12 Chordata

8/9/2019 12 Chordata

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Introduction to

Phylum Chordata


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Unifying Themes

1. Chordate evolution is a history of innovations that is built upon

major invertebrate traitsThey display many of the basic traits that first evolved in the

invertebrates: bilateral symmetry, cephalization, segmentation,

coelom, "gut" tube, etc.

2. Chordate evolution is marked by physical and behavioralspecializationsFor example the forelimb of mammals has a wide range of structural

variation, specialized by natural selection

3. Evolutionary innovations and specializations led to adaptive

radiations -the development of a variety of forms from a single

ancestral group


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Characteristics of the Chordates

Notochord, dorsal hollow nerve cord, pharyngeal gill slits,

blocks ofmuscle, postanal tail


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Characteristics of the Chordates

The notochordAll chordate embryos have a notochord, a stiff but flexible rod that

provides internal supportRemains throughout the life history of most invertebrate chordates;

among, present only in the embryos of vertebrate chordates


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Characteristics of the Chordates cont.

Dorsal Hollow Nerve Cord (=Spinal Cord)A fluid-filled tube of nerve tissue that runs the length of the

animal, dorsal to the notochordPresent in chordates throughout embryonic and adult life


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Characteristics of the Chordates cont.

Blocks ofMuscle - MyotomesSurrounding the notochord and nerve cord are blocks of muscle -

myotomes

Postanal TailThe notochord, nerve cord, and the myotomes extend to the tail

Found at some time during a chordate's development


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Invertebrate Chordates


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SubPhylum Urochordata

Marine animals; some species are solitary, others are colonial.Sessile as adults, but motile during the larval stages Possess all5 chordate characteristics as larvae

Settle head first on

hard substrates and

undergo a dramatic

metamorphosis

(e.g., tail,

notochord, muscle

segments, and nerve

cord disappear)


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SubPhylum Urochordata cont.

Adult body is covered by

an outer envelope or tunic;

composed of fibers of

tunicinembedded in a

mucopolysaccharide matrixTunic encloses a basket-

like pharynx,that is

perforated by gill slits

Tunicates are filter feeders; plankton is trapped in a sheet of mucus

and cilia later direct the food-laden mucus to the stomachWater leaves the animal via an excurrent siphon


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Chordate Metamerism

Body segmentation (i.e. metamerism) appears to have evolved in

two lineages of the chordates: the Cephalochordates and the

Vertebrates; probably occurred after divergence from the

UrochordatesHowever, segmentation in the chordates does not involve the

coelom

The cephalochordates and the chordates movement isaccomplished by contraction of muscle fibers that are arranged in

segmented blocks - myotomesPresumably, segmentation of muscles developed as an adaptation

for undulatory swimming and rapid burrowing


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SubPhylum Cephalochordata

Exclusively marine animals

Although they are capable of swimming, they usually are buried inthe sand with only their anterior end being exposed


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SubPhylum Cephalochordata cont.

All chordate characteristics are present throughout their life historyThey are filter feeders: inside of the oral hood is lined with cilia

-wheel organThese cilia, plus cilia in the pharynx help generate a water currentWater and suspended food particles pass through the oral hood,

equipped with projections called cirrithat strain larger particlesFeed by secreting a mucous net across the gill slits to filter outfood particles that are present in the water.


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Subphylum Vertebrata


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General Characteristics

Exhibit all 5 chordate characteristics at sometime in their life historyUsually well cephalized, including a well developed brain and a

number of anterior sensory structuresBrain is usually encased in a skull,made of hard bone or a cartilage.In most vertebrates, the embryonic notochord is replaced by a

vertebral column.

Possess a distinctive endoskeleton consisting of vertebral column,limb girdles, two pairs of jointed appendages, and a head skeletonMuscles are attached to the skeleton to provide movementOften have a muscular perforated pharynx

Closed circulatory system with a well developed muscular heart;blood is oxygenated as it flows through vascularized skin, gills or

lungs.


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Evolutionary Relationships of the Vertebrates

Earliest vertebrate fossils (jawless ostracoderm fishes; 500 mya)

share many of the novel structures observed in the living vertebratesQ. When and from where did these vertebrate characteristics evolve?May have evolved from an invertebrate chordate lineageThis idea is supported by the discovery of a fossilized mid-Cambrain

invertebrate chordate from the Burgess Shale formation - Pikaia

A ribbon shaped, somewhat fish-like creature about 5 cm in lengthIt possessed a notochord and the V-

shaped myomeresResembles Amphioxus, and may

very well be an early cephalochordate

E l i R l i hi f h V b


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Evolutionary Relationships of the Vertebrates cont.

Speculations regarding vertebrate ancestry have focused on living cephalochordatesand tunicatesOne hypothesis on the evolution of the vertebrates is Garstang's HypothesisIt suggests that sessile tunicates were an ancestral stock that evolved a motile larval

stageGarstang speculated that at some point larvae failed to metamorphose into an adult,

but developed gonads and reproduced in the larval stageAnd with continued larval evolution a new group of free swimming animals evolvedGarstang called this process paedomorphosis,a term that describes the presence (or

evolutionary retention) of juvenile or larval traits in the adult body

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12 Chordata