numbers of animals, it is better to classify them based on certain shared characteristics. Broadly, the animals are classified as invertebrates and vertebrates. These groups are further classified into different phyla, depending on similarities in certain characteristics such as the body symmetry, level of cell organisation, number of embryonic germ layers, and the patterns of anatomical systems. This article attempts to highlight the most distinguishing and important characteristics of the major phyla of the animal kingdom.
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Porifera (Sponges)
Primitive and sedentary multicellular animals.
The cellular-level organisation, diploblastic, no plane of symmetry, random body shapes.
Free-living, aquatic forms.
Pores (Ostia) are present on the animal.
An internal cavity (spongocoel) is present within the body, lined with special flagellated cells (choanocytes). Water passes out of the spongocoel through an opening called the osculum.
Numerous canals are present to ensure a steady supply of water.
An internal skeleton of calcareous/ siliceous spicules present.
Sequential hermaphrodites.
Examples− Sycon, Euspongia, Spongilla, etc.
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Coelenterata (Cnidaria)
Tissue level of organisation, diploblastic, acoelomates, radially symmetrical.
The digestive system is incomplete. A central gastrovascular cavity (coelenteron) opens to the outside via a single opening which functions as the mouth and anus.
Two forms exist- the polyp and the medusa. Poly is cylindrical and mostly sedentary, while the medusa is umbrella-like and free-swimming. Tentacles protrude around the mouth in both the forms.
The tentacles are equipped with cnidoblasts which perform functions of offense and defence.
Cnidoblast contains nematocyst, which contains a poisonous, stinging thread.
A non-centralized sensory “nerve net” serves sensory reception of stimuli .
Examples− Hydra, Adamsia (Sea Anemone), Physalia (Portuguese Man-o-War), box jellyfish, etc.
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Platyhelminthes (Flatworms)
Aquatic organisms.
Organ system level of organization, triploblastic, acoelomates, bilaterally symmetrical.
Dorso-ventrally flattened body.
No segmentation.
The nervous system consists of a brain comprised of a pair of ganglia. Two ventral nerve chords run through the body. Two primitive eyespots are present at the anterior end.
The excretory system comprises the protonephridia, which contain flame bulbs that excrete out excess water and nitrogenous waste.
The circulatory system is absent in platyhelminths. The flat surface of these animals helps in gaseous exchange via diffusion.
Three classes, viz., Turbellaria (flatworms), Cestoda (tapeworms), Trematoda (flukes).
Platyhelminthes are parasites of the human system. Tapeworms have special apparatus known as hooks/ suckers.
Reproductive units called proglottids are present at the posterior end of tapeworms, which break off and leave the hosts body with the faeces.
Hermaphrodites, internal fertilization.
Examples− Dugesia (Planarians), Schistosoma (blood fluke), Fasciola (liver fluke), Taenia (tapeworms), etc.
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Nematoda (Aschelminthes)
Found in soil and in aquatic environments.
Triploblastic, pseudocoelomates, bilaterally symmetrical.
Round, unsegmented worms with tapered ends.
Body wall equipped with cuticle.
Dioecious, internal fertilization.
Many species occur as internal parasites of humans and other higher animals.
Examples of diseases caused by nematodes include.
Ascariasis (roundworm disease) by Ascaris lumbricoides.
Filariasis by Wuchereria bancrofti.
Eyeworm disease by Loa loa.
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Annelida
Organ-system level of organisation, triploblastic, true coelomates, bilaterally symmetrical.
Metamerically segmented worms.
Ring-like grooves called annuli are present, which give this phylum its name. Each segment/ annulus consists of the same organs.
Setae (bristles) present in most forms.
Well developed excretory system of nephridia present for excretion and osmoregulation.
Closed circulatory system present.
The nervous system comprises a central paired ganglia as the brain and paired ganglia in each segment.
Some annelids use peristalsis for movement, while in others, parapodia are present for locomotion.
Examples− Nereis, Pheretima, Hirudinaria.
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Arthropoda
The largest and most successful phylum of the animal kingdom, with a whopping 1,00,000 plus species, which constitutes almost 84% of the entire animal kingdom.
Organ-system level of organisation, triploblastic, true coelomates, bilaterally symmetrical.
Metamerically segmented.
The body is divided into the cephalothorax and the abdomen or the head, the thorax and the abdomen.
An exoskeleton of chitinous cuticle is present. The cuticle is shed periodically by ecdysis or moulting.
Paired and jointed appendages present.
Open circulatory system, body cavity (haemocoel) filled with the body fluid called the haemolymph.
A well-developed nervous system of the brain and a nerve cord with gangliated nerves is present. A sensory system of the eyes, olfactory receptors, and antennae is present.
The respiratory system consists trachea in insects, booklungs in arachnids, and gills in crustaceans.
Malpighian tubules are present as an excretory and osmoregulatory system.
Sexes are separate, mostly internal fertilization is observed.
Examples− Arachnids (Scorpion, Spider, etc), Crustaceans (Crabs, Lobsters), Insects (Honey bee, Silkworm, etc), and centipedes and millipedes.
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Mollusca
With over a 100,00 species, it’s the second-largest group in the animal kingdom.
Organ-system level of body organisation, triploblastic coelomates, bilaterally symmetrical.
The soft body is often protected by a mantle.
Unsegmented body with a head, muscular foot, and a visceral hump.
A distinct feeding structure known as radula is present.
The respiratory pigment is haemocyanin. Respiration mainly occurs through the gills or lungs.
Open circulatory system present in all except the cephalopods.
Nephridia are present for the excretion of nitrogenous wastes.
Dioecious, fertilization is both internal and external, and oviparous development occurs.
A larval stage known as trochophore exists.
They are mostly marine, although some terrestrial forms also exist.
Examples− Pila, Sepia, Octopus, Dentalium, Chaetopleura, etc.
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Echinodermata
Exclusively marine, benthic animals with a spiny skin.
Organ-system level of organisation, triploblastic and coelomic, radially symmetrical.
A unique water vascular (ambulacral) system is present, comprised of tube feet that act as a water filter and serve feeding and attachment purposes as well.
A calcareous internal skeleton is present.
Dioecious, fertilization is external as gametes are released into the water.
Indirect development- bilaterally symmetrical larval stage is present.
The excretory system consists of specialised podocytes for ultrafiltration, connected to an opening known as madreporite.
Examples− Asterias, Echinus, Cucumaria, etc.
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Hemichordata
They are not true chordates.
Exclusively marine, worm-like animals.
Organ system level of body organisation, triploblastic, true coelomates, bilaterally symmetrical.
Soft, unsegmented bodies, divisible into an anterior proboscis, a collar and a posterior trunk.
The stomochord, which is hollow and endodermal in origin is present in the anterior end of the body.
Nerve tissues are present only in the collar.
Respiration occurs with the help of gill slits.
Open circulatory system is present.
The excretory system consists of a proboscis gland.
Dioecious, fertilization is external.
Development is indirect, the larval stage is known as tornaria.
Examples− Balanoglossus, Saccoglossus, Rhabospleura, Protoglossus, etc.
Chordata
Organ system level of organisation, triploblastic, true coelomates, bilaterally symmetrical.
The presence of a true notochord, a dorsal and hollow nerve cord, pharyngeal gill slites and a post-anal tail, at some stage of the lifecycle.
More than 60,000 species exist, across three sub-phyla viz., the Urochordata (Tunicata), the Cephalochordata (acrania) and the Vertebrata. Urochordata and Cephalochordata are often considered as protochordates.
Examples: Herdmania, Branchiostoma, Scoliodon, Dipnoi, Bufo, Ambystoma, Crocodylus, Columba, Macropus, Panthera, Homo sapiens, etc.
The animal kingdom is a diverse and large group of organisms that have been divided into several phyla based on certain shared characteristics.
Broadly, animals are classified as invertebrates and vertebrates, based on the presence of a backbone.
Porifera represents the simplest of animals, with a diploblastic development and cellular grade of organization.
Arthropods represent the largest and most diverse phylum of the animal kingdom.
Hemichordates are animals that are not true chordates, because of the presence of a stomochord instead of a notochord.
Chordates represent members of the animal kingdom characterised by a complex level of body organisation, the presence of a notochord, a dorsal and hollow nerve cord, pharyngeal gill slits, and a post-anal tail.
Q1. What is the function of the choanocytes in sponges?
Ans. These cells help maintain a steady current of water and also help capture food and remove wastes.
Q2. Do coelenterates exist only as a ploy or a medusa?
Ans. Coelenterates may exist either as a polyp or a medusa, or, as both. The latter phenomenon is known as alternation of generation, as the poly and the medusa alternately occur in the same animal.
Q3. What does a pseudocoelom mean?
Ans. Pseudocoelom refers to the body cavity that is not completely lined with mesoderm, instead, the mesoderm is scattered as pouches in the cavity.
Q4. Does the notochord persist in adult humans?
Ans. The notochord is present during embryonic development, and later develops into the intervertebral discs.
Q5. What is the difference between Urochordata, Cephalochordata and Vertebrata?
Ans. While in Urochordates the notochord is present only in the larval tail, it persists throughout the life of Cephalochordates. In Vertebrates, the notochord is present in the embryonic stage and is replaced by the vertebral column later.
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