Animal Kingdom | ISC Class 11 Biology Notes

This note covers animal classification, levels of organisation, body plans, symmetry, embryonic layers, body cavities, segmentation, the distinguishing features of non-chordate groups, chordate subdivisions and the major vertebrate classes.

How do levels of organisation and body plans help classify animals?

Classification groups animals using shared fundamental features. These include the arrangement of cells, symmetry, body cavity, segmentation and the organisation of digestive, circulatory and reproductive systems. A phylum is a major group within a kingdom; a class is a subdivision of a phylum.

Levels of organisation

All animals are multicellular, meaning that their bodies contain many cells. Their cells are not organised in the same way. A tissue is a group of cells performing a common function; an organ contains tissues working together for a particular function.

• Cellular level: sponges have loose aggregates of cells, with some division of labour, meaning that different cells carry out different activities.
• Tissue level: in cnidarians, cells performing the same function form tissues.
• Organ level: flatworms have tissues grouped into organs, each specialised for a particular function.
• Organ-system level: organs work together in systems, each concerned with a physiological function. Roundworms, annelids, arthropods, molluscs, echinoderms and chordates exhibit this organisation.

Body plans and digestive openings

A body plan describes the basic structural arrangement of an animal. The cell aggregate plan consists of loosely associated cells, as in sponges. The blind-sac plan has a digestive cavity with one opening. The tube-within-tube plan has a digestive tube within the outer body wall.

An incomplete digestive system has a single opening serving as both mouth and anus. A complete digestive system has separate mouth and anus openings. A cnidarian illustrates the blind-sac plan; a roundworm illustrates the tube-within-tube plan.

Keep level of organisation and body plan separate: one describes how cells, tissues and organs cooperate; the other describes the body's structural arrangement. Likewise, the cavity within the digestive tract is different from the body cavity between the gut and body wall.

Definition: Intracellular digestion occurs inside cells; extracellular digestion occurs outside cells. Sponges digest food intracellularly, whereas cnidarians and ctenophores use both forms of digestion.

How do symmetry and embryonic layers distinguish animals?

Symmetry patterns

Symmetry describes how body parts are arranged so that a plane can divide the body into matching halves. A plane is an imaginary flat surface used to make such a division. An axis is an imaginary line passing through the body.

Asymmetry means that no plane through the centre divides the body into equal halves. Sponges are mostly asymmetrical. With spherical symmetry, any plane passing through the body's centre gives equivalent halves. Radial symmetry instead concerns planes passing through a particular central axis.

In radial symmetry, any plane through the central axis divides the animal into identical halves. Cnidarians and ctenophores show this pattern. In bilateral symmetry, only one plane produces identical left and right halves, as in annelids and arthropods.

Adult echinoderms have radial symmetry, but their larvae have bilateral symmetry. A larva is a young developmental stage whose form differs from the adult. Classification must therefore consider the stage of development as well as the animal's group.

Embryonic layers

Germ layers are the cell layers formed during embryonic development, the early development of an animal. The ectoderm is the outer layer and the endoderm is the inner layer. Mesoderm, when present, lies between them.

Diploblastic animals have ectoderm and endoderm. An undifferentiated layer called mesoglea lies between these layers in cnidarians. Triploblastic animals have a third germ layer, the mesoderm. Flatworms through chordates have triploblastic organisation.

The paired circular sections label ectoderm and endoderm. The diploblastic section labels mesoglea between them; the triploblastic section labels mesoderm. Mesoglea must not be relabelled as mesoderm.

See Fig. 4.2 in your NCERT textbook

Symmetry and germ layers answer different questions. A cnidarian is both radially symmetrical and diploblastic. A flatworm is both bilaterally symmetrical and triploblastic. Neither description replaces the other when giving the animal's structural characteristics.

How do body cavities and segmentation differ?

Types of body cavity

The gut is the digestive tract. The space between the body wall and gut wall is considered when describing the body cavity. A coelom is a body cavity lined by mesoderm. Animals possessing it are coelomates, including annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.

Acoelomates, such as flatworms, lack this body cavity. Pseudocoelomates, such as roundworms, possess a cavity that is not lined by mesoderm; mesoderm occurs as scattered pouches between ectoderm and endoderm. This cavity is called a pseudocoelom.

A haemocoel is a blood-filled body space in which circulating fluid directly bathes organs. Haemocoelomate organisation is characteristic of arthropods. It must not be confused with a pseudocoelom: a haemocoel describes a space associated with open circulation, rather than a mesoderm-lined true coelom.

In an open circulatory system, blood pumped from the heart directly bathes cells and tissues. In a closed circulatory system, blood remains within vessels of varying diameters. Arthropods have open circulation, whereas annelids have closed circulation.

Three circular sections show a coelomate, a pseudocoelomate and an acoelomate. The first labels the coelom between mesodermal layers; the second labels the pseudocoelom; the third shows no intervening body cavity.

See Fig. 4.3 in your NCERT textbook

Segmentation

Metameric segmentation is the external and internal division of the body into serially repeated segments, with repetition of at least some organs. The phenomenon is called metamerism. Earthworms show this arrangement, and the repeated body units are called metameres.

Segmentation and coelom are independent features. Annelids are segmented coelomates, whereas molluscs are unsegmented coelomates. A body cavity therefore does not by itself indicate segmentation. Similarly, bilateral symmetry does not identify a particular cavity type: flatworms, roundworms and annelids are all bilateral.

What distinguishes Porifera and how does its canal system work?

Porifera comprises sponges. They are generally marine, meaning sea-dwelling, and mostly asymmetrical. They show cellular organisation, with loosely associated cells rather than the tissue organisation of cnidarians. Two examples are Sycon and Spongilla; Spongilla is a freshwater sponge.

Five distinguishing characteristics

• Cellular organisation: the multicellular body consists of loose cell aggregates with some division of labour.
• Canal system: pores and internal passages provide a route for water through the body.
• Choanocytes: collar cells line the central cavity and canals.
• Intracellular digestion: food is digested within cells.
• Supporting skeleton: spicules, the skeletal elements, or spongin fibres support the body.

Water transport in sequence

The small entry pores are ostia. The central cavity is the spongocoel, and the opening through which water leaves is the osculum. Water transport helps food gathering, respiratory exchange, meaning exchange of gases, and waste removal.

1. Water enters through the minute ostia in the body wall.
2. It follows the sponge's internal water-transport pathway.
3. It reaches the central cavity, the spongocoel.
4. It leaves the body through the osculum.

The spongocoel belongs to the water-transport system; it must not be mistaken for a mesoderm-lined coelom. The terms name different structures even though both contain the syllable “coel”.

Reproduction and development

Sponges are hermaphrodites: one individual produces both eggs and sperm. Gametes are these reproductive cells; fertilisation is their fusion. Sponges reproduce sexually through gametes and asexually, without gamete fusion, by fragmentation, in which the body breaks into pieces.

Fertilisation is internal, occurring within the body. Development is indirect, with a larval stage different in form from the adult. Direct development, by contrast, produces young resembling adults. These terms describe development and must not be confused with the location of fertilisation.

How can Cnidaria and Ctenophora be distinguished?

Cnidaria: stinging cells and body forms

Cnidaria, also called Coelenterata, includes aquatic animals, mostly marine. They may be sessile, meaning attached in place, or free-swimming. Five identifying features are radial symmetry, tissue organisation, diploblastic organisation, stinging cells and a central digestive cavity with one opening.

Cnidocytes, or cnidoblasts, contain stinging capsules called nematocysts. They occur on the body and tentacles, the projecting structures around the mouth, and function in anchorage, defence and prey capture. Digestion is extracellular and intracellular. Examples include Physalia and Adamsia, the sea anemone.

A polyp is a sessile, cylindrical body form, as in Hydra and Adamsia. A medusa is an umbrella-shaped, free-swimming form, as in Aurelia. Metagenesis is the alternation between asexual polyp reproduction and sexual medusa reproduction in animals such as Obelia.

1. The polyp phase reproduces asexually.
2. This asexual reproduction produces medusae.
3. The medusa phase reproduces sexually.
4. Sexual reproduction gives rise to the polyp phase again.

This sequence applies to cnidarians that possess both forms. It does not mean that every cnidarian alternates between a polyp and a medusa. Some cnidarians, such as corals, have a skeleton made of calcium carbonate.

Ctenophora: comb plates

Ctenophora includes comb jellies or sea walnuts, such as Pleurobrachia and Ctenoplana. Five distinguishing features are an exclusively marine habitat, radial symmetry, diploblastic tissue organisation, eight external rows of ciliated comb plates and well-marked bioluminescence.

Cilia are fine hair-like cell projections; the comb plates help locomotion, or movement from place to place. Bioluminescence is the ability of living organisms to emit light. Ctenophores carry out both extracellular and intracellular digestion.

Sexes are not separate, but reproduction occurs only by sexual means. Fertilisation is external, with gamete fusion outside the body, and development is indirect. Comb plates identify ctenophores, whereas cnidocytes identify cnidarians; their shared radial symmetry does not make them the same phylum.

How do flatworms and roundworms differ?

Platyhelminthes

Platyhelminthes are flatworms with bodies flattened from the dorsal, or back, surface towards the ventral, or belly, surface. They are mostly endoparasites, meaning parasites living inside a host animal. A host is the organism in or on which a parasite lives.

Five distinguishing characters are a flattened body, bilateral symmetry, three germ layers, absence of a body cavity and organ-level organisation. Taenia, the tapeworm, and Fasciola, the liver fluke, are examples. Parasitic forms have hooks and suckers; some absorb nutrients directly through their body surface.

Flame cells are specialised cells involved in excretion, the removal of metabolic wastes, and osmoregulation, the regulation of water and salt balance. Sexes are not separate. Fertilisation is internal, and development passes through many larval stages.

Some members, including Planaria, have a high capacity for regeneration, the replacement of lost body parts. The presence of Planaria also prevents the inaccurate statement that all flatworms are parasitic.

Nematoda or Aschelminthes

Nematoda, also called Aschelminthes here, comprises roundworms. Five useful characters are a circular cross-section, bilateral symmetry, three germ layers, a pseudocoelom and a complete digestive tract. The alimentary canal, another name for the digestive tract, includes a well-developed muscular pharynx, or throat region.

They may be free-living in water or on land, or parasitic in plants and animals. Ascaris and Wuchereria are examples. An excretory tube removes wastes through an excretory pore. They have organ-system organisation.

Roundworms are dioecious, meaning that males and females are separate individuals. Often females are longer than males. Fertilisation is internal, and development may be direct or indirect.

What distinguishes annelids from arthropods?

Annelida: segmented worms

Annelida includes aquatic and terrestrial animals that are free-living and sometimes parasitic. Five important features are bilateral symmetry, triploblastic organisation, a true coelom, metameric segmentation and a closed circulatory system. They have organ-system organisation.

Longitudinal muscles run along the body's length, while circular muscles encircle it; both help locomotion. Aquatic annelids such as Nereis have lateral appendages, or side projections, called parapodia, which help in swimming.

Nephridia are organs involved in excretion and osmoregulation. Nereis has separate sexes, whereas Pheretima, the earthworm, and leeches are monoecious, meaning that male and female reproductive structures occur in the same individual. Reproduction is sexual. Nereis and Pheretima provide two examples.

Arthropoda: jointed appendages

Arthropoda is the largest animal phylum. Five distinguishing characters are jointed appendages, a chitinous exoskeleton, a segmented body, bilateral symmetry and open circulation. An exoskeleton is an external supporting covering; chitin is the material forming this covering in arthropods.

They are triploblastic coelomates with organ-system organisation. Respiratory structures include gills, organs for gas exchange in water, or a tracheal system, an air-tube system. Malpighian tubules are excretory structures. Antennae are sensory appendages; statocysts are balancing organs.

Arthropods are mostly dioecious, fertilisation is usually internal, and they are mostly oviparous, meaning egg-laying. Development may be direct or indirect. Apis, the honey bee, and Bombyx, the silkworm, are examples of economically important insects within this phylum.

What identifies molluscs, echinoderms and hemichordates?

Mollusca

Mollusca is the second largest animal phylum. Molluscs occur on land and in marine or freshwater habitats. Five features are bilateral symmetry, triploblastic coelomate organisation, an unsegmented body, a muscular foot and a mantle covering the visceral hump, the body region containing internal organs.

The mantle is a soft, spongy skin layer. The mantle cavity lies between it and the visceral hump and contains feather-like gills with respiratory and excretory functions. The shell is calcareous, meaning composed of calcium carbonate, and is usually present.

The radula is a file-like feeding organ used for rasping. Molluscs are usually dioecious and oviparous with indirect development. Pila, the apple snail, and Pinctada, the pearl oyster, are two examples.

Echinodermata

Echinodermata includes Asterias, the starfish, and Ophiura, the brittle star. Five distinguishing features are an exclusively marine habitat, a calcareous endoskeleton, adult radial symmetry with bilateral larvae, a water vascular system and absence of an excretory system.

An endoskeleton is an internal supporting skeleton; its calcareous elements in echinoderms are called ossicles. These animals have triploblastic coelomate bodies and organ-system organisation. The water vascular system is a water-filled system helping locomotion, food capture and transport, and respiration.

Sexes are separate, reproduction is sexual and fertilisation is usually external. Development is indirect with a free-swimming larva. The symmetry difference between larva and adult is essential when describing this group.

Hemichordata

Hemichordata is a separate non-chordate phylum. Five identifying features are a marine worm-like body, division into proboscis, collar and trunk, bilateral symmetry, triploblastic coelomate organisation and respiration through gills. Balanoglossus and Saccoglossus are examples.

The proboscis is the anterior, or front, body region; the collar follows it, and the trunk forms the long remaining portion. The stomochord is a rudimentary structure in the collar region resembling a notochord. It does not make hemichordates true chordates.

Circulation is open, and the excretory organ is the proboscis gland. Sexes are separate, fertilisation is external and development is indirect. The division into three body regions is different from the repeated metameres of an annelid.

How are chordates classified using the notochord?

Fundamental chordate features

A notochord is a rod-like structure derived from mesoderm on the dorsal side during embryonic development. Chordates possess it at some stage. They also have a dorsal hollow nerve cord and paired pharyngeal gill slits, openings in the pharyngeal region.

A post-anal tail is the part of the body extending beyond the anus. Chordates possess this feature and have bilateral symmetry, three germ layers, a coelom and organ-system organisation. Their circulatory system is closed.

The elongated body drawing labels the nerve cord above the notochord, gill slits near the front and the post-anal part at the rear. Keep the nerve cord and notochord as separate labelled structures.

See Fig. 4.16 in your NCERT textbook

Three subphyla

A subphylum is a subdivision of a phylum. Chordata contains Urochordata, Cephalochordata and Vertebrata. Urochordata and Cephalochordata are often called protochordates and are exclusively marine.

• Urochordata or Tunicata: the notochord is present only in the larval tail. Examples are Ascidia and Salpa.
• Cephalochordata: the notochord extends from head to tail and persists throughout life. Branchiostoma, also called Amphioxus or lancelet, is an example; another is Asymmetron.
• Vertebrata: the notochord occurs during embryonic development. Adults possess a cartilaginous or bony vertebral column, the backbone.

Cartilage is a firm supporting tissue distinct from bone. Cartilaginous fishes retain the notochord throughout life, so its persistence must be remembered when studying that class. All vertebrates are chordates, but Urochordata and Cephalochordata show why all chordates are not vertebrates.

Major vertebrate divisions

Agnatha comprises jawless vertebrates, represented by Cyclostomata. Gnathostomata comprises vertebrates with jaws. Within the latter, Pisces includes the fin-bearing classes Chondrichthyes and Osteichthyes, while Tetrapoda includes Amphibia, Reptilia, Aves and Mammalia.

Vertebrates have a ventral muscular heart and kidneys for excretion and osmoregulation. Paired appendages, when present, may be fins or limbs. Their classes are distinguished by combinations of skeletal material, body covering, respiration, reproduction and temperature regulation.

How do cyclostomes, cartilaginous fishes and bony fishes differ?

Cyclostomata

Cyclostomata includes Petromyzon, the lamprey, and Myxine, the hagfish. Three distinguishing characters are a circular sucking mouth without jaws, absence of scales and paired fins, and a cartilaginous cranium and vertebral column. The cranium is the skeletal structure enclosing the brain.

The elongated body bears 6 to 15 pairs of gill slits. Cyclostomes are ectoparasites on some fishes, meaning parasites living on the host's exterior. They are marine but migrate to fresh water for spawning, the release of reproductive cells.

1. Marine cyclostomes migrate into fresh water for spawning.
2. Spawning takes place in fresh water.
3. The adults die within a few days after spawning.
4. The larvae undergo metamorphosis, a change in body form, and return to the ocean.

Chondrichthyes

Chondrichthyes comprises marine cartilaginous fishes. Three distinguishing features are a cartilaginous endoskeleton, separate gill slits without an operculum, or gill cover, and tough skin with minute placoid scales, the tooth-like scales characteristic of this group. Scoliodon, the dogfish, and Pristis, the sawfish, are examples.

The mouth is ventral, and the notochord persists throughout life. An air bladder is absent, so these fishes have to swim constantly to avoid sinking. Fertilisation is internal, and many are viviparous, meaning that they give birth to young.

Osteichthyes

Osteichthyes includes marine and freshwater bony fishes. Three distinguishing characters are a bony endoskeleton, four pairs of gills covered by an operculum on each side, and an air bladder regulating buoyancy, the tendency to remain supported in water.

The mouth is mostly terminal, meaning at the front end. Fertilisation is usually external; they are mostly oviparous and development is direct. Examples include Labeo, the rohu, and Catla, the katla.

Both fish classes have a two-chambered heart with one auricle, the receiving chamber, and one ventricle, the pumping chamber. They are poikilothermous, or cold-blooded, lacking the capacity to regulate body temperature.

How are amphibians and reptiles distinguished?

Amphibia

Amphibia includes animals able to live in aquatic as well as terrestrial habitats. Three distinguishing features are moist skin without scales, respiration through gills, lungs and skin, and a three-chambered heart with two auricles and one ventricle.

Most amphibians have two pairs of limbs. The body has a head and trunk, and a tail may occur in some. Eyes have eyelids. A tympanum, the eardrum, represents the ear. Bufo, the toad, and Rana, the frog, are examples.

The cloaca is a common chamber receiving the digestive, urinary and reproductive tracts and opening to the exterior. Amphibians are cold-blooded, with separate sexes, external fertilisation, egg-laying reproduction and indirect development.

Reptilia

Reptilia includes mostly terrestrial animals. Three useful characters are dry cornified skin with epidermal scales or scutes, internal fertilisation and direct development. Cornified means hardened by keratin; epidermal refers to the outer skin layer, and scutes are plate-like coverings.

Limbs, when present, occur in two pairs. Reptiles lack external ear openings; a tympanum represents the ear. Snakes and lizards shed their scales as a skin cast. Chelone, the turtle, and Testudo, the tortoise, are examples.

The heart is usually three-chambered, but crocodiles have a four-chambered heart. Reptiles are poikilothermous. Their sexes are separate, and they are oviparous. The crocodile exception must remain attached to any general comparison of reptilian and amphibian hearts.

Note: “Most amphibians have two pairs of limbs” and “reptilian limbs, when present, are two pairs” have different qualifications. Neither statement means that every member has four limbs. Ichthyophis is a limbless amphibian, and snakes lack limbs.

Compare the groups through several features together: skin covering, fertilisation and development give clearer distinctions than a shared cold-blooded condition. A three-chambered heart alone cannot distinguish a typical amphibian from a typical reptile.

What are the distinguishing features of birds and mammals?

Aves

Aves comprises birds. Three distinguishing features are feathers, forelimbs modified into wings and a fully bony endoskeleton with pneumatic long bones. Pneumatic bones are hollow bones containing air cavities. Most birds can fly, but flightless birds such as the ostrich are exceptions.

Birds possess a beak. Their hindlimbs generally bear scales and are modified for walking, swimming or clasping branches. Skin is dry and lacks glands except an oil gland at the tail base. Corvus, the crow, and Columba, the pigeon, are examples.

Birds have a completely four-chambered heart and are homoiothermous, or warm-blooded: they can maintain a constant body temperature. Respiration occurs through lungs, supplemented by connected air sacs. Sexes are separate, fertilisation is internal, reproduction is oviparous and development is direct.

Mammalia

Mammalia includes animals found in habitats ranging from polar ice caps to deserts, forests and caves. Some are adapted for flight or aquatic life. Three distinguishing features are milk-producing mammary glands, hair on the skin and different types of teeth in the jaw.

Mammary glands nourish the young with milk. Limbs are adapted for activities such as walking, running, burrowing, swimming or flying. The heart is four-chambered, body temperature is regulated, and respiration occurs through lungs.

Sexes are separate, fertilisation is internal and development is direct. Mammals are viviparous with few exceptions. Ornithorhynchus, the platypus, is an oviparous exception, while Macropus, the kangaroo, is viviparous; these provide two contrasting mammalian examples.

Both birds and mammals are warm-blooded and have four-chambered hearts, so those shared features cannot alone distinguish them. Feathers identify birds, while mammary glands identify mammals. Flight alone is insufficient because some mammals fly and some birds are flightless.

Glossary

• Cellular organisation — An arrangement in which cells form loose aggregates with some division of labour.
• Radial symmetry — A body arrangement allowing equal halves along any plane through the central axis.
• Bilateral symmetry — A body arrangement allowing identical left and right halves in only one plane.
• Diploblastic — Having two embryonic germ layers, an outer ectoderm and an inner endoderm.
• Triploblastic — Having mesoderm as a third embryonic layer between ectoderm and endoderm.
• Coelom — A mesoderm-lined body cavity located between the body wall and gut wall.
• Pseudocoelom — A body cavity not lined by mesoderm, characteristic of roundworm organisation.
• Metamerism — Serial body segmentation involving external and internal divisions and repetition of some organs.
• Choanocytes — Collar cells lining the spongocoel and canals of the sponge body.
• Cnidocytes — Cells containing stinging capsules, used for anchorage, defence and prey capture.
• Metagenesis — Alternation between asexually reproducing polyps and sexually reproducing medusae in certain cnidarians.
• Notochord — A mesoderm-derived rod-like structure formed dorsally during embryonic development in chordates.
• Poikilothermous — Describing animals that lack the capacity to regulate their body temperature.
• Homoiothermous — Describing animals able to maintain a constant body temperature despite environmental variation.
• Indirect development — Development involving a larval stage that differs in form from the adult.

Common errors and misconceptions

• Misconception: All sponges are marine and asymmetrical. Correct: Sponges are generally marine and mostly asymmetrical; Spongilla is a freshwater example.
• Misconception: Mesoglea is the third germ layer. Correct: Mesoglea lies between the two layers in diploblastic cnidarians; mesoderm is the third germ layer of triploblastic animals.
• Misconception: The spongocoel, pseudocoelom and coelom are interchangeable names. Correct: The spongocoel carries water, a pseudocoelom lacks a mesodermal lining, and a true coelom has that lining.
• Misconception: Echinoderms have radial symmetry throughout life. Correct: Adults are radially symmetrical, whereas larvae have bilateral symmetry.
• Misconception: Every chordate is a vertebrate. Correct: Urochordata and Cephalochordata belong to Chordata but are separate from Vertebrata.
• Misconception: All reptiles have three-chambered hearts. Correct: The reptilian heart is usually three-chambered, but crocodiles have four chambers.
• Misconception: All mammals give birth to young. Correct: Mammals are viviparous with few exceptions; Ornithorhynchus lays eggs.
• Misconception: Every bird can fly. Correct: Most birds can fly, but the ostrich is flightless. Feathers, rather than flight alone, distinguish birds.

Exam-style questions with model answers

Q1. Define intracellular digestion and extracellular digestion. [2 marks]

1. Intracellular digestion is the breakdown of food within cells, as seen in sponges.
2. Extracellular digestion is the breakdown of food outside cells; cnidarians use this as well as intracellular digestion.

Q2. Distinguish acoelomate, pseudocoelomate and coelomate organisation by the body cavity and give one animal group for each. [3 marks]

1. Acoelomate animals lack a body cavity between the body wall and gut wall. Platyhelminthes, the flatworms, illustrate this type of organisation.
2. Pseudocoelomate animals possess a body cavity that is not lined by mesoderm. Nematoda or Aschelminthes, the roundworms, provide an example.
3. Coelomate animals possess a body cavity lined by mesoderm between the body and gut walls. Annelida is one group with this organisation.

Q3. Describe the sponge water pathway from entry to exit in three stages, and state its functions as a fourth point. [4 marks]

1. Water enters the sponge through minute pores called ostia in the body wall, beginning its passage through the canal system.
2. The incoming water passes through the internal water-transport pathway into the central cavity, which is called the spongocoel.
3. Water then leaves the spongocoel and passes out of the sponge through the opening known as the osculum.
4. This water pathway helps in food gathering, respiratory exchange and removal of waste, linking transport with several essential functions.

Q4. Compare Chondrichthyes and Osteichthyes under five headings: skeletal material, mouth position, gill covering, air bladder and fertilisation. [5 marks]

1. Chondrichthyes has an endoskeleton made of cartilage, whereas Osteichthyes has a bony endoskeleton. Skeletal material is a central distinction between these two fish classes.
2. The mouth is ventral in Chondrichthyes. In Osteichthyes it is mostly terminal, so the qualification “mostly” must be retained in the comparison.
3. Chondrichthyes has separate gill slits without an operculum. Osteichthyes has four pairs of gills covered by an operculum on each side.
4. An air bladder is absent in Chondrichthyes. In Osteichthyes, an air bladder regulates buoyancy and helps support the fish in water.
5. Fertilisation is internal in Chondrichthyes, whereas it is usually external in Osteichthyes. External fertilisation should therefore not be stated as universal for bony fishes.

Q5. Describe the notochord in Urochordata and Cephalochordata, then state its embryonic occurrence in Vertebrata and its persistence in cartilaginous fishes. [3 marks]

1. In Urochordata, the notochord is present only in the larval tail. Ascidia and Salpa are examples of this exclusively marine subphylum.
2. In Cephalochordata, the notochord extends from the head to the tail and persists throughout life. Branchiostoma is an example of this organisation.
3. Vertebrates possess a notochord during embryonic development and have a cartilaginous or bony vertebral column. In cartilaginous fishes, the notochord persists throughout life.

Q6. Give three distinguishing features and two examples of Echinodermata. State each example as a separate point. [5 marks]

1. Echinoderms have a calcareous endoskeleton formed of ossicles. This is an internal supporting skeleton containing calcium carbonate, rather than the chitinous external covering of arthropods.
2. Adult echinoderms are radially symmetrical, whereas their larvae are bilaterally symmetrical. The description of symmetry therefore changes with the stage of development.
3. They have a distinctive water vascular system that assists locomotion, capture and transport of food, and respiration. These functions belong to the same characteristic system.
4. Asterias, commonly called the starfish, is one example of the phylum Echinodermata. It belongs to an animal group whose members are all marine.
5. Ophiura, commonly called the brittle star, is a second example of Echinodermata. It is a different example from Asterias and provides the second required name.

Q7. Contrast amphibian and reptilian skin, fertilisation and development. [3 marks]

1. Amphibian skin is moist and without scales. Reptilian skin is dry and cornified, with epidermal scales or scutes forming the body covering.
2. Amphibians have external fertilisation, with fusion of gametes outside the body. Reptiles have internal fertilisation, so fusion occurs within the body.
3. Amphibians show indirect development with a larval stage different from the adult. Reptiles show direct development, in which the young resemble adults.

Q8. State the distinctive feeding role of mammary glands and name the egg-laying mammal Ornithorhynchus by its common name. [2 marks]

1. Mammary glands produce milk with which mammals nourish their young.
2. Ornithorhynchus is the platypus, an egg-laying exception to the usual mammalian pattern of giving birth to young.

Key takeaways

• Animal classification combines organisation, body plan, symmetry, germ layers, body cavity, segmentation and notochord characters.
• Sponges have cellular organisation and a canal system; cnidarians have tissue organisation and specialised stinging cells.
• Flatworms are acoelomate, roundworms are pseudocoelomate, and annelids possess a true coelom with metameric segmentation.
• Jointed appendages distinguish arthropods, while the mantle, muscular foot and visceral hump characterise molluscan organisation.
• Echinoderms have a water vascular system, radial adults and bilateral larvae; hemichordates have proboscis, collar and trunk.
• Chordate subdivisions differ in the notochord's location, persistence and relationship to the adult vertebral column.
• Cartilaginous and bony fishes differ in skeleton, gill covering, air bladder and usual fertilisation pattern.
• Keep exceptions and qualifiers: most birds fly, crocodiles have four-chambered hearts, and a few mammals lay eggs.

Test yourself

What separates spherical symmetry from radial symmetry?

Spherical symmetry allows division into equivalent halves through any plane through the centre; radial symmetry concerns planes through a particular central axis.

What is a haemocoel?

A haemocoel is a blood-filled body space in which circulating fluid directly bathes organs, as in arthropods.

Which structures distinguish cnidarians from ctenophores?

Cnidarians have cnidocytes containing stinging capsules; ctenophores have eight external rows of ciliated comb plates.

Which excretory structures occur in flatworms and annelids?

Flatworms have flame cells, whereas annelids have nephridia. Both structures also participate in osmoregulation.

Why are hemichordates not classified as true chordates?

They possess a stomochord, a rudimentary collar-region structure resembling a notochord, rather than the true chordate notochord.

Which vertebrate class lacks jaws, scales and paired fins?

Cyclostomata lacks these features and has a circular sucking mouth; Petromyzon and Myxine are examples.

What qualification is necessary when describing reptilian hearts?

The heart is usually three-chambered, but crocodiles have a four-chambered heart.

Why does flight alone fail to distinguish birds from mammals?

Most birds fly, but some are flightless; some mammals also fly. Feathers and mammary glands distinguish the respective groups.