Animal Kingdom

More than a million animal species have been described, and they differ so much in shape, size and habit that a beginner can feel lost. Classification becomes manageable the moment you notice that a small set of features cuts across all of them. These features are chosen because they are fundamental — they are laid down early in embryonic development and do not change with habitat, food or season. Two animals can both be long, thin and worm-like and still belong to different phyla, because their internal body plan is built differently. So resist the urge to classify by looks; classify by plan.

Levels of organisation. Every animal is multicellular, but the cells are not always organised to the same degree. In sponges the cells are loosely arranged and largely independent — individual cells trap and digest their own food. This is the cellular level. In coelenterates and ctenophores, cells that do the same job are arranged together into tissues, giving the tissue level. In flatworms, tissues are grouped into definite organs such as a muscular pharynx or reproductive organs; this is the organ level. From roundworms onwards, organs are linked into complete working systems — a digestive system, an excretory system, a circulatory system — and this is the organ-system level.

Even inside the organ-system grade there is variation worth remembering. A digestive tract may be incomplete, with a single opening that serves as both mouth and anus, or complete, with a separate mouth at one end and anus at the other. A circulatory system may be open, where blood is pumped out of the heart into open spaces and bathes the tissues directly, or closed, where blood is always kept inside a network of vessels. Closed circulation gives finer control over how much blood reaches which organ.

Symmetry. Imagine slicing the animal through its centre. If no plane of slicing gives two equal halves, the animal is asymmetrical — many sponges are like this. If any plane passing through the central axis gives two identical halves, like slices of a round cake, the symmetry is radial; coelenterates, ctenophores and adult echinoderms show this. If only one particular plane divides the body into identical left and right halves, the symmetry is bilateral; annelids, arthropods, molluscs and chordates are built this way. Bilateral symmetry usually goes along with a definite head end, which is where sense organs and nerve tissue concentrate.

Germ layers. During development the embryo sets aside sheets of cells called germ layers, and every adult tissue traces back to one of them. Animals in which the cells are arranged in two such layers — an outer ectoderm and an inner endoderm, with a jelly-like undifferentiated layer called mesoglea in between — are diploblastic, as in coelenterates. Animals in which a third layer, the mesoderm, develops between ectoderm and endoderm are triploblastic; this includes platyhelminthes right through to chordates. The mesoderm matters enormously, because muscles, the lining of a true body cavity, and organs such as kidneys and the walls of the gonads all arise from it.

The coelom. A coelom is a body cavity that lies between the gut and the body wall and is lined on all sides by mesoderm. That lining is the whole test. Animals in which the cavity has this complete mesodermal lining are coelomates — annelids, arthropods, molluscs, echinoderms, hemichordates and chordates. In pseudocoelomates a body cavity is present, but mesoderm occurs only as scattered pouches lying between ectoderm and endoderm, so the cavity is not properly lined; roundworms such as Ascaris are the standard example. In acoelomates there is no body cavity at all, and the space between gut and body wall is packed with loose mesodermal tissue; flatworms such as Taenia belong here.

Why does a coelom matter so much that it is used to separate whole phyla? First, it gives room for organs to grow, fold and enlarge instead of being crushed against the gut. Second, the fluid inside is nearly incompressible, so muscles can push against it and use it as a hydrostatic skeleton for burrowing and crawling. Third, it frees the gut from the body wall, so food can be pushed along the gut by gut muscles while the animal moves in a completely different way. An acoelomate has to compromise between these jobs, which is one reason flatworms stay thin.

Segmentation. In some animals the body is divided, both externally and internally, into a series of repeating units, with at least some organs repeated serially in each unit. The earthworm is the classic example: its ring-like external divisions correspond to internal partitions, and structures such as nephridia and nerve ganglia repeat segment after segment. This pattern is called metameric segmentation and the phenomenon is metamerism. Do not confuse it with the simple external banding seen on some other worms — true metamerism must show internal repetition too. Arthropods are segmented as well, but in them the segments have become specialised and fused into distinct regions: head, thorax and abdomen.

Notochord. The notochord is a rod-like supporting structure that forms on the dorsal side during embryonic development and is mesodermal in origin. Animals that possess a notochord at some stage of life are chordates; animals that never form one are non-chordates. This single feature is the largest dividing line in the animal kingdom as you study it here. Note the careful wording: a chordate needs the notochord only at some stage, not for its whole life. In many chordates it appears in the embryo and is later replaced by a vertebral column, and in one group it appears only in the larval tail. Keep the phrase “at some stage of life” in the definition, and that group still fits the definition instead of contradicting it.

Porifera are the sponges. They are mostly marine, almost all asymmetrical, and sit at the cellular level of organisation. Their defining feature is a water transport or canal system: water enters through minute pores called ostia in the body wall, passes into a central cavity called the spongocoel, and leaves through a larger opening, the osculum. This flow brings in food and oxygen and carries away wastes. The canals and spongocoel are lined by flagellated collar cells called choanocytes, which drive the current and trap food particles. Digestion is intracellular, that is, inside individual cells. The body is supported by a skeleton of spicules made of calcium carbonate or silica, or by soft spongin fibres. Sponges are hermaphrodite, meaning eggs and sperms are produced by the same individual; fertilisation is internal and development is indirect, with a larva that looks nothing like the adult. Examples: Sycon (also called Scypha), Spongilla, a freshwater sponge, and Euspongia, the bath sponge.

Coelenterata, also called Cnidaria, are aquatic and mostly marine, either fixed in one place or free-swimming. They are radially symmetrical, diploblastic and at the tissue level. Their name-giving feature is the cnidoblast or cnidocyte, a stinging cell on the tentacles and body surface that contains a structure called a nematocyst. Cnidoblasts are used for anchorage, for defence and for capturing prey. There is a central gastro-vascular cavity with a single opening, the mouth, on a raised area called the hypostome; digestion is both extracellular and intracellular. Corals build a hard skeleton of calcium carbonate. Many coelenterates exist in two body forms — the polyp, which is cylindrical and fixed, such as Hydra and Adamsia, and the medusa, which is umbrella-shaped and free-swimming, such as Aurelia. Where both forms occur in one life cycle, polyps produce medusae asexually and medusae produce polyps sexually; this alternation is called metagenesis, seen in Obelia. Other examples: Physalia (Portuguese man-of-war), Pennatula (sea-pen), Gorgonia (sea-fan), Meandrina (brain coral).

Ctenophora, the comb jellies or sea walnuts, are exclusively marine, radially symmetrical, diploblastic and at the tissue level. The body carries eight external rows of ciliated comb plates, and the beating of these cilia is what moves the animal along. Digestion is extracellular and intracellular. Bioluminescence, the ability of the animal to give out light, is well marked here. Sexes are not separate, reproduction is only sexual, fertilisation is external and development is indirect. Examples: Pleurobrachia and Ctenoplana.

Platyhelminthes are the flatworms, and they mark several firsts: the first bilaterally symmetrical, triploblastic animals in this sequence, and the first with an organ level of organisation. They are acoelomate and dorso-ventrally flattened, which is why they are called flatworms. Many are endoparasites in animals including humans, and parasitic forms carry hooks and suckers to hold on to the host; some absorb nutrients directly through the body surface. Specialised cells called flame cells carry out excretion and osmoregulation. Sexes are not separate, fertilisation is internal, and development passes through several larval stages. Planaria is famous for its very high power of regeneration. Examples: Taenia, the tapeworm, and Fasciola, the liver fluke.

Aschelminthes are the roundworms, so called because a cut taken straight across the body gives a circular outline. They may be free-living in soil and water or parasitic in plants and animals. They are bilaterally symmetrical, triploblastic, pseudocoelomate and at the organ-system level. The alimentary canal is complete, with a well developed muscular pharynx, and an excretory tube carries wastes out through an excretory pore. Sexes are separate, and in many species the female is longer than the male. Fertilisation is internal and development may be direct or indirect. Examples: Ascaris, the roundworm, Wuchereria, the filaria worm, and Ancylostoma, the hookworm.

Annelida are aquatic or terrestrial, free-living or parasitic. They are the first phylum in this sequence to show a true coelom and true metameric segmentation. Longitudinal and circular muscles in the body wall work against the coelomic fluid to produce crawling and burrowing. Aquatic annelids such as Nereis carry lateral appendages called parapodia that help in swimming. The circulatory system is closed, excretion and osmoregulation are done by nephridia, and the nervous system has paired ganglia connected by lateral nerves to a double ventral nerve cord. Nereis has separate sexes, while the earthworm and the leech are hermaphrodite. Examples: Nereis, Pheretima (earthworm) and Hirudinaria, the blood-sucking leech.

Arthropoda is the largest phylum of the animal kingdom, taking in about two-thirds of all named animal species, insects being the biggest share. The body is covered by a chitinous exoskeleton and is divided into head, thorax and abdomen, with jointed appendages — the feature the name refers to. Respiration takes place through gills, book gills, book lungs or a system of air tubes called the tracheal system, depending on the group. The circulatory system is open. Sense organs include antennae, simple and compound eyes, and balance organs called statocysts. Excretion is carried out by Malpighian tubules. Most are dioecious, fertilisation is usually internal, and most are oviparous with direct or indirect development. Economically important examples are Apis (honey bee), Bombyx (silkworm) and Laccifer (lac insect); disease-carrying examples are Anopheles, Culex and Aedes; Locusta, the locust, is a crop pest that moves in swarms; and Limulus, the king crab, is called a living fossil.

Mollusca is the second largest phylum. Molluscs live on land and in water, and are bilateral, triploblastic, coelomate and at the organ-system level. The body is unsegmented and has three parts: a head, a muscular foot and a visceral hump that holds the organs. The soft body is usually protected by a calcareous shell. A soft, spongy layer of skin called the mantle covers the visceral hump, and the space between the hump and the mantle is the mantle cavity, in which feather-like gills lie; these gills serve both respiration and excretion. The head bears sensory tentacles, and the mouth contains a file-like rasping organ, the radula, used to scrape food. Molluscs are usually dioecious and oviparous, with development passing through a larval stage. Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea hare), Dentalium (tusk shell) and Chaetopleura (chiton).

Echinodermata means spiny-bodied, and the name comes from an endoskeleton of calcareous plates called ossicles. All echinoderms are marine — there is no freshwater or land echinoderm. They are at the organ-system level, triploblastic and coelomate. Their symmetry has to be stated in two parts: adults are radially symmetrical but the larvae are bilaterally symmetrical, which is one reason they are placed with the bilaterally symmetrical animals rather than with the coelenterates. The digestive system is complete, with the mouth on the lower surface and the anus on the upper surface. Their most distinctive feature is the water vascular system, a network of fluid-filled canals ending in tube feet, which is used for locomotion, for capturing and moving food, and for respiration. There is no excretory system. Sexes are separate, fertilisation is usually external, and development is indirect through a free-swimming larva. Examples: Asterias (star fish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber) and Ophiura (brittle star).

Hemichordata is a small phylum of worm-like marine animals. They are bilaterally symmetrical, triploblastic, coelomate and at the organ-system level. The cylindrical body has three regions: an anterior proboscis, a collar behind it, and a long trunk. Circulation is open, respiration takes place through gills, and the excretory organ is the proboscis gland. Sexes are separate, fertilisation is external and development is indirect. Examples: Balanoglossus and Saccoglossus. Note the name carefully — hemichordates are studied as a separate phylum of non-chordates, and the resemblance to chordates is only partial.

Chordata is defined by three features that must be present at some stage of life: a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits. Chordates are bilaterally symmetrical, triploblastic, coelomate and at the organ-system level. They also have a post-anal tail and a closed circulatory system. Compare this with non-chordates, where the nerve cord is solid, usually double, and lies on the ventral side, and the heart is dorsal or absent. In a chordate the heart is ventral.

Chordata is divided into three subphyla. In Urochordata the notochord is present only in the tail of the larva and is lost in the adult; examples are Ascidia, Salpa and Doliolum. In Cephalochordata the notochord runs from head to tail and persists throughout life; the example is Branchiostoma, also known as amphioxus or the lancelet. These two subphyla together are called protochordates and are exclusively marine. The third subphylum is Vertebrata, in which the embryonic notochord is replaced in the adult by a cartilaginous or bony vertebral column. Every vertebrate is a chordate, but not every chordate is a vertebrate, because protochordates never build a vertebral column. Besides the basic chordate features, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation, and paired appendages in the form of fins or limbs.

Vertebrata splits first into Agnatha, animals without jaws, represented by the class Cyclostomata, and Gnathostomata, animals with jaws. Gnathostomata in turn covers the fishes, grouped as Pisces (classes Chondrichthyes and Osteichthyes), and the four-limbed Tetrapoda (classes Amphibia, Reptilia, Aves and Mammalia).

Cyclostomata. Most are ectoparasites on fishes, clinging to the host with the sucking mouth. The elongated body carries 6 to 15 pairs of gill slits for respiration. The mouth is circular and sucking, and there are no jaws. The body has no scales and no paired fins. The cranium and vertebral column are cartilaginous, and circulation is closed. They are marine but migrate to fresh water to spawn, die within a few days of spawning, and their larvae return to the ocean after metamorphosis. Examples: Petromyzon (lamprey) and Myxine (hagfish).

Chondrichthyes. Marine, streamlined fishes with a cartilaginous endoskeleton. The mouth is on the ventral side and the notochord persists throughout life. Gill slits are separate and there is no operculum. The skin is tough and covered with minute placoid scales; the teeth are backwardly directed modified placoid scales, and the jaws are very powerful. There is no air bladder, so these fishes must keep swimming to avoid sinking. The heart has two chambers, one auricle and one ventricle, and they are cold-blooded. Sexes are separate, and in males the pelvic fins bear claspers. Fertilisation is internal and many are viviparous. Examples: Scoliodon (dog fish), Pristis (saw fish), Carcharodon (great white shark), Trygon (sting ray). Torpedo has electric organs, while Trygon has a poison sting — keep these two straight.

Osteichthyes. Marine and freshwater fishes with a bony endoskeleton. The mouth is mostly terminal, there are four pairs of gills covered by an operculum, and the skin bears cycloid or ctenoid scales. An air bladder is present and regulates buoyancy, so constant swimming is not needed. The heart has two chambers and they are cold-blooded. Fertilisation is usually external, most are oviparous and development is direct. Examples: Exocoetus (flying fish) and Hippocampus (sea horse) from the sea; Labeo (rohu), Catla (katla) and Clarias (magur) from fresh water; Betta (fighting fish) and Pterophyllum (angel fish) in aquaria.

Amphibia. They live both in water and on land. Most have two pairs of limbs, the body is divisible into head and trunk, and a tail may be present. The skin is moist and without scales. The eyes have eyelids and a tympanum represents the ear. The alimentary canal, urinary tract and reproductive tract open into a common chamber, the cloaca, which opens outside. Respiration is by gills, lungs and through the skin. The heart has three chambers and they are cold-blooded. Fertilisation is external, they are oviparous and development is indirect. Examples: Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander) and Ichthyophis, a limbless amphibian.

Reptilia. Mostly land-dwelling crawlers with a dry, cornified skin bearing epidermal scales or scutes. There are no external ear openings; the tympanum represents the ear. Limbs, when present, are two pairs. The heart is usually three-chambered, but crocodiles have a four-chambered heart. They are cold-blooded, and snakes and lizards shed their scales as a skin cast. Fertilisation is internal, they are oviparous and development is direct. Examples: Chelone (turtle), Testudo (tortoise), Chamaeleon (tree lizard), Calotes (garden lizard), Crocodilus (crocodile), Alligator (alligator), Hemidactylus (wall lizard), and the poisonous snakes Naja (cobra), Bungarus (krait) and Vipera (viper).

Aves. The defining feature is feathers, and most birds can fly, although some such as the ostrich cannot. Forelimbs are modified into wings, and the hindlimbs, which generally carry scales, are modified for walking, swimming or gripping branches. The skin is dry and lacks glands, except the oil gland at the base of the tail. The endoskeleton is fully ossified, and the long bones are hollow with air cavities, described as pneumatic bones. The digestive tract has extra chambers, the crop and the gizzard. The heart is completely four-chambered, and birds are warm-blooded, meaning they keep their body temperature steady. Respiration is by lungs, with air sacs connected to the lungs adding to it. Fertilisation is internal, they are oviparous and development is direct. Examples: Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin), Neophron (vulture).

Mammalia. The unique feature is the presence of mammary glands that produce milk to nourish the young. Two pairs of limbs are adapted for walking, running, climbing, burrowing, swimming or flying. The skin has hair, external ears or pinnae are present, and the jaws carry different kinds of teeth, a condition called heterodont. The heart is four-chambered, respiration is by lungs, and mammals are warm-blooded. Fertilisation is internal, and most are viviparous with direct development, though there are exceptions — Ornithorhynchus, the platypus, lays eggs. Other examples: Macropus (kangaroo), Pteropus (flying fox), Camelus (camel), Macaca (monkey), Rattus (rat), Canis (dog), Felis (cat), Elephas (elephant), Equus (horse), Delphinus (common dolphin), Balaenoptera (blue whale), Panthera tigris (tiger) and Panthera leo (lion).

The groups below differ from one another by only one or two features, so they are worth working through separately rather than in passing.

Radial does not always mean coelenterate. If a question gives radial symmetry alone, do not jump to Coelenterata. Check the other clues: if the animal is also triploblastic, coelomate and marine with a spiny skin, it is an echinoderm, whose adult is radial while its larva is bilateral. If it is diploblastic with stinging cells, it is a coelenterate; if diploblastic with eight rows of comb plates, it is a ctenophore.

Separate sexes or not. Sponges, flatworms and ctenophores are hermaphrodite. Among annelids, Nereis has separate sexes while the earthworm and leech are hermaphrodite — this split inside a single phylum is easy to get wrong. Roundworms, arthropods, molluscs, echinoderms and hemichordates have separate sexes.

Word your notochord answer carefully. Write that the notochord is a dorsal, rod-like, mesodermally derived structure formed during embryonic development, and that an animal is a chordate if it has one at any stage of life. Then the urochordate case, where the notochord sits only in the larval tail, follows naturally instead of contradicting you.

Excretory organs by phylum. Flame cells belong to flatworms, nephridia to annelids, Malpighian tubules to arthropods, the proboscis gland to hemichordates, gills within the mantle cavity handle much of it in molluscs, and echinoderms have no excretory system at all. Learn these six together as one list rather than separately.

Names that trip students. Limulus is an arthropod called the king crab and a living fossil. Pinctada is the pearl oyster, a mollusc, not an echinoderm. Balanoglossus is a hemichordate, not a chordate. Ichthyophis is a limbless amphibian, not a snake and not a fish, despite the name. Hippocampus, the sea horse, is a bony fish. Aptenodytes, the penguin, is a bird that cannot fly but still has feathers and a four-chambered heart.

How to structure a comparison answer. When asked to distinguish two groups, answer in fixed order: level of organisation, symmetry, germ layers, coelom, segmentation, and then the one or two special structures. Following the same order every time means you never leave out a point, and each difference stays easy to locate in your answer.