The Phylum Chordata

• This phylum has 3 subphyla- Urochordata

- Cephalochordata- Craniata

• Chordates are deuterostomes (sensu strictu)

• Possess four unique characteristics

1) Dorsal Hollow Nerve Cord• Forms from a dorsal ectoderm tube

• Forms the CNS (brain & spinal cord)

• Other animal phyla have ventral solid nerve cord(s)

2) Notochord• Longitudinal, flexible cartilagenous rod

• Located between nerve cord and gut

2) Notochord

• Extends most of the length of the organism

• A simple ENDOSKELETON

• For some chordates this is all they have (thus ‘invertebrate’ chordates)

• VERTEBRATE chordates develop a more complex jointed skeleton

• BUT vertebrates still have vestiges of the notochord (intervertebral discs !)

3) Pharyngeal Slits• Early chordates – these connected to the

digestive tract; used for FILTER FEEDING

• Later became modified for gas exchange

• Present (vestigial) in our early embryo stage

•

4) Postanal Tail

4) Postanal Tail

• Digestive tract extends most of the body length in the majority of non-chordates

• Tail extends beyond the anus in chordates

• Contains skeletal elements

• Contains muscles

• Major propulsive force in many aquatic chordates

• Other uses for the tail ?????

Subphylum Urochordata

• Invertebrate chordates• Called Tunicates (some called sea

squirts)• Tunic is made of a cellulose-like

carbohydrate called tunicin• Filter Feeders• Free swimming as larvae• Sessile as adults (adhere by ‘head’ region

to objects)

Subphylum Urochordata

• Highly modified as adults

• Scarcely resemble other chordates

• During metamorphosis they lose the tail, notochord & nerve cord (‘brain’ atrophies)

• Retain pharyngeal slits

• Closed circulatory system

• Possess heart & blood cells (no hemoglobin)

• Most primitive of the chordates (?????)

Subphylum Urochordata (larva)

Subphylum Urochordata (adult)

Subphylum Cephalochordata

• Lancelets

• Laterally compressed

Subphylum Cephalochordata

• Adults somewhat resemble urochordate larvae

• All 4 chordate traits persist

• Filter feeders; tentacles around mouth

• Marine; burrow tail first

• Feeble swimmers; fish-like movements (sinusoidal)

• Serially arranged muscle segments

Origin of Craniates & Vertebrates• Fossils – resembling Cephalochordates – Burgess

Shale of British Columbia ~550mya

• Craniates and Vertebrates first appear during the Cambrian “explosion” ~ 530mya

• Early Craniates and Vertebrates possessed all four chordate characteristics and were filter feeders

• Probably derived from a Urochordate-like ancestor similar to a tunicate larva

• Paedogenesis resulted in a larva achieving sexual maturity and it did not undergo metamorphosis

Paedogenesis• Precocious attainment of sexual maturity in

a morphologically juvenile organism

• If successful, natural selection would have reinforced the absence of metamorphosis

• These larvae were active and natural selection would favor the most active

• Actively foraging organisms benefit from good sense organs

• Cephalization is a benefit

Craniate / Vertebrate Characteristics• All possess the four basic chordate traits (at least

at some stage)• Cephalization with a highly specialized brain• The brain is covered by or enclosed by a skull• MOST Craniates possess a vertebral column that

encloses the nerve cord• MOST possess jaws• Skeleton is an endoskeleton

The Endoskeleton

• Composed of cartilage, bone or both

• Grows with the organism

• Cells secrete and rearrange the matrix

• AXIAL SKELETON - skull plus vertebral column (plus ribs & breastbone, if present)

• APPENDICULAR SKELETON may be present (supports fins or limbs)

Additional Traits (I)

• Closed circulatory system

• Heart with 2, 3 or 4 chambers

• Arteries, capillaries, veins

• Red blood cells (with hemoglobin) for O2 transport

• Blood oxygenated through skin or more commonly via gills or lungs

Additional Traits (II)

• Possess kidneys (compact excretory structures)• Reproduction usually sexual• Dioecious (= unisexual)• Gender control variable (XY, WZ,

environmental)• Some can change gender• Parthenogenesis found in most classes (but

not common)

Subphylum Craniata• Nine or ten extant classes• Mixini• Petromyzontidae• Chrondrichthyes• Antinopterygii, Actinistia, Dipnoi• Amphibia• Reptilia• Aves (?)• Mammalia

The Agnathan Classes (I)

• Oldest fossilized craniates were agnathans

• Mud-suckers/filter feeders

• Living forms lack paired appendages and external armor

• No covers for gill slits

• Two-chambered heart

• Class Mixini (hagfishes)

• Class Petromyzontida (lampreys)

The Agnathan Classes (II)

• Hagfishes (30 species) are eel shaped

• Lack rasping mouthparts

• Some feed on sick or dead fish or on worms

• Marine

• Skull of cartilage

• No vertebrae (are “craniate invertebrates”)

Hagfish

The Agnathan Classes (III)• Lampreys (about 35 species) are eel shaped• Young are usually suspension (filter) feeders• Young live in freshwater• Adults usually marine but some stay in freshwater• Adults have rasping mouth parts• Attach to live fish and are parasitic• Possess a cartilaginous tube around the notochord

with pairs of projections (Vertebrae); thus a “craniate vertebrate”)

Lamprey (I)

Lamprey (II)

Lamprey (III)

The First Gnathans• Classes Placodermi and Acanthodii (extinct)

• Possessed hinged jaws and paired fins

• Jaws developed from skeletal rods supporting some of the more anterior pharyngeal slits

• Jaws allow for more varied feeding habits

• Vertebrate jaws work UP and DOWN (arthropod jaws work Side to Side)

• Remaining slits functioned as major gas exchange sites

Basic Craniate (from text)

Development of Jaws(from arch supports 3 & 4)

Jaws and Supports

Acanthodian

Placoderm

Class Chondrichthyes (I)

• The cartilaginous fishes (about 750 species)

• Lack of bone in skeleton is considered a derived condition

• Sharks, rays, skates, and more

• No swim bladder

• Must swim to stay up in the water column

• Some have added buoyancy due to large amounts of oil stored in the liver

Class Chondrichthyes (II)• Marine animals – evolved in the sea• Paired fins, well-developed jaws; most are

carnivores (a few filter feeders)• No opercula (external gill covers)• Scaled; Teeth evolved from modified scales• Reproduce sexually; fertilization internal;

dioecious• Oviparous, Ovoviviparous or Viviparous• Claspers on male’s pectoral fins transfer

sperm into the female’s reproductive tract

Class Chondrichthyes (III)• Cloaca present

• Heart two-chambered

• Short digestive tract; spiral valve present

• Good vision but no color vision

• Sharks have a fusiform body

• Rays and skates are dorsoventrally flattened

Black Tipped Reef Shark

Ray

The “Bony” Fishes• Formerly in one class (Osteichthyes) but now

divided into three classes

• Cloaca absent in all three classes; dioecious; heart is 2-chambered

• Class Actinistia – coelacanths (believed to have been extinct for 65 million years) – Two (?) extant species; a lobe-fin group that evolved in freshwater and then moved to the ocean

• Class Dipnoi – lungfishes – another lobe-fin group; 3 genera and 7 species; Freshwater; use lungs for respiration; Gave rise to tetrapods

Coelacanth

Dipnoi (Australian Lungfish)

Class Antinopterygii• Largest group of vertebrates (~30,000 spp)

• Most of our familiar fish species

• Body covered by flattened bony scales

• Evolved in freshwater; many moved to the ocean; some have returned to freshwater

• A few species spend a portion of their lives in freshwater and in saltwater

• Possess a swim bladder for buoyancy – it developed from lungs

Rainbow Trout

Evolution of lungs & Swim Bladder

Gas Exchange in Fishes

• Chondrichthyes – swimming important – forces water into mouth and out over gills

• Bony Fish – water drawn into mouth and forced out over gills by movement of the OPERCULUM and contraction of muscles within the gill chambers