Biology - Diversity of Life - Classification of Animals


Animals are classified on the basis of certain common fundamental features, they are
1. Body symmetry
2. Nature of coelom
3. Body plan
4. Pattern of development
5. Segmentation of body
6. Presence or absence of notochord

Before discussing the various animal phyla, it is useful to introduce some of the terminology which will be used to describe these animals. The back or top side of an animal is its dorsal (dorso = back) side, and its belly or bottom side is its ventral (vent(er) = underside, belly) side. The head or front end is called the anterior (ante = before) end, and the tail or back end is the posterior (post = behind, after). Animals with radial symmetry (radia = spoke, radius; sym = with, together, metr, -metry = measure, measurement) do have distinct top and bottom sides, but have no distinct left and right. Starfish, jellyfish, and sea anemones are examples of animals with radial symmetry. Animals with bilateral symmetry (bi = two; later = side) do have distinct left and right sides, and most animals with which we are familiar, such as earthworms, ladybugs, and dogs, have bilateral symmetry.

The body of an animal is made up of several layers of tissue. Ectoderm (ecto = out, outer, outside; derm = skin) is the outer layer of tissue. The epidermis (epi = upon, over), or skin, other outer layers, and the nervous system in vertebrates (not in all animals) are formed from ectoderm tissue. Mesoderm (meso = middle) is the middle layers of tissue. Mesoderm forms the muscles and most other internal organs. Endoderm (endo = within, inner) forms the inner layers, including the lining of the digestive tract in all animals and the liver and lungs in vertebrates. Often animals have a space in their bodies between several of these layers. Animal groups, like flatworms, with no such space are referred to as acoelomates. Animals, such as roundworms, which have a space between the mesoderm and the endoderm, are called pseudocoelomates, and the space is called a pseudocoelom. Animals, including earthworms, insects, and humans, which have the space in between several of the mesoderm layers are called coelomates, and the space is called a coelom

Concentric germ layers develop which form the various tissues and organs as development continues. All Bilaterally symmetrical animals develop three germ layers (ectoderm, mesoderm, and endoderm) and are termed triploblastic. The radiata develop only two germ layers, (ectoderm and endoderm but no mesoderm) and are therefore termed diploblastic (e.g., phylum Cnidaria)


The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defining the primitive axis of the embryo. In lower vertebrates, it persists throughout life as the main axial support of the body, while in higher vertebrates it is replaced by the vertebral column. The notochord is found on the ventral surface of the neural tube.

Notogenesis is the development of the notochord The notochord forms during gastrulation and soon after induces the formation of the neural plate (neurulation), synchronizing the development of the neural tube In higher vertebrates, it extends throughout the entire length of the future vertebral column, and reaches as far as the anterior end of the midbrain, where it ends in a hook-like extremity in the region of the future


They have notochord. Central nervous system includes a single dorsal hollow nerve cord. Pharynx is perforated by gill slits. A post anal tail is present at some time of its life.


They never have a notochord. Gill slits are absent. A post-anal tail is absent.


Arthropods are the largest phylum of animals and include the insects, arachnids, crustaceans, and others. More than 80% of described living animal species are arthropods Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms.

Arthropods are characterised by the possession of a segmented body with appendages on each segment. They have a dorsal heart and a ventral nervous system. All arthropods are covered by a hard exoskeleton made of chitin, a polysaccharide, which provides physical protection and resistance to desiccation. Periodically, an arthropod sheds this covering when it moults.The success of arthropods is related to their hard exoskeleton, segmentation, and jointed appendages. The appendages are used for feeding, sensory reception, defence, and locomotion.

Aquatic arthropods use gills to exchange gases. These gills have an extensive surface area in contact with the surrounding water. Terrestrial arthropods have internal surfaces that are specialised for gas exchange. Insects and most other terrestrial species have tracheal systems: air sacs leading into the body from pores called spiracles in the epidermis cuticle. Others use book lungs, or gills modified for breathing air as seen in species like the coconut crab

Arthropods have an open circulatory system. Haemolymph containing haemocyanin, a copper-based oxygen-carrying protein, is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues.


1. Body metamerically segmented; symmetry bilateral.
2. Body wall with outer circular and inner longitudinal muscle layers; outer transparent moist cuticle secreted by epithelium.
3. Chitinous setae, often present on fleshy appendages called parapodia; setae absent in leeches.
4. Coelom (schizocoel) well developed and divided by septa, except in leeches; coelomic fluid supplies turgidity and functions as hydrostatic skeleton.
5. Blood system closed and segmentally arranged; respiratory pigments (hemoglobin, hemerythrin, or chlorocruorin) often present
6. Digestive system complete and not metamerically arranged.
7. Respiratory gas exchange through skin, gills, or parapodia
8. Excretory system typically a pair of nephridia for each metamere
9. Sensory system of tactile organs, taste buds, photoreceptor cells, and eyes with lenses (in some).
10.Hermaphroditic or separate sexes; asexual reproduction by budding in some.


Round worms have elongated cylindrical and vermiformed body, with tapering ends. Body unsegmented. Caudal end of the body is generally straight in case of females but coiled in case of males. Males are generally shorter than females. The body cavity is pseudocoel. Larval stage four, the third stage of larva is infective.


Cnidarians get their name from cnidocytes, which are specialized cells that carry stinging organelles called cnidocysts. The word Cnidaria also comes from the Greek word "cnidos", which means stinging needle. The corals, which are important reef-builders, belong here, as do the familiar sea anemones, jellyfish, sea pens, sea pansies and sea wasps.


Cnidarians are eumetazoan, possessing true tissues and organs. It has radial symmetry, meaning that whichever way it is cut along its central axis, the resulting halves will always be mirror images of each other. It is composed of two layers of tissue, known as the ectoderm and endoderm (or gastroderm), with a mesoglea in between them containing only scattered cells. Thus the organisms are considered to be diploblastic, though the mesoglea may be homologous with the mesoderm in other animals.
The ectoderm surrounds the cnidarian's 'stomach', or gastrovascular cavity which functions as both mouth and anus; it is used both to ingest food and excrete waste. It also serves along with the mesoglea as a hydrostatic supporting skeleton. Firm skeletons are only found among polyps, which produce lime for that purpose.


The cnidarian does not possess a true circulatory system. Respiration takes place through diffusion of oxygen directly through their tissues without specialized organs such as tracheae, gills or lungs. The gastrovascular system plays a role in the digestion and dispersion of food and the removal of metabolic waste: it surrounds the gastrovascular cavity as well as its extensions in the tentacles of polyps. Thus the gastrovascular system serves two separate functions, digestion and transport. Food particles are initially gathered by the feeding muscles of the gastroderm.


Cnidarians take their name from a specialised cell, the cnidocytes (nettle cell). Tentacles surrounding the mouth contain nematocysts, specialized stinging cells. The nematocysts are the cnidarians' main form of offence or defense and function by a chemical or physical trigger that causes the specialized cell to eject a barbed and poisoned hook that can stick into, ensnare, or entangle prey or predators, killing or at least paralyzing its victim.


Theoretically, members of Cnidaria have life cycles that alternate between asexual polyps and sexual, free-swimming forms called medusae.
Medusae have a hat or bell-shaped appearance and mostly swim passively with the current. Their tentacles hang freely below their bodies. However, they can actively swim by means of co-ordinated muscle contractions against the water contained in their gastrovacular cavity. Polyps, in contrast, are anchored to the substrate by their basal discs, although a few species can move in curious slow-motion somersaults. By nature they display their tentacles upwards, away from the substrate. Polyps often live in large colonies.


Ctenophores (Greek for "comb-bearers") have eight "comb rows" of fused cilia arranged along the sides of the animal, these cilia beat synchronously and propel ctenophores through the water. Some species move with a flapping motion of their lobes or undulations of the body. Many ctenophores have two long tentacles, but some lack tentacles completely.
Ctenophores, variously known as comb jellies, sea gooseberries, sea walnuts, or Venus's girdles, are voracious predators. Unlike cnidarians, with which they share several superficial similarities, they lack stinging cells. Instead, in order to capture prey, ctenophores possess sticky cells called colloblasts. In a few species, special cilia in the mouth are used for biting gelatinous prey.


The species which live in deep waters, such as the red tortuga, can appear brightly-colored, although usually with pigments that absorb blue light, making them appear dark in the sea. A deep-sea species informally called the “Tortugas Red” is bright red in color, presumably to absorb blue-light from its prey and the environment. Like many other ctenophores, can give off light by means of bioluminescence.

Ctenophora have an interesting form of symmetry, with many bilateral components, but a few asymmetrical structures such as the anal pores near the statocyst and sometimes the proportions of their auricles (ciliated lobe-like structures).

Ctenophorans are diploblastic (having only two body layers). The body consists of two transparent cell layers, which make up its outer skin (ectoderm) and inner skin (gastroderm). The ectoderm, made up of two cell layers, is mostly covered by a protective layer of slime, excreted by special glands. The gastroderm surrounds a cavity, which serves as a stomach and is only accessible by the mouth opening, connected by a long, narrow gullet. Captured quarry is pre-digested in the gullet by strong enzymes and fully decomposed in the stomach. There is no separate exit from the stomach apart from two 'anal pores', which despite their name appear to be only moderately used for excretion, so indigestible waste is principally expelled via the mouth.

The space between the inner and outer skin is taken up by the mesoglea, a thick, transparent, jelly-like layer made from collagen and connective tissue, pervaded by numerous small canals, which are used for transport and storage of nutrients. The position of the canals varies from species to species, but they mostly run directly underneath the tissues that they serve. The extracellular net of structural protein is kept upright by special cells similar to amoebas.

The mesoglea may also play a role in the lift of the creatures. Cilia found in the canals of the digestive system may serve to pump water in or out of the mesoglea, when osmotic water pressure changes, perhaps because the creature has swum out of saline sea water into coastal brackish water. Ctenophora do not possess a specific circulatory system, neither do they have any organs for breathing; gas exchange and the excretion of waste products of cell metabolism such as ammonia occur over the body's entire surface through simple diffusion.


The statocyst is a specialised system of the ctenophore that serves as a balancing organ and also controls its movement. It can be found on the end of the body opposite the oral opening and is formed by a collection of a few hundred calcareous cells balanced on four horizontal groups of serpentine flagella, known as the statolith


(Greek.echin = hedgehog + derma = skin)

Echinodermata includes the starfish, sea urchins, brittle stars, sea cucumbers and feather stars. Just like the name says they have spines or spicules on their skins to a varying degree in the different groups. The usually have a radial symmetry with no anterior or posterior, but radiating out from a central point. Radial symmetry is present only in the adult form (larvae bilateral). They reproduce sexually and most of the species release eggs and sperm into the water where the fertilization then occurs. The huge number of gametes produced compensates for the wastefulness of this type of fertilization. The larvae are attractive, planktonic creatures that are transparent and has long slender arms.


1. Symmetry is usually radial in adults, bilateral in larvae. Triploblastic. Most of the organs are ciliated. No segmentation.
2. Body surface of five symmetrical radiating areas, or ambularca, from which the tube feet project.
3. Body covered by a delicate epidermis over a firm mesodermal endoskeleton of moveable or fixed calcareous plates, usually in a definite pattern. Often with spines (skin leathery and plates usually microscopic in Holothuroidea).
4. No head. The body is arranged on a oral, aboral axis.
5. The Coelom is enterocoelous, large and lineated with ciliated peritoneum and subdivided during development to give rise to the unique water-vascular system. They poses tubular feet for motion, food handling and respiration. A complex hemal system present.
6. Respiration by minute dermal branchiae (skin gills) or papulae protruding from the coelom by tube feet and in Holothuroidea by coral respiratory trees.
7. The nervous system is diffuse and composed typically of three rings centered on the mouth region with radiating branches.
8. The sexes are separate, with rare exceptions, and all are alike externally. The gonads are large with simple ducts. Eggs are abundant and are usually fertilized in the sea. The larvae are bilateral, microscopic, ciliated, transparent and usually free swimming, with conspicuous metamorphosis.
9. No excretory system is present.