Animalia

10Introduction to Kingdom Animalia

The name Animalia comes from the Latin word anima, meaning breath or soul. All the animals of the world are included in the kingdom Animalia. Animals are found almost in all types of habitats. They may be free living and motile, sessile (fixed in one place) or parasites. They range in size from worms seen only with a microscope to blue whales.

Animal

A eukaryotic, multicellular, ingestive heterotroph that lacks cell walls.

Distinct features of animals

• Eukaryotic: the cells have a true nucleus and membrane-bound organelles.
• Multicellular: the body is made of many cells.
• Ingestive heterotrophs: they cannot make their own food, so they take in (ingest) food made by other organisms.
• No cell walls: animal cells are bounded only by a cell membrane.

Two major groups

Animals are divided into two major groups, invertebrates and vertebrates. This unit first studies the criteria used to classify animals, then the eight phyla of invertebrates, and then the phylum Chordata, which includes the vertebrates.

10.1Criteria for animal classification

The book notes that these criteria are not given in the curriculum, but students need to know them. Animals can be classified according to four criteria: (a) the presence or absence of tissues, (b) the number of tissue layers, (c) the type of body symmetry and (d) the type of body cavity.

10.1.1Classification based upon organization of tissues

Animals can be classified according to the organization of their tissues into two subkingdoms: Parazoa and Eumetazoa.

Parazoa

The simplest multicellular animals. They lack tissue organization but show division of labour. They are asymmetrical. The group includes all the sponges.

Eumetazoa

Animals in which similar cells are grouped together into a highly coordinated unit called a tissue. The tissues are assembled into larger functional units called organs, and different organs operate together as an organ system.

10.1.2Classification based upon number of tissue layersOften examined

Animals can be classified according to the number of tissue layers (germ layers) as diploblastic and triploblastic.

Diploblastic animals

Animals whose body consists of two germ layers of cells, the ectoderm and endoderm. They have a tissue level of organization. A jelly-like mesoglea lies between the two germ layers. There are no specialized organs. A neuron net is present and there is only one cavity, the gastrovascular cavity, with only one opening. Example: animals of phylum Cnidaria.

Triploblastic animals

Animals whose body consists of three germ layers: ectoderm, mesoderm and endoderm.

10.1.3Classification based upon type of body symmetryOften examined

Animals can be classified according to body symmetry. The subkingdom Eumetazoa is divided into two grades: grade Radiata and grade Bilateria. Sponges, the Parazoa, lack symmetry (they are asymmetrical).

Grade Radiata

Radial symmetry

The body has a top and bottom and similar body parts arranged as spokes, radiating from a central body axis. Examples: jellyfish and sea anemone.

Radial symmetry is considered an adaptation for a sessile life: an animal that stays in one place meets food and danger equally from every side.

Grade Bilateria

Bilateral symmetry

A plane through the midline divides the body into roughly equivalent right and left halves that are mirror images.

• The front or anterior end generally has a head; the rear or posterior end may be equipped with a tail.
• There are well-defined dorsal (upper) and ventral (lower) surfaces.
• All the animals included in grade Bilateria are triploblastic.
• Bilateral symmetry is considered an adaptation to motility (active movement): the head end meets the environment first.
• In echinoderms the larval stages show bilateral symmetry and the adult secondarily develops radial symmetry.

10.1.4Classification based upon type of body cavityOften examined

Animals can be grouped according to the type of body cavity or coelom.

Coelom

A fluid-filled space between the outer body wall and the digestive tube.

Acoelomate

There is no body cavity between the digestive tract and the outer body wall, so these animals are called acoelomate. The mesoderm is packed solidly between the ectoderm and endoderm. Example: Platyhelminthes (flatworms).

Pseudocoelomates

If the body cavity develops between the mesoderm and endoderm, it is called a pseudocoelom (false cavity). Animals with this type of cavity are pseudocoelomates. Example: Aschelminthes (nematodes).

Coelomate

If the body cavity forms within the mesoderm and is completely lined by mesoderm, the body cavity is a true coelom. It is filled with coelomic fluid. Animals with a true coelom are called coelomates. Animals from annelids to chordates are coelomate.

Protostomes and deuterostomes

Coelomates can be divided into two groups, protostomes and deuterostomes. These groups reflect two main lines of evolution based on their pattern of early development.

10.2Diversity in animals: invertebratesOften examined

Animals are diverse in structure. There are vast differences in structural complexity among organisms, ranging from the simplest sponges to humans.

Invertebrates

Invertebrates

The animal species that lack a backbone. They account for 95% of known animal species.

The invertebrates have been divided into eight major phyla: Porifera, Cnidaria, Platyhelminthes, Aschelminthes, Mollusca, Annelida, Arthropoda and Echinodermata. Each phylum is studied in turn below.

10.2.1 (1)Phylum Porifera (pore bearer)Often examined

Phylum Porifera means "pore bearer". These are the sponges.

General characteristics

• Sponges are sessile, attached to the rocks at the bottom of water. The larvae are motile.
• They are all aquatic, mostly marine, and some are found in freshwater.
• They range in size from a few millimetres wide to more than a metre long.
• The body is multicellular and not organized as tissues or organs. The body lacks symmetry.
• The body is perforated by many pores called ostia. There is a single cavity inside the body called the spongocoel.
• Sponges depend on food coming along with water currents.
• The skeleton is formed by spicules of various shapes. These are needle-like and may be calcareous or siliceous. The bath sponge has spongin fibre.

Reproduction and regeneration

• Asexual reproduction takes place by budding or by gemmules. Buds develop into new sponges.
• Sexual reproduction takes place by egg and sperm. Sexes may be separate or hermaphrodite. Embryo development includes free-swimming ciliated larval stages.
• Sponges have a remarkable ability of regeneration from a small fragment.
• Sponges have evolved from the protists called choanoflagellates.

Examples of sponges are Spongilla and Sycon.

Importance of sponges

• The skeletons of sponges are used for washing and bathing.
• Sponges have a great capacity to absorb water, so they are used in surgical operations for absorbing fluid and blood.
• Sponges are used for sound absorption in buildings.

10.2.1 (2)Phylum Cnidaria (stinging celled organisms)Often examined

Phylum Cnidaria means "stinging celled organisms".

General characteristics

• Cnidarians are entirely aquatic, mainly marine; a few are found in freshwater, for example Hydra.
• Most species are sessile (Hydra), while others are free living and motile (jellyfish). Some are colonial (Obelia).
• They range in size from microscopic (Hydra) to two metres in length (jellyfish).
• The body shows radial or biradial symmetry.
• They are diploblastic, having ectoderm, endoderm and mesoglea in between the two.
• They have a sac-like internal gastrovascular cavity with only one opening, the mouth, often surrounded by tentacles.
• The tentacles and body carry stinging cell organelles called nematocysts.
• The nervous system is present in the form of a net.
• Asexual reproduction takes place by budding and sexual reproduction by gametes.

Zooids and colonies

Cnidarians also occur in the form of colonies. The units of the colonies are called zooids. There are two main types of zooids: polyps, which are feeding zooids, and medusae, which are reproductive zooids for sexual reproduction.

Polymorphism

Polymorphism

The condition in which a cnidarian colony shows a number of zooids of different forms that take up different functions. It denotes division of labour among the zooids of the colony.

A polymorphic colony contains many individuals called zooids, mainly of two types: polyp and medusa. The polyp is sedentary and shows the mouth and tentacles at its free end. The medusa is free swarming. A single polyp form can perform all functions. Obelia-like animals show two forms, the polyp as the nutritive zooid and the medusa as the reproductive zooid. This is called dimorphic organization. In a colony of Obelia three types of zooids are present: (1) hydranth (a polyp stage), (2) blastostyle (asexually reproducing zooid) and (3) medusae.

Importance of cnidarians

• Coral reefs protect shores from erosion by tidal waves.
• Corals are used in jewellery, and others are used in aquaria, rock gardens and so on.
• Some cnidarians have poisonous stings, for example jellyfish and sea anemone.

10.2.1 (3)Phylum Platyhelminthes (flatworms)Often examined

Phylum Platyhelminthes means "flatworms".

General characteristics

• The flatworms are free living (for example Planaria) or parasites (for example tapeworm). They are found in freshwater, marine habitats, animal gut and liver.
• The body is soft and flattened dorsiventrally.
• They are triploblastic and show bilateral symmetry.
• Coelom is absent, and the spaces are filled with mesenchyme tissue (they are acoelomate).
• Eyespots are present in some flatworms.
• Free-living forms are motile. They move by cilia present on the underside of the animal, for example Planaria. In parasitic forms movement is restricted.
• Reproduction takes place both by asexual and sexual means. Asexual reproduction is by fission. Most forms are monoecious.
• The reproductive system is complex, usually with well-developed gonads, ducts and accessory organs. Fertilization is internal.

The examples of flatworms are Dugesia (planaria), Fasciola (liver fluke) and Taenia (tapeworm).

Importance of Platyhelminthes

The parasitic forms of flukes and tapeworms are very harmful for man, for example tapeworm, liver fluke and the blood fluke of cattle.

10.2.1 (4)Phylum Aschelminthes (roundworms) / NematodaOften examined

Phylum Aschelminthes means "roundworms"; they are also called Nematoda (nematodes).

General characteristics

• Roundworms are free living or parasites. They live in soil, roots, human and animal intestine and muscles.
• Most roundworms are less than five cm long and many are microscopic, but some parasitic roundworms are more than one metre in length.
• The worms exhibit bilateral symmetry, having three germ layers (triploblastic).
• The body is cylindrical, tapering at both ends.
• The muscular layer is not continuous. It is divided into four longitudinal quadrants: two dorsolateral and two ventrolateral.
• The body cavity is a pseudocoelom.
• Most nematodes are dioecious (separate sexes). Fertilization is internal.

Importance of Aschelminthes

• Ascaris lumbricoides is an intestinal parasite of man.
• Pinworms (Enterobius vermicularis) are parasites in the human caecum, colon and appendix, causing insomnia and loss of appetite.

10.2.1 (5)Phylum Mollusca (soft bodied animals)Often examined

Phylum Mollusca means "soft bodied animals".

General characteristics

• Molluscs are free living or sessile and live in freshwater, marine habitats and on land (in moist places).
• They exhibit bilateral symmetry, are triploblastic, coelomate, soft and unsegmented animals.
• The body is divided into two parts: the head ventral muscular foot and the dorsal visceral region.
• The whole animal is covered in an envelope called the mantle. It secretes the shell and covers the visceral mass.
• The mantle cavity functions in excretion, gas exchange, elimination of digestive wastes and release of reproductive products.
• The shell may be external (snail), internal (cuttlefish) or even absent (octopus).
• The mouth cavity may have a tongue-like structure called the radula, for example in cuttlefish and snail.
• The coelom is divided into cavities surrounding the heart, nephridia and gonads.
• Sexes may be separate (Unio) or united (Helix).
• Development takes place through trochophore larvae.

The examples of molluscs are snail, slug, oyster, freshwater mussel and octopus.

Economic importance of molluscs

• The shells of freshwater mussels are used in the button industry. Shells are also used for making ornaments.
• Some oysters make valuable pearls, for example the pearl oyster.
• Clams, oysters and mussels are a source of food in the Far East, Europe and America.
• Slugs are injurious in garden and cultivation. Toredo, a shipworm, damages wooden parts in ships.

10.2.1 (6)Phylum Annelida (segmented worms)Often examined

Phylum Annelida means "segmented worms".

General characteristics

• Annelids are called segmented worms. They are free living (earthworm) or ectoparasite (leech). They are found in soil, freshwater and marine habitats, for example Nereis.
• The body is metamerically segmented.
• The coelom is a true coelom. It is separated into compartments. The coelomic fluid of the adjacent chamber is mixed. The coelomic fluid also serves as a hydrostatic skeleton.
• The body wall contains circular and longitudinal muscles which help in locomotion.
• The organs of locomotion are chitinous chaetae or setae. Parapodia are present in the body wall of Nereis.
• The common mode of reproduction is sexual. Most annelids are hermaphrodite (earthworm, leech); sexes are separate in some annelids (Nereis). Fertilization is external.
• They have a closed circulatory system and a ventral nerve cord with ganglia.
• Development is direct or indirect through trochophore larvae. Regeneration is common in annelids.

The examples of annelids are Nereis, Pheritima (earthworm) and Hirudo (leech).

Economic importance of annelids

• Polychaetes form an important food item for many edible fish.
• Earthworms help in soil improvement.
• The leech is an ectoparasite to man and cattle.

10.2.1 (7)Phylum Arthropoda (animals with jointed legs)Often examined

Phylum Arthropoda means "animals with jointed legs". The arthropods are called joint footed animals.

General characteristics

• Arthropods are free living or parasites and are found in all types of habitat.
• The body is segmented. Some are worm-like and others are flying insects. Segments are modified, specialized and fused.
• Symmetry is bilateral; head, thorax and abdomen are variously distinct or fused.
• The body is covered by chitin. It is flexible at many places to allow articulation.
• There are several pairs of appendages.
• Coelom is not present as the main body cavity. It is reduced and is called the haemocoel, because it is connected with the blood vascular system.
• The alimentary canal has two openings, the mouth and the anus.
• The skeleton is an exoskeleton, formed chiefly of chitin. Muscles are attached to the exoskeleton for locomotion.
• Sexes are separate. The male and female arthropods are often unlike. The reproductive organs and ducts are paired: testes produce sperms and ovaries produce eggs. Fertilization is mostly internal.
• Development takes place through metamorphosis.

Importance of arthropods

10.2.1 (8)Phylum Echinodermata (spiny skinned animals)Often examined

Phylum Echinodermata means "spiny skinned animals".

General characteristics

• Echinoderms are free living; some are attached to the substratum. They are exclusively marine. Most are found at the bottom along the shorelines in shallow seas.
• The body is covered by a delicate epidermis.
• Echinoderms are triploblastic coelomates. They show radial symmetry in the adult; the larva has bilateral symmetry.
• They have an endoskeleton consisting of spine-bearing calcium-rich plates. The spines stick out through the delicate skin, which gives the phylum its name.
• The mouth is on the oral side and the anus on the aboral side. There is a central disc from which arms radiate.
• The nervous system consists of a nerve, a nerve ring and radial nerves.
• The sexes are separate. Fertilization is external.
• The larvae, such as bipinnaria and brachiolaria, are complex and exhibit bilateral symmetry, autotomy and regeneration. Regeneration is shown by the adult and larval stages.

Other examples are the sea lily. The photographs in the book (Figure 10.15 and Figure 10.16) show a starfish, cake urchin, sea urchin, sea cucumber and brittle star.

Water vascular system

Water vascular system

The part of the coelom of an echinoderm that is made up of canals and spaces. It is composed of water-filled canals used in locomotion, attachment, gas exchange and feeding. The organs of locomotion are the tube feet, which are present along the edges of grooves in the arms.

Importance of echinoderms

• Many echinoderms are used as food.
• The dried skeleton of echinoderms is used as fertilizer because of its high percentage of calcium and nitrogen.
• Starfishes act as scavengers and thus clean sea water.
• They cause damage to oyster beds. The stinging sea urchins are poisonous.

10.2.1 (9)Phylum Hemichordata (acorn worm)

Phylum Hemichordata means "acorn worm". Hemichordates show characteristics of both echinoderms and chordates, and both phyla belong to the deuterostome branch of the animal kingdom. Hemichordates are also called prochordates because of their close relationship to chordates. Examples: Balanoglossus and Saccoglossus.

General characteristics

• The hemichordates are called acorn worms. All hemichordates are marine. Some are solitary, naked and slow moving; others are sedentary.
• The body is soft, unsegmented and has a worm-like form. The body has three distinct regions: proboscis, collar and trunk.
• Symmetry is bilateral and hemichordates are triploblastic. The body cavity is a true coelom.
• The excretory system comprises a glomerulus situated in the proboscis and connected with blood vessels. There are no nephridia.
• Sexes may be separate or united. Fertilization is external. Development may include a free-swimming larval stage.

10.2.1 (10)Phylum ChordataOften examined

The representatives of the phylum Chordata, called chordates, are the most familiar, adaptable and successful and the most widely distributed animals, showing diversity of form, habitat and habits. The phylum includes the vertebrates.

Characteristics of chordates

All the chordates possess four basic characteristics: (a) notochord, (b) dorsal hollow nervous system, (c) gill slits and (d) post-anal tail.

10.2.1 (10)Classification of chordatesOften examined

The phylum Chordata has been subdivided into two groups: (a) Protochordata (Acrania), in which the brain is not enclosed in a bony case, and (b) Craniata, in which the brain is enclosed in a bony case and the notochord has been replaced by the vertebral column.

10.2.1 (10a)Group Protochordata (Acrania)

Group Protochordata (Acrania) is the group of chordates in which the brain is not enclosed in a bony case. It has been divided into two subphyla: (i) subphylum Urochordata and (ii) subphylum Cephalochordata.

Subphylum Urochordata

The larva has a mid-dorsal supporting rod, the notochord, in the tail, so the group has been named urochordata. The notochord usually disappears during metamorphosis, so that the adult has no skeleton. The examples of urochordates are Ascidia and Halosymthia (tunicates).

Subphylum Cephalochordata

The body is fish-like. It has no head but a tail is present. The notochord extends the entire length of the body. There is no organ for respiration. The example of a cephalochordate is Amphioxus (lancelet).

10.2.1 (10b)Group Craniata: subphylum Vertebrata

Group Craniata has one subphylum, Vertebrata.

Subphylum Vertebrata

The characteristics that give the members of this group the names "vertebrata" and "craniata" are the spinal column of vertebrae, which forms the chief skeletal axis of the body, and a brain case or cranium. The subphylum Vertebrata is divided into two super classes: (a) Agnatha and (b) Gnathostomata.

10.2.1 (10c)Super class Agnatha (jawless fishes)

Super class Agnatha means jawless fishes.

General characteristics

• It is divided into two classes: Myxini and Cephalaspidomorphi.
• The body is slender, eel-like and rounded with naked skin.
• The sexes are separate. Fertilization is external and there is no larval stage.
• The examples of agnatha are hagfish and lamprey.

10.2.1 (10d)Super class Gnathostomata

Super class Gnathostomata is divided into six classes: Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia.

10.2.1 (10e1)Class Chondrichthyes (cartilaginous fishes)Often examined

Class Chondrichthyes means cartilaginous fishes.

General characteristics

• The body is laterally compressed and spindle (fusiform) shaped.
• The mouth is ventral.
• The olfactory sacs are not connected to the mouth cavity.

Examples: the dogfish shark (Squalus acanthias), the eagle ray (Myliobatis) and the skate (Raja).

10.2.1 (10e2)Class Osteichthyes (bony fishes)Often examined

Class Osteichthyes means bony fishes.

General characteristics

• The body is usually spindle-shaped and streamlined for active movement through water.
• The endoskeleton is partly or wholly bony.
• Vertebrae are numerous.
• The pelvic girdle is often absent.
• The notochord persists in a greatly reduced form.

Examples in the book: Labeo rohita (rohu, Figure 10.24), the sea horse (Figure 10.25) and the lobe-finned fish Latimeria, a living fossil (Figure 10.26).

10.2.1 (10e3)Class AmphibiaOften examined

Class Amphibia includes frogs, toads, salamanders, caecilians and mud puppies.

General characteristics

• The body varies considerably in form and is divisible only into head and trunk.
• Most have two pairs of pentadactyl limbs with 4 to 5 or fewer digits. Some are without legs (caecilians). Webbed feet are often present (frogs).
• The skin is often smooth, moist and rich in glands. It is highly vascular. Scales are generally absent. In some, glands are poisonous. Chromatophore pigment cells are present in the skin.
• In the larval stage respiration takes place by gills; in the adults by lungs and skin.
• The heart is three chambered (two atria and one ventricle). Sinus venosus and truncus arteriosus are present. Double circulation takes place through the heart.
• The sexes are separate. The gonads are paired. Fertilization may be external or internal. Most forms are oviparous.
• Development takes place through metamorphosis.
• Amphibians are anamniotes (without amnion).
• Body temperature is variable, that is poikilothermic (ectotherms), and most forms undergo hibernation in winter.

The examples of amphibians are frogs, toads, salamanders, caecilian, mud puppy (Necturus) and so on.

10.2.1 (10e4)Class ReptiliaOften examined

Class Reptilia includes tortoise, lizard, snake, crocodile and alligator.

General characteristics

• There are two pairs of pentadactyl limbs, each typically with five digits.
• The skin is rough, cornified and dry, which is adapted to land life.
• The heart is incompletely four chambered, having two atria and a partly divided ventricle. Crocodiles have a completely four chambered heart.
• Reptiles are cold blooded animals, that is poikilothermic (ectotherms), and hibernate in winter.
• The sexes are separate. The gonads are paired. Fertilization is internal. Most forms are oviparous.
• The eggs are large, amniotic and have large yolk. They are enclosed by a leathery or limy shell for protection.
• The embryo is protected by three embryonic membranes known as amnion, allantois and chorion.

The examples of reptiles are tortoise, lizard, snake, crocodile and alligator.

10.2.1 (10e5)Class AvesOften examined

Class Aves includes the birds.

General characteristics

• The body is streamlined and boat shaped. It is divisible into a head, a neck, a trunk and a tail. The neck is very long and the tail very short.
• There are two pairs of pentadactyl limbs. The forelimbs are modified to form wings. The hind limbs are large, strong and adapted for perching, walking or swimming. Each foot usually bears four toes armed with horny claws.
• The skin is covered by an epidermal horny exoskeleton of feathers all over the body and scales on the feet.
• Due to air spaces the skeleton is light. The skull has large sockets. The jaws extend into a horny beak. Teeth are absent.
• The heart is four chambered, having two atria and two ventricles. There is only the right aorta. It curves to the right side and then bends backward.
• Birds are endothermic. Respiration takes place only by lungs. A system of thin-walled air sacs lying among the viscera maintains the supply of fresh air through the lungs.
• The voice box, the syrinx, lies at the junction of the trachea and bronchi.
• The alimentary canal has a muscular structure called the gizzard, which is used for crushing food.
• The excretory system consists of a pair of kidneys. The ureters open into the cloaca and the urinary bladder is absent. The urine is semisolid and uric acid is the main nitrogenous waste.
• The sexes are separate. Fertilization is internal. The eggs are large with much yolk. Only one ovary and oviduct is functional.
• Some birds have secondarily lost the power of flight and are called running birds, for example ostrich and kiwi.

The book's Figure 10.29 shows a kiwi, penguin, duck and parrot.

10.2.1 (10e6)Class MammaliaOften examined

Class Mammalia includes the mammals, from the egg-laying monotremes to man.

General characteristics

• The body is variously shaped and divisible into a head, a neck, a trunk and a tail.
• There are two pairs of pentadactyl limbs. These are variously adapted for walking, running, burrowing and swimming or flying.
• The skin is glandular, mostly covered by hair.
• The coelom is completely divided into an anterior smaller thoracic cavity and a posterior larger cavity by a muscular partition, the diaphragm, which is present only in mammals.
• The endoskeleton is fully ossified. The skull has two occipital condyles and a large cranium.
• The external ear (pinna) is present. There is a chain of three bones in the ear: incus, malleus and stapes.
• Mammals have deciduous and permanent teeth.
• The heart is four chambered. Only the left aortic arch is present. Red blood cells are non-nucleated.
• Mammals are warm blooded (endothermic) animals.
• The voice apparatus is well developed and consists of the larynx and epiglottis.
• Mammals give birth to their young ones and feed them on milk produced by the mammary glands of the mother.

Subclasses of mammals

Mammals are classified into three subclasses: (1) Prototheria, the egg-laying mammals; (2) Metatheria, the pouched mammals; and (3) Eutheria, the placental mammals.

The tetrapod classes at a glance

10.2.1 (10e6a)Sub class Prototheria (the monotremes)Often examined

The subclass Prototheria is a connecting link between reptiles and mammals and provides evidence of the evolution and origin of mammals from reptiles.

• Certain members of this subclass are adapted for aquatic life, for example the duckbill platypus, which has a bill similar to that of a duck and has webbed toes.
• The mammalian feature of the monotremes is that the female has mammary glands and they feed their young.
• The reptilian features include the presence of a cloaca and cloacal opening (instead of separate openings for the digestive and urinogenital systems).
• Monotremes are found in Australia.

The examples of monotremes are the duckbill platypus and the echidna (spiny anteater).

10.2.1 (10e6b)Sub class Metatheria (the marsupials)Often examined

The subclass Metatheria are the marsupials.

• The females have an abdominal pouch, the marsupium, where they rear their young.
• The young ones when born are immature. The nipples are in the pouch.
• The mother feeds the young ones and carries them in the pouch till they are matured enough.
• The marsupials are found in Australia and America.

The examples of marsupials are the opossum, kangaroo and koala.

10.2.1 (10e6c)Sub class Eutheria (the placentalis)Often examined

The subclass Eutheria are the placentalis, the placental mammals.

• Development of the young takes place inside the body of the mother.
• The young ones are born fully developed, at a relatively advanced stage.
• Developing placental mammals depend on the placenta, an organ of exchange between maternal and fetal blood. So these mammals are called placental mammals.
• Nutrients are supplied to the growing offspring, and wastes are passed to the mother for excretion.
• All the placental mammals have maximum mammalian characteristics. In some, hair has been modified into scales in pangolin and spines in porcupine.

The examples of placentalis are man, whale, elephant, horse, rat, mice, bat, dolphin, cat, tiger, lion, monkey, gorilla and so on. The book's Figure 10.32 shows a whale, panda, tiger, bat, goat and elephant.

Placenta

An organ of exchange between maternal and fetal blood, by which a developing placental mammal receives nutrients and passes wastes to the mother.

Key terms

Kingdom Animalia

The kingdom that includes all animals; the name comes from the Latin word anima, meaning breath or soul.

Animal

A eukaryotic, multicellular, ingestive heterotroph that lacks cell walls.

Parazoa

The subkingdom of the simplest multicellular animals, which lack tissue organization but show division of labour; asymmetrical; includes all the sponges.

Eumetazoa

The subkingdom of animals in which similar cells are grouped into tissues, tissues into organs and organs into organ systems.

Diploblastic animals

Animals whose body consists of two germ layers, ectoderm and endoderm, with a jelly-like mesoglea between them (for example cnidarians).

Triploblastic animals

Animals whose body consists of three germ layers: ectoderm, mesoderm and endoderm.

Mesoglea

The non-living jelly-like layer between the ectoderm and endoderm of diploblastic animals.

Radial symmetry

Body symmetry in which similar body parts are arranged like spokes around a central body axis; an adaptation for a sessile life (grade Radiata).

Bilateral symmetry

Body symmetry in which a plane through the midline divides the body into roughly equivalent right and left mirror-image halves; an adaptation to motility (grade Bilateria).

Coelom

A fluid-filled space between the outer body wall and the digestive tube.

Acoelomate

An animal with no body cavity between the digestive tract and the body wall; the mesoderm is packed solidly between ectoderm and endoderm (flatworms).

Pseudocoelomate

An animal whose body cavity develops between the mesoderm and endoderm (a false cavity), for example roundworms.

Coelomate

An animal whose body cavity is a true coelom, formed within the mesoderm and completely lined by mesoderm.

Blastopore

The opening formed when a group of cells of the blastula move inward; it becomes the mouth in protostomes and generally the anus in deuterostomes.

Protostomes

Coelomates in which the blastopore develops into the mouth (molluscs, annelids, arthropods); the name means first, the mouth.

Deuterostomes

Coelomates in which the blastopore develops into the anus and the mouth forms later (echinoderms and chordates).

Invertebrates

Animals that lack a backbone; they make up 95% of known animal species and are divided into eight phyla.

Pinacoderm

The outer dermal layer of a sponge, made of thin flat cells called pinacocytes.

Choanoderm

The inner layer of a sponge, made of flagellated collar cells called choanocytes.

Ostia

The many pores in the body wall of a sponge through which water enters.

Spongocoel

The single central cavity of a sponge.

Osculum

The large main opening of a sponge through which water goes out.

Spicules

Needle-like calcareous or siliceous pieces that form the skeleton of sponges.

Cnidocytes

Special stinging cells of cnidarians that give rise to nematocysts.

Nematocyst

The stinging cell organelle of cnidarians.

Gastrovascular cavity

The sac-like internal cavity of cnidarians (and flatworms) with only one opening, the mouth.

Zooids

The units of a colony of cnidarians; the two main types are polyps (feeding) and medusae (reproductive).

Polymorphism

The condition in which a colony has zooids of different forms and functions, showing division of labour among them.

Polyp

The sedentary, feeding zooid of cnidarians, with the mouth and tentacles at the free end.

Medusa

The free-swarming, reproductive zooid of cnidarians.

Platyhelminthes

The phylum of flatworms: triploblastic, bilateral and acoelomate; free living (Planaria) or parasitic (liver fluke, tapeworm).

Aschelminthes

The phylum of roundworms (nematodes): cylindrical, tapering at both ends, pseudocoelomate, triploblastic and bilateral.

Mantle

The envelope covering the body of a mollusc; it secretes the shell and covers the visceral mass.

Radula

A tongue-like structure in the mouth cavity of some molluscs, for example snail and cuttlefish.

Trochophore larva

The larval stage through which development occurs in molluscs and (indirectly) in annelids.

Metamerism

Segmentation of the body into a series of similar segments, as in annelids.

Chaetae (setae)

Chitinous bristles that are the organs of locomotion in annelids.

Parapodia

Paired fleshy outgrowths of the body wall in the annelid Nereis.

Chitin

The tough, flexible substance that forms the exoskeleton of arthropods.

Haemocoel

The reduced coelom of arthropods, connected with the blood vascular system.

Metamorphosis

The change of form during development, as in arthropods and amphibians.

Water vascular system

The part of the coelom of echinoderms made of water-filled canals, used in locomotion, attachment, gas exchange and feeding.

Tube feet

The organs of locomotion of echinoderms, present along the edges of grooves in the arms.

Notochord

A solid unjointed rod in the mid-dorsal line between the gut and the nerve cord; the axial endoskeleton of chordates.

Dorsal hollow nerve cord

The single tubular, fluid-filled, nonganglionated central nervous system of chordates, above the notochord.

Pharyngeal pouches (gill slits)

Paired perforations on the sides of the anterior body leading from the pharynx to the exterior; present in all chordates at some stage.

Post-anal tail

A tail extending beyond the anus, present at least in the embryo of chordates.

Protochordata (Acrania)

The group of chordates with no cranium, made of Urochordata (tunicates) and Cephalochordata (lancelets).

Craniata

The group of chordates with a brain case (cranium); it has one subphylum, Vertebrata.

Agnatha

The super class of jawless fishes, with hagfish and lamprey as examples.

Gnathostomata

The super class of jawed fishes and all tetrapods, divided into six classes.

Anamniotes

Animals whose embryo has no amnion, for example amphibians.

Amniotes

Reptiles, birds and mammals, whose embryos develop with an amnion (a fluid-filled private pond).

Poikilothermic (ectotherms)

Animals whose body temperature varies with the surroundings, for example amphibians and reptiles.

Endothermic

Warm blooded; the body temperature is kept constant by the body itself, as in birds and mammals.

Diaphragm

The muscular partition that divides the mammalian coelom into a thoracic and an abdominal cavity; present only in mammals.

Prototheria

The subclass of egg-laying mammals (monotremes) with a cloaca, such as the duckbill platypus and echidna.

Metatheria

The subclass of pouched mammals (marsupials) whose immature young are reared in a marsupium.

Eutheria

The subclass of placental mammals whose young develop inside the mother and are born fully developed.

Placenta

An organ of exchange between maternal and fetal blood in placental mammals.

FBISE 2025 and 2026 questions

Questions from the FBISE HSSC-I 2026 (1st Annual) and FBISE HSSC-I 2025 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

Exam focus

1. Give three distinguishing characteristics and an example of each of the invertebrate phyla (Cnidaria, Arthropoda, Mollusca, Porifera, Platyhelminthes, Annelida). (long)
   A 6-mark Section C question in both 2025 sets (Set B Q.6 OR: Cnidaria, Arthropoda, Mollusca; Set D Q.6 OR: Porifera, Platyhelminthes, Annelida), and the same as the book's long question 1. Learn 3 features and an example for every phylum.
   Go to answer
2. State characteristics of the phyla Porifera and Cnidaria (body layers, pores and osculum, spicules, diploblastic, nematocysts, polyp and medusa). (short)
   Sponges in 2026 Set B Q.2 (iii) and cnidarians in 2026 Set D Q.2 (iii), both 3 marks; book short questions 2 and 3.
   Go to answer
3. Four basic characteristics of chordates, notochord and its fate in vertebrates. (short)
   2026 Set B Q.2 (xi) (three common characteristics), 2026 Set D Q.2 (xi) (notochord and its fate) and 2025 Set B MCQ 15 (notochord common to all chordate embryos).
   Go to answer
4. Echinoderms: radial symmetry, spiny skin and water vascular system with tube feet. (mcq)
   Asked as an MCQ in 2026 Set D (MCQ 12) and 2025 Set D (MCQ 12); book exercise MCQ 13 also tests it.
   Go to answer
5. Arthropods: jointed appendages, chitin exoskeleton, haemocoel, metamorphosis, and why insects are so successful. (mcq)
   2026 Set B MCQ 12 (jointed appendages), 2025 Set B Q.6 OR and the book's short question 7 and long question 4.
   Go to answer
6. Differentiate diploblastic from triploblastic, radial from bilateral symmetry, and coelomate, acoelomate and pseudocoelomate animals. (short)
   No paper question in 2025 or 2026 asked it directly, but it is the base of animal classification and the book's short question 19 (differences) and MCQs 7 and 8.
   Go to answer
7. Animals as eukaryotic, multicellular, ingestive heterotrophs (holozoic nutrition) without cell walls. (mcq)
   2026 Set B MCQ 10 asked which group is holozoic; book short question 1 asks the distinct features of animals.
   Go to answer
8. Characteristics of Amphibia, Reptilia, Aves and Mammalia, and the three mammalian subclasses. (long)
   The papers of 2025 and 2026 asked no vertebrate class question, but the book's exercise has short questions 10 to 12, 16, 17 and long questions 2 and 3; likely future 5 to 7 mark questions.
   Go to answer