animal classification [part 2]

ANIMAL CLASSIFICATION [Part 2]

TOPIC OUTLINE

A) Some general features of animalsB) Evolution of the animal body planC) CnidariaD) PlatyheminthesE) AnnelidaF) ArthropodaG) MolluscaH) Echinodermata I) Chordata

PHYLUM ARTHROPODA

Class: Insecta/ Hexapodabee

Class: Arachnida spiders

Subphylum: Crustacea water flea Daphnia, prawns, barnacles, crabs, lobsters, woodlice

Daphnia

Barnacles

Phylum Arthropoda contains more species than any other phylum

80%

~ 1 million arthropod species described

ubiquitous habitats [exist everywhere]

Phylum Arthropoda

The class insecta / hexapoda is the largest class.

1st major innovation in body structure among coelomates

Segmentation among annelids:

The development of jointed appendages : an even more profound

innovation

Jointed appendages mark the origin of the body plan

characteristic of the most successful of all animal groups.

include:

Jointed appendages :

legsantennae

mouthparts

are used for a variety of functions: feeding, locomotion and sensory purposes

the functional flexibility provided by such a broad array of appendages has made arthropods the most successful of animal groups

Arthropod bodies are segmented like those of annelids

the individual segments: often exist only during early development fuse into functional groups as adults

Head: 5-7 segments

Thorax: 3 segments

Abdomen: 11 segments

Specialisation of body parts: Tagmata

the arthropod body has undergone various forms of regional specialisation to produce segment groups specialised for different functions

these specialised body regions are called tagmata (singular, tagma) (e.g. the head, thorax and abdomen)

this fusing process is known as tagmatization, is of central importance in the evolution of arthropods

the head possesses sensory receptors: eyes antennae feeding appendages

the brain is much larger than in annelids

cephalisation is much more pronounced

Question: SEP 2013 Paper 2

Use your knowledge of biology to explain the evolutionary importance of the following:

tagmatisation in insects. (5)

Exoskeleton is secreted by the epidermis contains chitin, a nitrogen-containing

polysaccharide which resembles cellulose

Exoskeleton chitin has high tensile strength (it is difficult to break

by pulling from both ends)

the properties of the exoskeleton can be altered by the addition of other chemicals like in crustaceans, calcium salts makes it harder

a range of hardness, flexibility and stiffness is thus possible

flexibility is important at joints

Advantages of the exoskeleton:

1. support, particularly on land;2. it provides an anchor for the muscles internally;3. protection from physical damage;4. addition of a layer of wax from special glands in the epidermis

helps prevent desiccation on land;5. insect flight and the jumping ability of fleas and grasshoppers

depends on the presence of a remarkably elastic protein in the exoskeleton;

6. it has a low density, which is important for flying animals;7. flexible joints are possible between segments;8. it can be modified to form hard jaws for biting, piercing, sucking

or grinding;9. it can be transparent in places allowing, for example , entry of

light into eyes and camouflage in water.

TWO disadvantages associated with the exoskeleton:

Final body size is limited:

either not be able to support its own weight

or the exoskeleton would have to be so massive that it would not be able to move

An arthropod the size of an elephant would:

[The other important restriction on size of insects is their breathing mechanism which

works mainly by diffusion through tube called tracheae]

It restricts growth: so periodic moulting (ecdysis) is required if

the animal is to grow however, the arthropod is very vulnerable to

attack by predators at this period

The haemocoel

Coelom is a reduced in :

arthropods

Their main body cavity is the:  

haemocoel

molluscs

develops from the cavities of the blood vascular system and is therefore filled with blood

the blood is generally circulated in the haemocoel, and through several attached blood vessels

the major organs are bathed in blood

The haemocoel

the coelom still exists but is: small confined to cavities of excretory organs and

the reproductive ducts

the high blood volume to body volume in arthropods enables them to maintain a high metabolic rate, allowing them to be very active animals

Flight in insects have greatly increases opportunities

for:finding food escaping predators

Cleidoic egg egg with a relatively impermeable shell is an adaptation to conserve water as it

prevents loss of moisture also found in birds and reptiles shell is porous to air shell may be flexible or calcerous it frees the animal from returning to water to

lay eggs

Question: MAY 2012 Paper 2

Use your knowledge of biology to explain the evolutionary importance of the following:

the cleidoic egg. (5)

Main features of the Arthropoda (Arthropods)

1. Triploblastic, coelomate.2. Metameric segmentation as in annelids but segments not

separated from each other by septa, definite in number, and showing varying degrees of specialisation.

3. Bilateral symmetry.4. Exoskeleton of chitin.5. Each segment typically bears a pair of jointed appendages

used for locomotion or feeding or sensory purposes.6. Coelom much reduced, main body cavity a haemocoel.7. Some groups have a compound eye for vision.8. Cuticle shed periodically (moulting or ecdysis) to allow for

growth.[diagnostic features: Exoskeleton; jointed appendages]

Main features of the class Insecta (Insects)

1. Mainly terrestrial.2. Well-defined head, thorax and abdomen.3. One pair of antennae.

Main features of the class Insecta (Insects)

4. Usually three pairs of mouthparts. 5. A pair of compound eyes and simple eyes.6. Three pairs of legs on thorax, one pair per

segment. Usually one or two pairs of wings on thorax.

7. Life cycle commonly involves metamorphosis either ‘complete’ or ‘incomplete’, with a larval stage.

8. No gills in adult. Gaseous exchange by tracheae

Subphylum: Crustacea

Spider crabCan weigh 19 kg.

1. Mainly aquatic.2. Cephalothorax (head and thorax not distinctly separate).

Main features of the subphylum Crustacea (Crustaceans)

A crustacean Carcinus maenas, the shore crab. Dorsal view.

Main features of the subphylum Crustacea (Crustaceans)

3. Two pairs of antennae.4. At least three pairs of mouthparts.5. A pair of compound eyes raised on stalks.6. Appendages often modified for swimming, as they

are mainly aquatic; number of legs variable, sometimes 10.

7. Larval form occurs.8. Typical gaseous exchange by gills – outgrowths of

the body wall or limbs.9. Dorsal side of body usually protected by a shield-like

carapace.

Main features of the class Arachnida (Arachnids)

1. Terrestrial.2. Cephalothorax; thorax separated from

abdomen by a narrow waist-like constriction (pedicel).

Spider

pedicel

3. No antennae.

Spider – dorsal view.

4. No true mouthparts but one pair of appendages used in capturing prey and one pair of sensory palps.

5. Simple eyes only (no compound eyes).6. Four pairs of walking legs.7. No larval forms.8. Gaseous exchange by book lungs or

tracheae.

Question: MAY 2007 Paper 2

“Arthropods are considered to be the most successful animals on Earth”. Discuss. (25)

Question: SEP 2002 Paper 2

Why are insects the most successful animals on Earth? Insects  are  successful  animals  as  they  have  various  adaptations  to conserve  water  thus  enabling  them  to  inhabit  a  terrestrial environment. They excrete uric acid which does not require water for its  elimination,  from  the  Malpighian  tubules.  Their  exoskeleton  is covered by a thin layer of wax, making it waterproof. Their spiracles are  guarded  by  valves,  which  close  to  reduce  water  loss  from  the tracheal system when the insect is at rest. Hairs at the spiracle have the  same  role.  Embryos  are  enclosed  in  a  relatively  impermeable shell  (cleidoic egg), allowing development outside the body without loss of water.  They have diverse mouthparts allowing  them  to  feed on solid or liquid food, thus reduce competing for food. Some possess wings which are an asset both to escape predators and to find a mate or new locations with bountiful resources.

Question: SEP 2005 Paper 2

Use your knowledge of biology to explain the following statement.

Scorpions and woodlice have very different appearances, yet are classified in the same phylum. (5)

TOPIC OUTLINE

A) Some general features of animalsB) Evolution of the animal body planC) CnidariaD) PlatyheminthesE) AnnelidaF) ArthropodaG) MolluscaH) Echinodermata I) Chordata

Slow-moving snails and slugs

Relatively sedentary bivalves such as clams

Highly active cephalopods (cuttlefish, squid, octopus)Types of

molluscs

PHYLUM MOLLUSCA Class: Gastropoda

slugs

snails

limpets

Mollusca: 2nd largest animal phylum

with almost 100 000 species

the giant squid is the largest non-vertebrate animal: weighs several

tonnes measures 16 m in

length

Molluscs were able to colonise :

aquatic & terrestrial environments by :

The more active molluscs show a

reduction or loss of the shell because:

2. The formation of a protective shell

1. The use of gills for gaseous exchange

A shell hinders locomotion.

Body plan of the Molluscabilateral symmetry

Visceral mass: is covered with a soft

epithelium contains the organs

of: digestion excretion reproduction

A muscular foot that is used in locomotion

Molluscs may have a differentiated head at the anterior end of the body: octopus

The muscular foot may be adapted for:

locomotion attachment food capture (in squids

& octopuses)

The mantle : is a thick epidermal sheet that covers the dorsal

side of the body bounds the mantle cavity

The mantle :

Secretes the calcium carbonate of the shell in those molluscs with a shell

The mantle cavity has a role in breathing:

Terrestrial molluscs:the mantle cavity forms a lung sac

Aquatic molluscs:the gills or ctenidia

project into the mantle cavity

Ctenidiaconsist of filaments rich in blood vessels:

greatly increase the surface area capacity for gas exchange

mollusc gills are very efficient

can extract 50% or more of the dissolved oxygen from the water that passes through the mantle cavity

this water: brings in oxygen

and, in the case of the bivalves, food;

carries waste materials and gametes

in aquatic molluscs, a continuous stream of water passes into and out of the mantle cavity, drawn in by the cilia on the gills

Locomotion some molluscs e.g. slugs secrete

mucus, forming a path that they glide along on their foot

in cephalopods (squids and octopuses) the foot is divided into arms, also called tentacles

in some pelagic forms (molluscs that are perpetually free-swimming), the foot is modified into wing-like projections or thin fins

Feeding

the radula: a rasping, tongue-like

organ used for feeding

one of the most characteristic features of all the molluscs except the bivalves

radula

food

Scraps of food

dozens of microscopic, chitinous teeth arranged in rows

The radula consists primarily of :

gastropods use their radula to:

Scrape algae & other food

materials off their substrates

… then to convey this food

to the gut

Circulationthe circulatory system of all molluscs except

cephalopods consists of: a heart an open

circulation

Cephalopods:a closed circulatory

system aids their active form of life

The coelomis primarily represented by a small cavity

around the heart

Coelom

Main features of the Mollusca1. Bilaterally symmetrical.2. Unsegmented, triploblastic coelomates.3. Body soft and fleshy and divided into a head,

ventral muscular foot and dorsal visceral hump.4. The mantle secretes a calcereous shell.5. Main body cavity is the haemocoel.6. No limbs.7. Gaseous exchange effected by one or more pair/s

of ctenedial gills housed in the mantle cavity.[Diagnostic feature: Body soft and fleshy and divided into a head, ventral muscular foot and dorsal visceral

hump.]

Main features of the class Gastropoda

1. Terrestrial, marine and freshwater.2. Asymmetrical.3. Shell of one piece, usually coiled due to

rotation of hump during growth.

Main features of the class Gastropoda

4. Large flat foot used in locomotion.5. Head, eyes and sensory tentacles.6. Radula, a rasping tongue-like structure used

in feeding.7. Anus at anterior.

Operculum : a plate present in most marine

gastropods that the animal can pull to close the shell

most adult land gastropods

most gastropods have a pair of tentacles with eyes

these tentacles have

been lost in some of the more advanced forms of the class

EYE

TOPIC OUTLINE

A) Some general features of animalsB) Evolution of the animal body planC) CnidariaD) PlatyheminthesE) AnnelidaF) ArthropodaG) MolluscaH) Echinodermata I) Chordata

Phylum: Echinodermata

deuterostomate, triploblastic animals

have no proper circulatory system

Class: Asteroidea (starfish e.g. Asterias)

Class: Echinoidea (sea urchins e.g. Echinocardium

all marine are largely bottom-dwellers (benthic) inhabiting

shorelines and shallow seas

Echinoderm body plan undergoes a fundamental shift during development

all echinoderms have secondary radial symmetry: larvae are bilaterally symmetrical but adults

become radially symmetrical

Question: SEP 2012 paper 2

Use your knowledge of biology to explain the following statements

echinoderms exhibit secondary radial symmetry. (5)

show pentamerous symmetry (a form of radial symmetry)

Adult echinoderms have a five-part body plan

Endoskeleton ‘echinoderm’ means spiny skin

refers to an endoskeleton composed of hard calcium-rich plates just beneath the delicate skin

in adults, the plates frequently fuse, forming a hard shell

The plates in certain portions of the body of some echinoderms are perforated: tube feet, part of the water

vascular system [a unique feature of this phylum] extend

TS arm of starfish

Tube feet

Ossicles in the skin provide sessile animals with:support protection from predators + wave action

Ossicles in starfish

The water vascular system

is a hydraulic system which aids in:MovementRespirationExcretionFeeding

the water vascular system radiates from a ring canal that encircles the oesophagus

Five radial canals extend into each of the five parts

of the body and determine the basic

symmetry

Madreporite: a sieve-like plate on the animal’s surface location through which water enters the water vascular system

and flows to the ring canal through a tube, or stone canal

The five radial canals in turn extend out through short side branches into the hollow tube feet

Suckers may be present at the end of each tube

foot

Modified tube feet around the mouth cavity of certain echinoderms

are used in feeding

Body cavity the coelom:

is connected with a complicated system of tubes

helps provide circulation & respiration

the digestive system is simple but usually complete, consisting of :

gut

mouth

anus

coelom

Reproductionmany echinoderms are able to regenerate

the lost part

some echinoderms, especially starfish, drop various parts when under attack

Reproductionin a few echinoderms, asexual reproduction

takes place by splitting [Fragmentation] the broken parts of sea stars can

sometimes regenerate whole animals

Reproduction

most reproduction in the phylum is:

sexual external

sexes are separate

[free-swimming – form part of plankton]

Metamorphose into sedentary adults

Male Female

Fertilised egg

Bilaterally symmetrical larva

Main features of Echinoderms1. Triploblastic, coelomate.2. Skin contains calcareous ossicles (endoskeleton)

and spines.3. Exclusively marine.4. Adult with five-rayed symmetry, but larva is

bilaterally symmetrical.5. Mouth generally on lower (oral) side; anus on

upper (aboral) side.6. A coelomic water vascular system which contains

and is in direct contact with sea water and which operates hydraulically the locomotory tube feet (podia).

7. No proper circulatory system.8. No differentiated head.9. No excretory organs.10. A through gut.11. Sexes are separate.12. Nervous system diffuse, decentralised,

allowing them to engage in their environment equally from all sides.

[Diagnostic features: pentamerous radial symmetry; tube feet]

Characteristics of the Class Asteroidea (starfish)

characterised by a flattened body which grades into 5 or sometimes more (up to 40) arms

the animals are not attached and freely mobile

few calcareous plates in body wall; movable spines

Characteristics of the Class Asteroidea (starfish)Pedicellariae: modified spines which act as scissors or

forceps to remove organisms that try to settle down on their body surface

Characteristics of the Class Asteroidea (starfish)starfish are mostly

carnivorous

digestion is mostly extracellular

anus has little function

Main features of the Class Echinoidea (sea urchin)

spherical, free-living echinoderms which lack arms

numerous calcareous plates in the body wall, attached to each other to form a rigid test

the test bears:pedicellariae movable spines

Main features of the Class Echinoidea (sea urchin)

the mouth bears a distinctive grazing apparatus, called Aristotle’s lantern used for chewing

Teeth of Aristotle’s lantern

Question: SEP 2005 Paper 2

Use your knowledge of biology to explain the following statement.

Starfish and sea-urchins have very different appearances, yet are classified in the same phylum. (5)

TOPIC OUTLINE

A) Some general features of animalsB) Evolution of the animal body planC) CnidariaD) PlatyheminthesE) AnnelidaF) ArthropodaG) MolluscaH) Echinodermata I) Chordata

Phylum: CHORDATA [Chordates]

Sub phylum: VertebrataClass: Osteichthyes / Teleosta (bony fish -

teleosts) Class: Amphibia (amphibians: newts,

salamanders, frogs and toads) Class: Reptilia (reptiles) Class: Aves (birds) Class: Mammalia (only this class in syllabus)

Four principal features characterise chordates:• Notochord • Dorsal hollow nerve cord

• Pharyngeal slits• Muscular, postanal tail.

runs just beneath the dorsal surface of the animal

hollow means it is filled with fluid in vertebrates, the dorsal nerve cord

differentiates into the:

1. A single, hollow nerve cord

brain spinal cord

2. A flexible rod, the notochord

is present at some stage in the life cycle in all chordates

forms on the dorsal side of the primitive gut in the early embryo

during embryological development in most vertebrates by the vertebral column that runs around the nerve cord.

The notochord becomes surrounded and then replaced

connect the pharynx, a muscular tube that links the mouth cavity and the gullet, with the outside

in most vertebrates, the slits do not actually connect to the outside and are better termed pharyngeal pouches

3. Pharyngeal slits

pharyngeal pouches are: present in the embryos of all vertebrates but are lost later in the development of terrestrial

vertebrates

the presence of these structures in all vertebrate embryos provides a clue to the aquatic ancestry of the group.

extends beyond the anus, at least during their embryonic development

nearly all other animals have a terminal anus

4. The postanal tail

All chordates have all four of these characteristics at some stage in their lives e.g. 

Human adults retain only the: nerve cord one pair of pharyngeal

slits which are the Eustachian tubes that connect the throat to the middle ear

Human embryoshave all four features

In addition to these four principal features, a number of other

characteristics distinguish the chordates fundamentally from other

animals: chordates have a more or less segmented body plan,

and distinct blocks of muscles can often be clearly seen in embryos of this phylum

most chordates have an internal skeleton against which the muscles work

either this internal skeleton or the notochord makes possible the extraordinary powers of locomotion that characterise the members of the group

Main features of Chordates1. Notochord present at some stage in the life history. 2. Post-anal tail (tail starts posterior to anus).3. Bilateral symmetry.4. Pharyngeal (visceral) clefts present (slits in the

pharynx).5. Dorsal, hollow nerve cord.6. Triploblastic coelomate.7. Segmental muscle blocks (myotomes) on either side

of the body.8. Limbs formed from more than one body segment.

[diagnostic features : 1-4]

Main features of the subphylum Vertebrata

1. Notochord replaced in adult by a vertebral column (backbone), a series of vertebrae made either of bone or cartilage.

2. Well-developed central nervous system including brain. Skull protects the brain. A high degree of cephalisation.

3. A living endoskeleton.4. Pharyngeal clefts (gill slits) few in number.5. Two pairs of fins or limbs. These are attached to the

rest of the skeleton by girdles, pectoral and pelvic.6. Closed circulatory system.

Reptiles, birds and mammals:the amniotes

possess extraembryonic membranes, including an amnion, during development

Wastes are discharged into another sac called

the allantois.

The embryo obtains food from a yolk sac attached to its gut.

Within the shell are several extraembryonic membranes that protect the embryo from drying and assist gas exchange and excretion of nitrogen.

The pentadactyl limb has five digits on the hand & foot has a specific pattern of bones

Penta = 5 Dactyl = Fingers

The structure of a human arm includes

A bone between the shoulder and the elbow: humerus.

Below the elbow, are two other bones: radius ulna

A set of wrist bones and then the five-digit fingers,

follow.

1

23

The pentadactyl limb is found in all classes of tetrapods

The fundamental structures of pentadactyl limbs are the same in all tetrapods indicating that they

originated from a common ancestor.

But in the course of evolution these fundamental structures have been

modified they have become:

superficially different unrelated structures

this phenomenon is shown in the forelimbs of mammals

to serve different functions in

adaptation to different environments & modes of

life

The pentadactyl limb is a homologous structure

Homologous structures have the same basic plan but are adapted for different functions

Basic pentadactyl

pattern

WalkingRunningFlying

Swimming

Question: MAY 2003

Write brief notes on the evolutionary significance of each of the following structures: [five marks]The pentadactyl limbTypical limb of the mammals, birds, reptiles, and amphibians. The limb has three parts a hand/foot with five digits (fingers/toes), a lower limb containing two bones, and an upper limb containing one bone. This basic pattern has persisted in all the terrestrial vertebrates, and those aquatic vertebrates (such as seals) which are descended from them.  Natural selection has modified the pattern to fit different ways of life. In flying animals (birds and bats) it is greatly altered and in some vertebrates, such as whales and snakes, the limbs are greatly reduced or lost. Pentadactyl limbs of different species are an example of homologous organs.

Main features of Mammalia

1. Skin bears hair with two types of glands, sebaceous and sweat.2. Bony skeleton.3. Two pairs of pentadactyl limbs.4. Visceral clefts never develop gills.5. External ear.6. Only two genera lay eggs, the spiny anteater and the duck-billed platypus.7. Embryo develops in mother. Young born in a more mature state, i.e.

mammals are viviparous.8. Mother has mammary glands which produce milk for the newborn.

Internal fertilisation.9. Muscular diaphragm.10. Well-developed sense organs.11. A highly developed brain and intelligent behaviour involving learning and

memory.12. Heterodont dentition (various types and sizes of teeth)13. A bony roof to the mouth called the palate.14. Endothermic.

Question: MAY 2008 Paper 2

Use your knowledge of biology to describe the selective advantage of each of the following adaptations:

viviparity (live birth) in mammals. (5)

Evolutionary trends shown in vertebrates towards adaptation to

a terrestrial mode of life

Creatures that make the transition from life in water to life on land, must come to terms with FOUR new exigencies: they must

1. find ways to cope with gravity;2. breathe air;3. avoid desiccation;4. evolve senses that are appropriate to air

rather than water