Animal Kingdom

COMPREHENSIVE CHAPTER NOTES: ANIMAL KINGDOM (CLASS 11 CBSE)

BASIS OF CLASSIFICATION

**Classification of animals is essential** because over a million species have been described, and systematic organization helps assign positions to newly discovered species. Animals share fundamental features in cell arrangement, body symmetry, coelom nature, and patterns of digestive, circulatory, and reproductive systems that form the basis of classification.

LEVELS OF ORGANISATION

**Definition**: The degree of complexity in cell arrangement and functional organization within animal bodies determines five levels of organization.

**Cellular Level of Organisation**

Seen in **Porifera (sponges)**

Cells are arranged as loose aggregates with minimal division of labour

No specialized tissues present

Cells perform individual functions

**Tissue Level of Organisation**

Seen in **Coelenterata (Cnidaria)**

Cells performing same function grouped into **tissues**

More complex arrangement than cellular level

Examples: nervous tissue, epithelial tissue

**Organ Level of Organisation**

Seen in **Platyhelminthes and some higher phyla**

**Tissues grouped to form organs**, each specialized for particular function

Single digestive system opening serves as both mouth and anus (incomplete digestive system)

Examples: brain, digestive organs

**Organ-System Level of Organisation**

Seen in **Annelida, Arthropoda, Mollusca, Echinodermata, Chordata**

**Organs organized into functional systems** each concerned with specific physiological function

Examples: digestive system, circulatory system, nervous system, reproductive system

**Complete vs. Incomplete Digestive Systems**:

**Incomplete**: Single opening serving as mouth and anus (Platyhelminthes)

**Complete**: Two openings—mouth and anus (Annelida onwards)

Complete system allows sequential processing of food

**Circulatory System Types**:

**Open Type**: Blood pumped from heart, directly bathes cells and tissues; no distinct vessels (Arthropoda, Mollusca)

**Closed Type**: Blood circulates through vessels of varying diameters—arteries, veins, capillaries (Annelida, Chordata)

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SYMMETRY

**Definition**: The balanced proportional arrangement of body parts around central axis or plane.

ASYMMETRY

Body cannot be divided into identical halves by any plane

Seen in **Porifera (sponges)**

Least organized body plan

Example: Most sponges lack symmetrical structure

RADIAL SYMMETRY

**Definition**: Any plane passing through central axis divides body into two identical halves

Body parts arranged around central axis in circular pattern

Seen in: **Coelenterata, Ctenophora, Echinoderms (adult form)**

Adapted for sessile or free-floating lifestyle

Examples: Hydra (polyp), Aurelia (medusa), starfish

Allows organism to respond equally to stimuli from all directions

BILATERAL SYMMETRY

**Definition**: Body divided into identical left and right halves in only ONE median plane

Dorsal (upper) and ventral (lower) surfaces differ

Anterior (head) and posterior (tail) ends differ

Seen in: **Annelida, Arthropoda, Mollusca, Hemichordata, Chordata**

Adaptation for directional movement

Associated with cephalization (anterior concentration of sensory organs)

Examples: Earthworm, insect, fish, human

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DIPLOBLASTIC AND TRIPLOBLASTIC ORGANISATION

DIPLOBLASTIC ANIMALS

**Definition**: Body composed of **two embryonic germ layers** during development

**Layers**:

External: **Ectoderm** (outer layer)

Internal: **Endoderm** (inner layer)

Between layers: **Mesoglea** (undifferentiated, jelly-like, non-contractile)

Tissue level of organization

Seen in: **Coelenterata and Ctenophora**

Example: Hydra, jellyfish

Mesoglea provides structural support but lacks muscle and nerve cells

TRIPLOBLASTIC ANIMALS

**Definition**: Body composed of **three embryonic germ layers** during development

**Layers**:

External: **Ectoderm** (gives rise to nervous system, epidermis)

Middle: **Mesoderm** (gives rise to muscles, connective tissue, circulatory system)

Internal: **Endoderm** (gives rise to digestive and respiratory systems)

Mesoderm is **differentiated and organized**, allowing muscle and organ formation

Organ and organ-system level organization

Seen in: **Platyhelminthes to Chordata**

Examples: Flatworms, earthworms, insects, vertebrates

**Significance**: Mesoderm enables greater complexity and internal organization

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COELOM (BODY CAVITY)

**Definition**: Fluid-filled body cavity lined entirely by **mesoderm-derived peritoneum**, located between body wall and internal organs.

**Functions**:

Accommodates organ development

Allows organ movement

Distributes nutrients and oxygen

Absorbs shock

Aids in reproduction and excretion

COELOMATE ANIMALS

**Definition**: Possess true **coelom lined by mesoderm**

Body cavity clearly separated from gut

Organs suspended in fluid for independent movement

Phyla: **Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata**

Examples: Earthworm, crayfish, squid, starfish, fish

PSEUDOCOELOMATE ANIMALS

**Definition**: Body cavity **NOT lined by mesoderm**; instead, mesoderm present as **scattered pouches** between ectoderm and endoderm

Incomplete body cavity

False coelom—less developed than true coelom

Phylum: **Aschelminthes (Nematoda)**

Examples: Roundworms, hookworms, pinworms

Organs not fully suspended; restricted movement

ACOELOMATE ANIMALS

**Definition**: **No body cavity present** between body wall and gut

Body composed of solid tissue

Organs packed directly in parenchyma (tissue mass)

Diffusion-dependent for nutrient transport (limited size)

Phylum: **Platyhelminthes**

Examples: Flatworms, tapeworms, planaria

Organ space limited, restricting complexity

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SEGMENTATION (METAMERISM)

**Definition**: Division of body into **similar repeating units called segments or metameres**, with **serial repetition of at least some organs**.

**Characteristics**:

**External segmentation**: Visible grooves/rings on body surface

**Internal segmentation**: Repetition of organs (nervous ganglia, blood vessels, excretory organs)

Not all organs repeat in each segment

Allows flexibility and specialized segment functions

**Seen in**: **Annelida (prominent), Arthropoda**

**Examples**:

**Earthworm (Pheretima)**: 100+ segments, each with repeated nephridia, blood vessels, nerves

**Insects**: Head, thorax (3 fused segments), abdomen (multiple segments)

**Evolutionary Significance**: Segmentation allows:

Specialization of different body regions

Increased body size (more segments = larger animal)

Greater locomotor efficiency

Independent function of segments if one damaged

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NOTOCHORD

**Definition**: **Rod-like, mesodermally-derived supportive structure** formed on dorsal side during embryonic development.

**Characteristics**:

Made of turgid cells enclosed in sheath

Provides axial support during development

Lies between spinal cord and digestive tract

Hydrostatic skeleton function

NOTOCHORD IN VERTEBRATES

Present in **embryonic stage only** in most vertebrates

Replaced by **cartilaginous or bony vertebral column** (backbone/spine) in adults

Persists throughout life in **Cephalochordates (Amphioxus)**

Persists in larval form only in **Urochordates**

CLASSIFICATION BASIS

**Chordates**: Animals with notochord at some developmental stage

**Non-chordates**: Animals without notochord (Porifera through Echinodermata)

**Exam Point**: All vertebrates are chordates, but **NOT all chordates are vertebrates**. Protochordates (Urochordata, Cephalochordata) lack true vertebrae.

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MAJOR PHYLA OF KINGDOM ANIMALIA: COMPREHENSIVE CLASSIFICATION

PHYLUM PORIFERA (SPONGES)

**Definition**: Primitive multicellular aquatic animals, mostly marine, with cellular level organization and water canal system.

**Key Characteristics**:

**Body Organization**: Cellular level (no true tissues or organs)

**Symmetry**: Mostly asymmetrical

**Skeleton**: Spicules (calcium carbonate/silica) or spongin fibres

**Digestion**: Intracellular (food engulfed by cells)

**Reproduction**: Both asexual (fragmentation, budding) and sexual (gamete formation)

**Fertilization**: Internal

**Development**: Indirect (free-swimming larva morphologically different from adult)

**Sexes**: Hermaphroditic (both male and female organs in same individual)

**Water Transport System**:

Water enters through **ostia (minute pores)**

Passes into **spongocoel (central cavity)**

**Choanocytes (collar cells)** line spongocoel and canals—filter feed and create water current

Water exits through **osculum (large opening)**

Functions: Food gathering, respiratory gas exchange, waste removal

**Examples**:

**Sycon (Scypha)**: Vase-shaped, marine

**Spongilla**: Freshwater sponge

**Euspongia**: Bath sponge, commercially used

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PHYLUM COELENTERATA (CNIDARIA)

**Definition**: Aquatic animals (mostly marine) with tissue-level organization, radial symmetry, diploblastic body plan, and stinging cells.

**Key Characteristics**:

**Body Organization**: Tissue level

**Symmetry**: Radial (can be divided into identical halves by multiple planes)

**Germ Layers**: Diploblastic (ectoderm, mesoglea, endoderm)

**Special Cells**: **Cnidocytes/Cnidoblasts** containing nematocysts (stinging capsules)

Function: Prey capture, defense, anchorage

**Digestive System**: Gastro-vascular cavity with single opening (mouth on hypostome)

**Digestion**: Both extracellular and intracellular

**Skeleton**: Some have calcium carbonate skeleton (corals)

**Body Forms**:

**Polyp**: Sessile, cylindrical (Hydra, Adamsia, Pennatula)

**Medusa**: Free-swimming, umbrella-shaped (Aurelia, jellyfish)

**Reproduction**: Asexual (polyps) and sexual (medusae)

**Metagenesis (Alternation of Generations)**: Polyps asexually produce medusae; medusae sexually produce polyps (Example: Obelia)

**Sexes**: Hermaphroditic

**Examples**:

**Physalia**: Portuguese man-of-war

**Adamsia**: Sea anemone (polyp form)

**Aurelia**: Jellyfish (medusa form)

**Meandrina**: Brain coral (skeleton-forming)

**Pennatula**: Sea-pen

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PHYLUM CTENOPHORA (SEA WALNUTS/COMB JELLIES)

**Definition**: Exclusively marine, radially symmetrical diploblastic organisms with tissue-level organization and comb plates for locomotion.

**Key Characteristics**:

**Habitat**: Exclusively marine

**Body Organization**: Tissue level

**Symmetry**: Radial

**Germ Layers**: Diploblastic

**Locomotion Organelles**: **Eight external rows of ciliated comb plates (ctenes)**

Create metachronal wave for movement

Refract light creating iridescence

**Digestion**: Both extracellular and intracellular

**Bioluminescence**: Well-developed light emission ability

**Reproduction**: Sexual only (no asexual reproduction)

**Fertilization**: External

**Development**: Indirect (free-swimming larva)

**Sexes**: Hermaphroditic

**Examples**:

**Pleurobrachia**: Egg-shaped, common in plankton

**Ctenoplana**: Flattened comb jelly

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PHYLUM PLATYHELMINTHES (FLATWORMS)

**Definition**: Bilaterally symmetrical, triploblastic, acoelomate animals with organ-level organization, mostly parasitic, with flattened dorso-ventral body.

**Key Characteristics**:

**Body Organization**: Organ level

**Symmetry**: Bilateral

**Germ Layers**: Triploblastic

**Coelom**: Acoelomate (no body cavity; body mass is solid parenchyma)

**Body**: Dorso-ventrally flattened (hence "flatworms")

**Habitat**: Mostly endoparasites; some free-living

**Parasitic Adaptations**:

Hooks and suckers for attachment

Direct nutrient absorption through body surface from host

**Excretion**: **Flame cells** (cyrtocytes) create water current for osmoregulation and waste removal

**Nervous System**: Primitive (ladder-like with cerebral ganglia)

**Digestive System**: Incomplete (single opening)

**Reproduction**: Sexual

**Fertilization**: Internal

**Development**: Indirect (multiple larval stages)

**Sexes**: Hermaphroditic

**Special Ability**: Some possess high regeneration capacity (Planaria can regenerate from small body parts)

**Examples**:

**Taenia**: Tapeworm (endoparasite in human intestine)

**Fasciola**: Liver fluke (endoparasite in sheep/human liver)

**Planaria**: Free-living flatworm with regeneration ability

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PHYLUM ASCHELMINTHES (ROUNDWORMS/NEMATODES)

**Definition**: Bilaterally symmetrical, triploblastic, pseudocoelomate worm-like animals with complete digestive system and organ-system level organization.

**Key Characteristics**:

**Body Organization**: Organ-system level

**Symmetry**: Bilateral

**Germ Layers**: Triploblastic

**Coelom**: Pseudocoelomate (body cavity not fully lined by mesoderm; mesoderm as scattered pouches)

**Body Shape**: Circular in cross-section (hence "roundworms")

**Body Length**: Unsegmented

**Habitat**: Free-living (aquatic, terrestrial) or parasitic (plants, animals)

**Digestive System**: **Complete** with well-developed muscular pharynx, straight intestine

**Mouth to Anus**: Direct straight tube enabling efficient food processing

**Excretion**: **Excretory tube** removes wastes through excretory pore (anterior)

**Reproduction**: Sexual

**Sexes**: Dioecious (separate males and females—**sexually dimorphic**)

**Sexual Dimorphism**: Females typically longer than males

**Fertilization**: Internal

**Development**: Direct (young resemble adults) or indirect

**Body Wall**: Cuticle (non-living outer covering with layers)

**Examples**:

**Ascaris lumbricoides**: Large roundworm (human intestinal parasite)

**Wuchereria bancrofti**: Filarial worm (causes elephantiasis)

**Ancylostoma**: Hookworm (parasitic)

**Caenorhabditis elegans**: Free-living nematode (model organism)

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PHYLUM ANNELIDA (SEGMENTED WORMS)

**Definition**: Bilaterally symmetrical, triploblastic, coelomate animals with metamerically segmented body, organ-system level organization, and closed circulatory system.

**Key Characteristics**:

**Body Organization**: Organ-system level

**Symmetry**: Bilateral

**Germ Layers**: Triploblastic

**Coelom**: Coelomate (true coelom present, lined by peritoneum)

**Segmentation**: Metamerically segmented (body divided into repeating segments/metameres)

Each segment contains repeated set of organs

External and internal segmentation visible

**Body Wall**: Muscular (longitudinal and circular muscle layers) enabling locomotion

**Habitat**: Aquatic (marine and freshwater) or terrestrial; free-living or parasitic

**Circulatory System**: **Closed type** (blood confined to vessels)

Dorsal and ventral blood vessels

Aortic arches connecting them

Efficient oxygen transport

**Respiration**: Through body surface (cutaneous) or gills in aquatic forms

**Excretion**: **Nephridia** (paired in each segment)

Function: Osmoregulation and nitrogenous waste removal

Similar to vertebrate nephrons

**Nervous System**:

**Paired ganglia** in each segment

**Double ventral nerve cord** connecting ganglia

Lateral nerves to muscles and sensory organs

**Reproductive System**:

**Nereis**: Dioecious (separate sexes)

**Pheretima (Earthworm)**: Monoecious (hermaphroditic)

**Hirudinaria (Leech)**: Monoecious

**Reproduction**: Sexual

**Special Features in Aquatic Annelids**:

**Parapodia**: Lateral appendages in forms like Nereis, aid in swimming

Setae/chaetae: Bristle-like structures for anchoring

**Examples**:

**Nereis**: Marine polychaete worm with parapodia

**Pheretima**: Earthworm (oligochaete, terrestrial)

**Hirudinaria**: Blood-sucking leech (parasitic)

**Arenicola**: Lugworm (marine)

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PHYLUM ARTHROPODA (JOINTED-LEGGED ANIMALS)

**Definition**: Largest animal phylum, comprising over two-thirds of all named species, with organ-system level organization, chitinous exoskeleton, jointed appendages, and diverse habitats.

**Key Characteristics**:

**Body Organization**: Organ-system level

**Symmetry**: Bilateral

**Germ Layers**: Triploblastic

**Coelom**: Coelomate (greatly reduced, mainly hemocoel)

**Segmentation**: Metamerically segmented (fused into distinct regions)

**Body Regions**: Head, thorax, abdomen

**Exoskeleton**: **Chitinous cuticle** (protein-chitin complex)

Non-living, molted periodically for growth

Provides shape, support, protection

Site for muscle attachment

**Appendages**: **Jointed legs and modified appendages** (antennae, mouthparts)

Arthros (joint) + Poda (feet)

Allow diverse movement and manipulation

**Respiration**:

Gills (aquatic arthropods)

Book gills/book lungs (spiders, scorpions)

Tracheal system (insects—tubes branching to cells)

**Circulatory System**: **Open type** (hemocoel—blood bathes organs directly)

**Sensory Organs**:

Antennae (chemoreception, touch)

Eyes (compound and simple ocelli)

Statocysts/balancing organs

Tympana (hearing in insects)

**Excretion**: **Malpighian tubules** (in insects)

Outgrowths of digestive tract

Filter hemolymph

Conservation of water

**Reproduction**: Sexual

**Sexes**: Mostly dioecious

**Fertilization**: Usually internal

**Development**: Direct or indirect (with larval stages)

**Oviparous**: Mostly egg-laying

**Metamorphosis**: Complete (holometabolous) or incomplete (hemimetabolous)

**Economic Importance Examples**:

**Apis mellifera**: Honey bee (pollination, honey production)

**Bombyx mori**: Silkworm (silk production)

**Laccifer lacca**: Lac insect (lac/shellac production)

**Vectors of Disease**:

**Anopheles**: Mosquito (malaria vector)

**Culex**: Mosquito (filariasis, Japanese encephalitis)

**Aedes aegypti**: Mosquito (dengue, yellow fever)

**Pest**:

**Locusta migratoria**: Locusts (gregarious pest, crop destruction)

**Living Fossil**:

**Limulus polyphemus**: King crab (horseshoe crab, 450 million years old, unchanged)

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PHYLUM MOLLUSCA (SOFT-BODIED ANIMALS)

**Definition**: Second-largest animal phylum with organ-system level organization, bilaterally symmetrical, unsegmented body with distinct head, muscular foot, and calcareous shell.

**Key Characteristics**:

**Body Organization**: Organ-system level

**Symmetry**: Bilateral

**Germ Layers**: Triploblastic

**Coelom**: Coelomate (reduced, mainly around heart)

**Segmentation**: Unsegmented (no metameric segmentation)

**Body Regions**:

**Head**: Anterior region with sensory tentacles

**Muscular Foot**: Ventral locomotor appendage (varies by class)

**Visceral Hump**: Contains organs

**Mantle**: Soft, spongy skin layer covering hump

Secretes shell

Creates mantle cavity (between hump and mantle)

**Shell**: **Calcareous** (made of calcium carbonate)

Protective function

Secreted by mantle

Absent in some species (Octopus)

**Mantle Cavity**: Space between mantle and hump

Contains **feather-like gills**

Functions: Respiration and excretion

Water current created for gas exchange

**Feeding Organ**: **Radula** (file-like rasping organ in mouth)

Rows of keratinous teeth

Scrapes food from surfaces

Unique to molluscs

**Circulatory System**: Open type (hemocoel present)

**Nervous System**: Three pairs of ganglia (cerebral, pedal, visceral)

**Reproduction**: Sexual

**Sexes**: Usually dioecious

**Fertilization**: External or internal

**Development**: Indirect (free-swimming larvae)

**Habitat**: Terrestrial or aquatic (marine or freshwater)

**Major Classes**:

1. **Gastropoda**: Snails, slugs (muscular foot for creeping)

2. **Bivalvia**: Clams, oysters, mussels (foot for burrowing, shell in two halves)

3. **Cephalopoda**: Squid, octopus, cuttlefish (foot modified into tentacles, high intelligence)

**Examples**:

**Pila globosa**: Apple snail (freshwater gastropod)

**Pinctada fucata**: Pearl oyster (bivalve, pearl production)

**Sepia officinalis**: Cuttlefish (cephalopod, intelligent)

**Loligo**: Squid (cephalopod, fast swimmer)

**Octopus vulgaris**: Octopus/Devil fish (highly intelligent, color change)

**Aplysia**: Sea-hare (gastropod, marine)

**Dentalium**: Tusk shell

**Chaetopleura**: Chiton (primitive, living fossil)

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PHYLUM ECHINODERMATA (SPINY-SKINNED ANIMALS)

**Definition**: Marine animals with internal calcareous skeleton made of ossicles, distinctive water vascular system, organ-system level organization, and unique larval development.

**Key Characteristics**:

**Body Organization**: Organ-system level

**Symmetry**:

**Radial in adults** (five-fold radial symmetry)

**Bilateral in larvae** (evidence of evolutionary origin)

**Germ Layers**: Triploblastic

**Coelom**: Coelomate (spacious)

**Skeleton**: **Endoskeleton** made of calcareous **ossicles** (bone-like plates)

Internal location (unlike arthropod exoskeleton)

Provides support and protection

**Habitat**: Exclusively marine

**Digestive System**: **Complete** (mouth ventral, anus dorsal)

Allows sequential processing of food

**Unique Feature—Water Vascular System (Ambulacral System)**:

**Definition**: Network of water-filled tubes

**Functions**:

Locomotion (tube feet/podia extend and contract for movement)

Capture and transport of food

Respiration (gas exchange through thin tube feet walls)

**Structure**: Radial canals connect to ring canal; lateral canals to tube feet

**Pressure System**: Ampulla (bulb) and sucker discs work hydraulically

**Circulation**: Open circulatory system; no blood vessels

**Respiration**: Through tube feet and dermal branchiae (thin body wall outgrowths)

**Excretion**: **No excretory system** (wastes diffuse through body surface)

**Reproduction**: Sexual

**Sexes**: Dioecious (separate sexes)

**Fertilization**: Usually external

**Development**: Indirect with **free-swimming larva**

Larva bilaterally symmetrical (ophiopluteus, bipinnaria, auricularia types)

Metamorphosis to radially symmetrical adult (major transformation)

**Examples**:

**Asterias rubens**: Starfish (five arms, active predator)

**Echinus**: Sea urchin (globular, movable spines)

**Antedon**: Sea lily (stalked, filter feeder)

**Cucumaria**: Sea cucumber (elongated, holothurian)

**Ophiura**: Brittle star (slender arms, rapid movement)

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PHYLUM HEMICHORDATA

**Definition**: Small group of worm-like marine animals formerly considered sub-phylum of Chordata, now separate phylum, possessing rudimentary notochord-like structure (stomochord).

**Key Characteristics**:

**Body Organization**: Organ-system level

**Symmetry**: Bilateral

**Germ Layers**: Triploblastic

**Coelom**: Coelomate (divided into compartments)

**Body Regions** (Three-part):

**Proboscis**: Anterior muscular extension (boring function)

**Collar**: Middle region (contains stomochord—notochord-like rod)

**Trunk**: Posterior elongated section (contains most organs)

**Stomochord**: Notochord-like structure in collar (not true notochord)

Suggests evolutionary relationship to chordates

Provides support

**Circulatory System**: Open type (hemocoel present)

**Respiration**: Through **gill slits/pharyngeal clefts** (chordate characteristic)

Opens to exterior

Water current for respiration

**Excretion**: **Proboscis gland** (specialized excretory organ)

**Nervous System**: Distributed (dorsal and ventral nerve cords)

**Reproduction**: Sexual

**Sexes**: Dioecious

**Fertilization**: External

**Development**: Indirect (free-swimming tornaria larva, similar to echinoderm larvae)

**Evolutionary Significance**: Bridge between non-chordates and chordates; shares chordate features (gill slits) and non-chordate features (open circulation)

**Examples**:

**Balanoglossus**: Acorn worm (common, burrowing habit)

**Saccoglossus**: Another acorn worm genus

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PHYLUM CHORDATA

**Definition**: Animals fundamentally characterized by presence of notochord, dorsal hollow nerve cord, and paired pharyngeal gill slits at some developmental stage, showing bilateral symmetry, triploblastic organization, coelomate body plan, and organ-system level organization.

**Key Chordate Characteristics**:

1. **Notochord**: Rod-like structure on dorsal side

Embryonic or adult (depending on subphylum)

Provides axial support

Lies between spinal cord and digestive tract

2. **Dorsal Hollow Nerve Cord**:

Single, tubular, dorsal to notochord

Distinguishes from non-chordates (ventral solid cord)

Develops from ectoderm (neural plate)

3. **Pharyngeal Gill Slits**:

Paired openings in pharynx (throat region)

Function: Water passage for respiration/filter feeding

May be modified or lost in adults (lungs in tetrapods)

4. **Post-anal Tail**:

Body extension posterior to anus

Not present in non-chordates

May be reduced/absent in some adults

5. **Closed Circulatory System**: Blood confined to vessels

6. **Bilateral Symmetry**: Left-right organization

7. **Triploblastic**: Three germ layers

8. **Coelomate**: True body cavity lined by mesoderm

9. **Organ-system Level Organization**

**Comparison: Chordates vs. Non-Chordates** (Table 4.1)

| Feature | Chordates | Non-Chordates |

|---------|-----------|---------------|

| Notochord | Present (embryonic or adult) | Absent |

| Nerve Cord | Dorsal, hollow, single | Ventral, solid, double |

| Gill Slits | Pharyngeal gill slits present | Gill slits absent |

| Heart Position | Ventral (below digestive tract) | Dorsal (above digestive tract if present) |

| Post-anal Tail | Present | Absent |

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SUBPHYLA OF CHORDATA

SUBPHYLUM UROCHORDATA (TUNICATA)

**Definition**: Marine chordates with notochord present only in larval tail; adults lack notochord; covered by tunic (cellulose-like covering).

**Key Characteristics**:

**Habitat**: Exclusively marine

**Notochord**: Present only in larval stage (in tail)

**Adult Form**: Sessile (non-motile), bag-like

**Tunic**: Cellulose-like outer covering (unique among animals)

**Larva**: Free-swimming, tadpole-like, with notochord in tail (resembles vertebrate larva)

**Metamorphosis**: Dramatic transformation from mobile larva to sessile adult

**Gill Slits**: Numerous

**Classification**: Protochordates (primitive chordates)

**Examples**:

**Ascidia**: Common tunicate

**Salpa**: Barrel-shaped, planktonic

**Doliolum**: Planktonic tunicate

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SUBPHYLUM CEPHALOCHORDATA

**Definition**: Marine chordates with persistent notochord extending from head to tail throughout life; small, fish-like, filter-feeding organisms.

**Key Characteristics**:

**Habitat**: Exclusively marine (shallow sandy areas)

**Notochord**: **Persistent throughout life** (head to tail), well-developed

**Body**: Small, fish-like, elongated (2-3 cm)

**Gill Slits**: Numerous pharyngeal slits

**Feeding**: Filter feeder (amphioxus)

**Similarity to Fish**: Body structure resembles simplified fish

**Classification**: Protochordates

**Evolutionary Significance**: Closest living relative to vertebrate ancestor; shows all chordate characters clearly

**Examples**:

**Branchiostoma lanceolatum**: Amphioxus/Lancelet (classic example in textbooks)

**Asymmetron**: Another cephalochordate

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SUBPHYLUM VERTEBRATA

**Definition**: Chordates in which embryonic notochord is **replaced by cartilaginous or bony vertebral column** (backbone) in adults; possess true vertebrae; more