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Deutrostomia. PHYLUM: CHORDATA. This phylum includes the vertebrates, the hagfish, the tunicates, lancelets and maybe a few extinct groups World wide distribution in aquatic and terrestrial habitats

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Phylum chordata l.jpg

  • This phylum includes the vertebrates, the hagfish, the tunicates, lancelets and maybe a few extinct groups

  • World wide distribution in aquatic and terrestrial habitats

  • The most important characteristics that make a chordate a chordate are a notochord, and visceral clefts and arches

  • The earliest chordate found so far are from the Cambrian around 525 Myo

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  • These are the tunicate animals

  • World wide distribution in aquatic and terrestrial habitats

  • The larval stage has a notochord, a dorsal hollow nerve cord and pharyngeal slits and arches

  • Adults are mostly sessile filter feeders that use a mucous food trap to feed

  • There are at least four clades one of which is discussed below although new evidence suggests that Ascidiacea is an artificial group

    • Clade one- the Styelid and pyurid ascidians

    • Clade two - molgulid ascidians

    • Clade three - phlebobranch and thaliaceans

    • Clade four - the larvaceans

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  • World wide distribution in aquatic and terrestrial habitats

  • E.g. Pyura is a solitary tunicate each animal is an independent unit

  • Each individual animal is enclosed in a thick protective test which is attached to the substrate by a holdfast

  • Water is drawn in through the buccal cavity and filtered by the pharynx, and passes out through the atrial siphon

  • The larva is free swimming and similar to amphioxus.

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  • Branchiostoma (amphioxus) is a small fish-like marine animal (70 mm in length)

  • occurs in tropical and temperate waters

  • normally found partly buried in sand

  • free swimming

  • possess a pronounced notochord

  • have major blood vessels that are similar to that of a fish

  • have multiple gill slits and a pharynx that is highly specialized for filtering.

  • no lateral paired fins has a dorsal fin that extends the entire length of the body, a caudal fin and a ventral fin

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  • The body walls are divided into V shaped muscle segments called myotomes

  • Contraction of the myotomes produces lateral body movements necessary for swimming

  • The protruding mouth is surrounded by an oral hood from which project ciliated oral tentacles (cirri)

  • The anus opens on the left side of the body near to the base of the caudal fin

  • The skeletal system is composed of a well developed notochord

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  • Amphioxus food consists of minute organisms that are filter fed into the mouth by water currents produced by the cilia on the gills

  • The digestive system consists of a mouth, a pharynx, a stomach, and a straight intestine which leads to the anus.

  • The circulatory system is similar to that of a fish

  • Respiration occurs in the pharynx which is attached dorsally and hangs in to a cavity called the atrium which is lined with ectodermal epithelium

  • The exchange of respiratory gases occurs through the gill slits

  • The excretory system is composed of simple nephridia that are situated dorsally-laterally

  • The nervous system consists of a central nerve cord that runs dorsal to the digestive tract and resting on the notochord

  • Sense organs include:

    • 1) Olfactory pit

    • 2) Epidermal sensory cells on the oral and velar tentacles

    • 3) Rows of simple eyes (1 ganglion and 1 pigment cell)

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  • The sexes are separate

  • The reproductive system is composed of paired gonads that project into the atrium

  • Fertilization is external

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  • Includes all animals with a skull

  • The animals within the craniata fall into two major clades

    • the Hyperotreti (hagfish)

    • Vertebrata (lampreys and vertebrates)


  • Sister group to the lamprey and vertebrates

  • 20 known species in four genera

  • Live in the cold northern and southern marine waters of the world

  • Look a bit like eels in shape

  • They are jawless parasitic or predatory.

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  • This group ranges from Lampreys to humans.

  • Now most of the vertebrate have jaws but around 400 million years ago most vertebrates were jawless fish (the ostracoderms)

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  • External Features

  • Body cylindrical with a laterally compressed tail possessing a caudal fin

  • The skin is without scales, but has an epidermal gland which produces the slimy secretions

  • There is a row of sensory pits forming a lateral line.

  • The mouth consists of suctorial disc, a mouth that is held open by a ring of cartilage, bordered by sensory papillae and horny “teeth” like structures

  • From the mouth protrudes a piston-like tongue.

  • There is a pair of eyes

  • A single nasal opening

  • seven gill slits

  • anus is on the ventral surface just behind the urogenital opening

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    Skeletal System

    • A notochord persists throughout

    • The axial skeleton is composed of small cartilaginous neural arches, with the head supported and protected by a cartilage skull

      Muscular System

    • Short segmented muscles which appear to have a “Z” shape in side view.

    • Alternate contractions produce rhythmic lateral movement of the flanks which propel the animal forward

    • Radial muscles surround the mouth and retractor muscles occur in the tongue.

      Digestive System

    • Oral disc and its horny teeth attach the Lamprey to other fish. The tongue rasps into the flesh of the fish using its retractor muscles. The digestive system consists of an oesophagus, a ventral pharynx, no differentiated stomach, a long strait intestine which has internal spirally arranged folds (typhlosole) which increases surface area for absorption. The digestive system terminates in an anus. A Liver is present in association with the digestive system.

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    Circulatory System

  • Consists of heart, arteries, veins and lymphatic sinuses. The heart lies in the pericardial cavity and consists of an atrium which receives blood and ventricle which pumps blood.

    Respiratory System

  • Seven pairs of gill pouches which open internally into the pharynx and externally via the gill slits. There are numerous gill filaments supplied with capillaries. Gaseous exchange occurs as water passes through the gill pouches. Water is drawn into the gill pouch through the gill slits (an adaptation to a parasitic life requiring that the mouth be attached to a host). The Larval Lamprey (called an amnocoete) is not parasitic, and water passes through the mouth and over the gills (the pattern that occurs in bony fish).opening.

    Excretory System

  • Linked with the reproductive system (urogenital system). The kidneys lie along the dorsal wall of the body. Urine passes via a urinary duct into the genital sinus which opens to the exterior via a urogenital opening.

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    Nervous System

  • The brain is primitive, dominated by forebrain with a large pair of olfactory lobes. The cerebral hemispheres are attached to a diencephalon, and there is a ventrally attached broad infundibulun and above it a pineal structure. The midbrain possesses a large pair of optic lobes, and the hind brain has rudimentary cerebellum. There are ten pairs of cranial nerves.

    Sense Organs

  • The olfactory sac is contained in the nasal capsule and is connected to the outside through the single nasal opening occurring on the dorsal surface. There is a balancing organ which lies in the auditory capsule and consists of two semicircular canals. The eyes are primitive, yet allow good vision. Posterior to the nasal opening is the pineal eye with a lens and a pigmented retina. The pigments of the eye vary with the Lamprey switching from Rhodopsin under marine conditions to Porphyropsin under freshwater conditions. Lampreys are also able to produce electrical fields.

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  • The sexes are separate, with the gonad being protandric, that is first producing male gametes and then female gametes within the same individual. A single large long gonad fills the abdominal cavity. There are no genital ducts, with egg or sperm being discharged into abdominal cavity and passes through paired genital pores into the urogenital sinus and the passes outside the body external fertilization takes place. Spawning always occurs in freshwater, with a female lamprey producing up to 100,000 eggs. After spawning the female Lampreys die. The eggs hatch into a larva called the amnocoete and remain in this condition for 3 to 12 years. The Larva obtains its food through inspiration of water and using mucous to entangle the food and pass it to the small intestine (in a very similar fashion to that of amphioxus). The amnocoete larva undergoes metamorphosis to become an adult Lamprey and the returns to the sea. The lamprey differs from the hagfish which has a mouth that is surrounded by sensory tentacles and also has a varying number of gill pouches (five to fifteen

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  • These animals make up the majority of the vertebrates from the middle Devonian up until now.

  • The Gnathostomes are split into two extant clades,

    • the Chondrichthyes and the

    • Osteichthyes

  • as well as two extinct clades

    • the Placodermi and

    • the Acanthodii.

  • They differ from other animals by having jaws.

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  • These are Large, heavily armoured fish possessing jaws, and were possibly the first vertebrates to possess them (this may, however, be in dispute).

  • Most placoderms were predatory fish and some reached 10 metres in length.

  • The most characteristic feature is the heavy armour plates on the outside,

  • The placoderms were far removed from the main evolutionary line of the vertebrates.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

  • Currently the most primitive vertebrates.

  • Divided into two clades, Elasmobranchii and Holocephali

  • They possess

    • Complete and separate vertebrates

    • Movable jaws

    • Paired appendages

  • They are less advanced than the bony fish (Class Osteichthyes) because

    • Skeleton is made only of cartilage

    • Placoid scales only

    • Separate gill clefts

    • Possesses a pair of spiracles connected to the pharynx

    • Has no swim bladder

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

  • An example is the dog fish (Squalus) whose eternal features include

    • Blunt pointed head

    • Trunk spindle shaped

    • Two separate median dorsal fins

    • A median caudal fin

    • Two pairs of lateral fins (pectoral and pelvic)

    • Mature males have a clasper on their pelvic fins

    • Anal fin

    • Head

    • Two nostrils

    • Lateral eyes without eyelids

    • Five oval gill slits open anteriorly to each pectoral fin

    • Spiracle (gill cleft) opens behind each eye

    • Body covering diagonal rows of minute placoid scales

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

  • Skeleton

  • Cartilage reinforced with limy deposits

  • Axial skeletons: skull/vertebral column, with each vertebra having a spool shaped centrum, concaved at both ends, a neural arch housing the nerve cord. The notochord persists in spaces between the vertebrae.

  • The skull is composed of a cranium housing the brain, possesses paired capsules for olfactory, optic and auditory organs. The visceral skeleton consists of jaws, the hyoid arch and five pairs of branchial arches supporting the gill region.

  • The appendicular skeleton includes a V – shaped pectoral girdle supporting the pectoral fins, a flatter pelvic girdle with fins and a smaller jointed cartilage which provides internal support for the lateral fins.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Muscular System

  • Body and tail muscles segmented

    Digestive System

  • Mouth

    • Teeth

    • Flat tongue

    • On the sides of the wide pharynx are openings leading to separate gill slits and spiracles.

    • Short oesophagus

    • J shaped stomach

    • Intestine follows and is connected to a cloaca and anus.

    • Liver

    • Pancreas

    • Stomach, intestine, and other internal organs lie in the large body cavity or coelom.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Digestive System

  • It is lined with a smooth membrane: the peritoneum and this covers the organs

  • The peritoneum is supported from the mid dorsal wall of the coelom by thin mesenteries

  • The transverse septum separates coelom from the cavity containing the heart

    Circulatory System

  • The heart consists of a thin-walled dorsal sinus venosa that receives blood, followed by the atrium, then the thick walled ventricle and the conus arteriosus.

  • Blood passes anteriorly into the ventral aorta where five pairs of afferent branchial arteries distribute to the capillaries in the gills for aeration, four pairs of efferent branchial arteries collect blood into the dorsal aorta which extends along mid-dorsal wall of the coelom.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Circulatory System

  • Principal arteries are:

    • Paired external/internal carotids -. Head

    • Paired subclavians -. pectoral fins

    • Coelic -. stomach, liver, intestine

    • Anterior mesenteric -. spleen and posterior intestine

    • Posterior mesenteric -, to rectal gland

    • Renal and gonadic -. to kidneys/reproductive organs

    • Paired iliacs -. pelvic fins

  • Principal veins are:

    • Paired renal portal  kidneys

    • Paired postcardinal  paralleling the kidneys

    • Paired Lateral abdominal veins  either side of the body cavity

    • Jugular and anterior cardinal veins (sinuses) return blood from the head region

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Respiratory System

  • By opening and closing the mouth, sharks draw in water and pass it out through the gill slits and spiracles.

  • The gills are composed of many parallel, slender filaments that contain capillaries.

  • Blood from the ventral aorta passes through these capillaries, discharges carbon dioxide and absorbs oxygen dissolved in water, then continues in the dorsal aorta.

    Excretory System

  • The two slender kidneys lie immediately above the coelom on either side of the dorsal aorta.

  • Urine from each kidney is collected in a series of segmental tubules that join a longitudinal duct, the ureter, leading posteriorly; the two ureters empty through a single urogenital papilla, located dorsally in the cloaca.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Sense organs

  • Two nostrils (olfactory sacs)

  • Pharynx: scattered taste buds

  • Eyes: moved by three pairs of muscles attaching the eyeball to its socket and are without lids. Retina composed of rods only; therefore no colour vision is possible.

  • “Ear” Organ of balance. Three semi-circular canals

  • Lateral line - A fine groove along each side of the line trunk and line tail. Contains a slender canal with many small openings to the surface. Within the canal are sensory hairs connecting to a branch of the tenth cranial nerve. They respond to low frequency pressure stimuli in the surrounding water. On the head are other sensory canals opening by senses pores; each pore leads to a small chamber (Ampulla of Lorenzini) containing an electroreceptor with sensory hairs connecting to nerve fibres.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Nervous System

  • More advanced than the lamprey.

  • Forebrain

    • Two olfactory sacs

    • Large olfactory tracts  olfactory lobe

    • Paired cerebral hemispheres on the diencephalons

    • Dorsal pineal body

    • Ventral infundibulun and hypophysis

  • Midbrain

    • Two rounded lobes dorsally situated

  • Hindbrain

    • Large median dorsal cerebellum over the open topped medulla oblongata

    • Eleven cranial nerves (one to ten plus zero)

    • Spinal nerve protected by vertebrae

    • Paired spinal nerves to each body segment

    • Sympathetic nervous system: series of ganglia, roughly segmental above the post cardinal veins.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Reproductive System

  • Sexes are separate

  • Male sperm develops in long testes from each testis vasa deferens runs posteriorly on the ventral surface of the kidney and empties into the urogenital papilla.

  • At mating claspers are placed close together, and they are inserted into the cloacae of the females. Seminal fluid flows down the canal formed by the adjacent clasper grooves.

  • Females have two large ovaries, two oviducts extend the length of the body cavity, their anterior ends join in a single large funnel through which the eggs enter. The forward part of the each duct is dilated as a shell gland, and in ovoviviparous species, like the dog fish the posterior part is enlarged as an “uterus” to contain the young during development. The oviducts open separately into the cloaca.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Classification of Chondrichthyes


  • upper jaws are not fused to their skulls

  • anal and urogenital openings are the same

  • have many sharp and replaceable teeth.

  • SUPER ORDER: BATOIDEA (rays and skates)

  • “Bottom living forms” flattened dorsoventrally,

  • pectoral fins enlarged and locomotion is achieved through undulations,

  • lost the caudal fins;

  • no lateral thrashing of the tail.

  • Mouth on bottom.

  • Water brought in via spiracles on dorsal surface (behind the eyes)

  • ORDER: RAYIFORMES (common rays and skates)

  • These are the true rays and skates and have flat bodies with a cartilaginous skeleton.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Classification of Chondrichthyes

  • ORDER: PRISTIFORMES (saw fish)

  • These have a long snout with teeth like projections and no swim bladder

  • ORDER: TORPEDINIFORMES (electric rays)

  • capable of producing an electric discharge used to hunt prey


  • This group contains the hand size pigmy shark right up to the 12 m whale shark

  • ORDER: HEXANCHIFORMES (6/7 gilled sharks)

  • These are the most primitive types of shark. There are five species living today. They have only one dorsal fin and no nictitating membrane

  • ORDER:SQUALIFORMES (2 dorsal finned sharks e.g. dogfish)

  • There are about 80 species in this clade. They have two dorsal fins, no anal fin and no nictitating membrane.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Classification of Chondrichthyes


  • one family and two genera

  • all have a long snout with tooth like projections

  • ORDER: SQUATINIFORMES (angel sharks)

  • one family,16 species

  • look like rays and skates with a flattened body.

  • ORDER: HETERODONTIFORMES (bullhead/ horn sharks)

  • These are primitive modern sharks.

  • nine species, one genus

  • all small (50-150 cm) bottom feeders.

  • ORDER: ORECTOLOBIFORMES (carpet sharks e.g. whale and nurse sharks)

  • 32 species including the Whale and nurse shark.

  • They have two dorsal fins and many have barbells.

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  • CLASS: CHONDRICHTHYES (Cartilaginous fish)

    Classification of Chondrichthyes

  • ORDER: CARCHARHINIFORMES (ground sharks/ blue/ tiger/ bull)

  • over 270 species

  • have a nictitating membrane

  • two dorsal fins, an anal fin and five gill slits.

  • ORDER: LAMNIFORMES (mackerel sharks/ goblin and basking sharks)

  • two dorsal fins, an anal fin and five gill slits without a nictitating membrane.


  • Shark-like but with crushing tooth plates.

  • Most forms are extinct.

  • ORDER CHIMAERIDA Chimaeras or ratfish,

  • Flap of skin covering the separate branchial slits

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  • May have been the earliest gnathostomes

  • They were small fish which superficially resembled sharks.

  • Typified by the genus Climatius

  • Although Climatius represents a primitive ancestral form the acanthodians diversified in the same ways that the bony fish have; namely long slender eel - like forms to deep-bodied fast swimming forms.

  • Acanthodian were most abundant in the Devonian and were extinct by the end of the Palaeozoic

  • The phylogenetic relationship of these fish to the other jawed fishes is unclear.

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  • Characteristics

  • Skin with many mucous glands, usually with embedded bony dermal scales

  • Median and paired fins are present

  • No limbs

  • Mouth usually terminal and possessing teeth

  • Jaws well developed and articulated to the skull

  • Eyes are usually well developed possessing rods and cones (the latter for colour vision) and not possessing eyelids.

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  • Characteristics

  • Skeleton

    • chiefly of bone (cartilage in primitive sturgeons).

    • Vertebrae are many and distinct;

    • tail usually homocercal

  • Heart is two chambered (one atrium and one ventricle).

  • Respiration

    • by pairs of gills on bony gill arches contained within a common chamber on each side of the pharynx

    • covered by a bony operculum.

  • Usually possess a swim bladder which sometimes has a duct connecting to the pharynx.

  • Cranial nerves zero to ten are present.

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  • Characteristics

  • Body temperature dependent on environment and behaviour

  • Gonads typically paired;

  • usually oviparous

  • fertilization usually external

  • Eggs are usually minute but can be large

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  • Currently the most numerous group of vertebrates

  • characterized by

    • ganoid scales.

    • large eyes

    • good olfactory senses

    • lateral line system.

  • Evolutionary History of the Actinopterygians

    • Isolated scales were found in the late Silurian,

    • the first body fossils were found in the Middle Devonian.

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  • early fish

    • small

    • heavy scales

    • complex, fully ossified skulls

    • Tooth bearing bones characterize the upper and lower jaws.

    • eyes extremely large

    • the mouths were long.

    • heterocercal tail

    • a single dorsal fin

    • an anal fin

    • Pectoral and pelvic fins were widely separated.

    • vertebrae incompletely ossified and the notochord very conspicuous.

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  • CLADISTIA (polypteriformes, reedfishes)

  • Sister group of recent actinopterygii

  • CHONDROSTEI (sturgeons, paddlefish)

  • abundant in the Palaeozoic

  • generally small fish with uptilted (heterocercal) tails and scales covered with ganine

  • E.g. Sturgeon fish (Acipenser) 16 species

    • generally lost their ganoid scales

    • The skeleton is mostly cartilaginous.

  • E.g Paddle fish

  • lost most of their ganoid scales

  • jaws have degenerated

  • tend to scavenge with their mouths open.

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  • fast swimming fish

  • have elongated jaws

  • HALECOSTOMI (bowfins and teleosts)

  • E.g. Bowfin (Amia),

  • scales have lost their ganoid covering.

  • Originated in the middle to late Mesozoic.

  • pectoral fins are positioned high up on the lateral surfaces

  • pelvic fins are positioned far forward.