Read chapter 2 of the text. Phylum Chordata The chordates are a group of particular interest to us as we belong to it, being members of the subphylum

Read chapter 2 of the textRead chapter 2 of the text

Phylum ChordataPhylum Chordata

The chordates are a group of particular The chordates are a group of particular interest to us as we belong to it, being interest to us as we belong to it, being members of the subphylum Vertebrata.members of the subphylum Vertebrata.

The chordates include all of the The chordates include all of the vertebrates (fish, amphibians, reptiles, vertebrates (fish, amphibians, reptiles, mammals and birds), but also two non-mammals and birds), but also two non-vertebrate subphyla: the Urochordata and vertebrate subphyla: the Urochordata and the Cephalochordata.the Cephalochordata.

Phylum ChordataPhylum Chordata

The chordates were in the 19The chordates were in the 19 thth century century considered to have been derived from considered to have been derived from protostome ancestors (the annelid, mollusc, protostome ancestors (the annelid, mollusc, arthropod group).arthropod group).

However, a better understanding of embryology However, a better understanding of embryology shows that chordates are deuterostomes and shows that chordates are deuterostomes and the invertebrates they are most closely related to the invertebrates they are most closely related to are the Echinodermata (sea stars, sand dollars, are the Echinodermata (sea stars, sand dollars, sea urchins) and the Hemichordata (acorn sea urchins) and the Hemichordata (acorn worms).worms).

Protostomes and DeuterostomesProtostomes and Deuterostomes

Within the eucoelomates there are two Within the eucoelomates there are two major evolutionary lineages that split early major evolutionary lineages that split early in the history of animals and follow quite in the history of animals and follow quite different developmental pathways.different developmental pathways.

These are the protostomes “mouth first” and These are the protostomes “mouth first” and deuterostomes “mouth second”.deuterostomes “mouth second”.

Important differences in development Important differences in development between protostomes and deuterostomesbetween protostomes and deuterostomes

The differences in development that distinguish The differences in development that distinguish the protostomes and deuterostomes include:the protostomes and deuterostomes include: Whether cleavage of cells in the early zygote is spiral Whether cleavage of cells in the early zygote is spiral

or radial.or radial. Whether or not, if the early blastomere is separated, Whether or not, if the early blastomere is separated,

each cell can develop into a normal larva or not.each cell can develop into a normal larva or not. Whether the blastopore ultimately forms the mouth or Whether the blastopore ultimately forms the mouth or

anus of the organism.anus of the organism. Whether or not the organism possesses a coelom and Whether or not the organism possesses a coelom and

how that coelom is formed.how that coelom is formed.

Protostomes and DeuterostomesProtostomes and Deuterostomes

Protostomes include the annelids, Protostomes include the annelids, mollusks, and arthropods.mollusks, and arthropods.

Deuterostomes include the echinoderms Deuterostomes include the echinoderms and Chordates, which includes the and Chordates, which includes the vertebrates.vertebrates.

Figure 23.02

Characteristics of the ChordataCharacteristics of the Chordata

Chordates are:Chordates are: bilaterally symmetricalbilaterally symmetrical triploblastictriploblastic have a well developed coelom have a well developed coelom have a complete digestive systemhave a complete digestive system

Germ layers of a triploblastic Germ layers of a triploblastic organismorganism

EndodermEndoderm: innermost germ layer of an embryo. Forms : innermost germ layer of an embryo. Forms the gut, liver, pancreas.the gut, liver, pancreas.

EctodermEctoderm: Outer layer of cells in early embryo. : Outer layer of cells in early embryo. Surrounds the blastocoel. Forms outer epithelium of Surrounds the blastocoel. Forms outer epithelium of body and nervous system.body and nervous system.

MesodermMesoderm: Third germ layer formed in gastrula between : Third germ layer formed in gastrula between ectoderm and endoderm. The ectoderm and endoderm. The coelomcoelom forms in the forms in the mesoderm. Mesoderm gives rise to connective tissue, mesoderm. Mesoderm gives rise to connective tissue, muscle, urogenital and vascular systems and muscle, urogenital and vascular systems and peritoneum.peritoneum.

CoelomsCoeloms

The coelom is a cavity entirely surrounded The coelom is a cavity entirely surrounded by mesoderm. by mesoderm.

A coelom provides a tube-within-a-tube A coelom provides a tube-within-a-tube arrangement which has many advantages:arrangement which has many advantages: Allows visceral organs to grow independently Allows visceral organs to grow independently

of the body wallof the body wall fluid-filled coelom acts as a hydrostatic fluid-filled coelom acts as a hydrostatic

skeleton in some animals (e.g. earthworms).skeleton in some animals (e.g. earthworms). In mammals the pericardial, peritoneal, and In mammals the pericardial, peritoneal, and

pleural cavities are formed from the coelom.pleural cavities are formed from the coelom.

CoelomsCoeloms

Triploblastic organisms (organisms with three Triploblastic organisms (organisms with three germ layers including mesoderm fall into one of germ layers including mesoderm fall into one of three different coelomic states:three different coelomic states: AcoelomateAcoelomate: mesoderm fills the blastoceol, no cavity : mesoderm fills the blastoceol, no cavity

occurs in the mesoderm. Flatworms and nemerteans.occurs in the mesoderm. Flatworms and nemerteans. PseudocoelomatePseudocoelomate: mesoderm lines only outer edge : mesoderm lines only outer edge

of blastocoel. No peritoneal lining develops. of blastocoel. No peritoneal lining develops. Nematodes and rotifers.Nematodes and rotifers.

EucoelomateEucoelomate: Have a true coelom derived from : Have a true coelom derived from mesoderm and lined with peritoneum. Arthropods, mesoderm and lined with peritoneum. Arthropods, annelids, mollusks, echinoderms, vertebrates.annelids, mollusks, echinoderms, vertebrates.

Five distinctive characteristics of Five distinctive characteristics of the chordatesthe chordates

Five distinctive characteristics separate the Five distinctive characteristics separate the chordates from all other phyla:chordates from all other phyla: NotochordNotochord Single, dorsal, tubular nerve cordSingle, dorsal, tubular nerve cord Pharyngeal pouches or slitsPharyngeal pouches or slits EndostyleEndostyle Postanal tailPostanal tail

Not all of these characteristics are apparent in Not all of these characteristics are apparent in adult organisms and may appear only in the adult organisms and may appear only in the embryonic or larval stages.embryonic or larval stages.

NotochordNotochord

NotochordNotochord: the notochord is a flexible, : the notochord is a flexible, rodlike structure. It extends the length of rodlike structure. It extends the length of the body and is an anchor point for the body and is an anchor point for muscles. muscles.

The notochord bends without shortening The notochord bends without shortening so it permits the animal to undulate.so it permits the animal to undulate.

Figure 23.01

NotochordNotochord

In nonvertebrates and the jawless In nonvertebrates and the jawless vertebrates the notochord is present vertebrates the notochord is present throughout life.throughout life.

However, in the jawed vertebrates it is However, in the jawed vertebrates it is replaced by the vertebral column; the replaced by the vertebral column; the remnants of the notochord being found in remnants of the notochord being found in the intervertebral disks.the intervertebral disks.

Single, dorsal, tubular nerve Single, dorsal, tubular nerve cordcord

In most invertebrates the nerve cord, if present, In most invertebrates the nerve cord, if present, is ventral to the gut.is ventral to the gut.

In chordates, in contrast, the nerve cord is In chordates, in contrast, the nerve cord is dorsal to the gut and notochord. The nerve cord dorsal to the gut and notochord. The nerve cord passes through the neural arches of the passes through the neural arches of the vertebrae, which protect it. vertebrae, which protect it.

The nerve cord is enlarged in vertebrates into a The nerve cord is enlarged in vertebrates into a brain, which is surrounded by a bony or brain, which is surrounded by a bony or cartilaginous cranium.cartilaginous cranium.

Pharyngeal pouches and slitsPharyngeal pouches and slits

Pharyngeal slits occur in aquatic chordates and Pharyngeal slits occur in aquatic chordates and lead from the pharyngeal cavity to the outside. lead from the pharyngeal cavity to the outside.

The pharyngeal slits are used as a filter feeding The pharyngeal slits are used as a filter feeding device in protochordates (i.e., Urochordata device in protochordates (i.e., Urochordata (Tunicates)) and Cephalochordata (lancelets (Tunicates)) and Cephalochordata (lancelets e.g. Amphioxus). e.g. Amphioxus).

Water containing food is drawn in through the Water containing food is drawn in through the mouth by cilia and exits via the pharyngeal slits mouth by cilia and exits via the pharyngeal slits where the particles are trapped in mucus.where the particles are trapped in mucus.

Figure 23.09b

Amphioxus


In vertebrates the pharyngeal arches have In vertebrates the pharyngeal arches have been modified into gills by the addition of a been modified into gills by the addition of a rich blood supply and thin gas permeable rich blood supply and thin gas permeable walls. walls.

The contraction of muscles in the pharynx The contraction of muscles in the pharynx drive water through the gills.drive water through the gills.


In amniotes an opening may not form and In amniotes an opening may not form and rather than slits only grooves called rather than slits only grooves called pharyngeal pouches develop.pharyngeal pouches develop.

In tetrapods these pouches give rise In tetrapods these pouches give rise during development to a variety of during development to a variety of structures including the middle ear cavity, structures including the middle ear cavity, eustachian tube, and tonsils.eustachian tube, and tonsils.

Endostyle or thyroid glandEndostyle or thyroid gland

The endostyle is found in protochordates The endostyle is found in protochordates and in lamprey larvae. It is located on the and in lamprey larvae. It is located on the floor of the pharynx and secretes mucus, floor of the pharynx and secretes mucus, which is used to trap particles. which is used to trap particles.

The endostyle works with the pharyngeal The endostyle works with the pharyngeal slits in filter feeding.slits in filter feeding.

Endostyle or thyroid glandEndostyle or thyroid gland

Some cells in the endostyle secrete Some cells in the endostyle secrete iodinated proteins and are homologous iodinated proteins and are homologous with iodinated-hormone secreting thyroid with iodinated-hormone secreting thyroid gland, which is found in adult lampreys gland, which is found in adult lampreys and vertebrates.and vertebrates.

Postanal tailPostanal tail

The postanal tail, some musculataure and the The postanal tail, some musculataure and the notochord enable larval tunicates and notochord enable larval tunicates and amphioxus to swim.amphioxus to swim.

The postanal tail evolved to allow organisms to The postanal tail evolved to allow organisms to swim and its efficiency has been enhanced by swim and its efficiency has been enhanced by the addition of fins. The postanal tail is present the addition of fins. The postanal tail is present only in vestigial form in humans (the coccyx) only in vestigial form in humans (the coccyx) although tails as a whole are widespread among although tails as a whole are widespread among vertebrates. vertebrates.

Figure 23.09b

Amphioxus

Classification of the ChordataClassification of the Chordata

There are three subphyla in the Chordata:There are three subphyla in the Chordata: Subphylum Urochordata: tunicatesSubphylum Urochordata: tunicates Subphylum Cephalochordata: lanceletsSubphylum Cephalochordata: lancelets Subphylum Vertebrata: fish, amphibians, Subphylum Vertebrata: fish, amphibians,

reptiles, birds, mammals, etc.reptiles, birds, mammals, etc.

Subphylum UrochordataSubphylum Urochordata

The Urochordata (“tunicates” named for the The Urochordata (“tunicates” named for the tough tunic that surrounds the adult) look like tough tunic that surrounds the adult) look like most unpromising candidates to be chordates most unpromising candidates to be chordates and relatives of the vertebrates.and relatives of the vertebrates.

The largest group, the ascidians or sea squirts The largest group, the ascidians or sea squirts (Class Ascidiacea) as adults are marine, sessile, (Class Ascidiacea) as adults are marine, sessile, filter feeding organisms that live either solitarily filter feeding organisms that live either solitarily or in colonies. or in colonies.

Ciona intestinalis (a solitary sea squirt)

0147.jpg

Synoicum pulmonaria a colonial sea squirt

AscidiansAscidians

Adult ascidians lack a notochord and there Adult ascidians lack a notochord and there is only a single ganglion in place of the is only a single ganglion in place of the dorsal nerve cord. dorsal nerve cord.

Of the five characteristics of chordates Of the five characteristics of chordates adults possess only two: pharyngeal gill adults possess only two: pharyngeal gill slits and an endostyle, both of which they slits and an endostyle, both of which they use in filter feeding.use in filter feeding.

AscidiansAscidians

The adult sea squirt draws water in The adult sea squirt draws water in through an incurrent siphon and pushes it through an incurrent siphon and pushes it back out an excurrent one. back out an excurrent one.

Food particles are filtered out in the Food particles are filtered out in the pharyngeal slits with mucus from the pharyngeal slits with mucus from the endostyle used to trap particles.endostyle used to trap particles.

15.4

Figure 23.04

Larval AscidianLarval Ascidian

Even though the adult ascidian hardly Even though the adult ascidian hardly resembles a chordate its larva does.resembles a chordate its larva does.

Larval ascidians are very small and Larval ascidians are very small and tadpole-like and possess all five chordate tadpole-like and possess all five chordate characteristics previously outlined. characteristics previously outlined.

Young larval ascidian

Larval AscidianLarval Ascidian

The larval ascidian’s role is to disperse The larval ascidian’s role is to disperse and to achieve this it is free swimming. and to achieve this it is free swimming. However, it has only a short larval life However, it has only a short larval life (minutes to a couple of days) and does not (minutes to a couple of days) and does not feed during this time. feed during this time.

Instead it searches for a place to settle Instead it searches for a place to settle and then attaches and metamorphoses and then attaches and metamorphoses into an adult. into an adult.

Ascidian metamorphosisAscidian metamorphosis

During metamorphosis the notochord During metamorphosis the notochord disappears, the nerve cord is reduced to a disappears, the nerve cord is reduced to a single nerve ganglion and a couple of single nerve ganglion and a couple of nerves.nerves.

15.5

Figure 23.06

Other Urochordate classesOther Urochordate classes

Besides the ascidians there are two other Besides the ascidians there are two other classes of the Urochordata: the Larvacea classes of the Urochordata: the Larvacea and Thaliacea.and Thaliacea.

Both are small, transparent planktonic Both are small, transparent planktonic forms. Thaliaceans are cylindrical or forms. Thaliaceans are cylindrical or spindle shaped whereas larvaceans are spindle shaped whereas larvaceans are tadpolelike and resemble an ascidian tadpolelike and resemble an ascidian larva. larva.

Garstang’s hypothesis of chordate Garstang’s hypothesis of chordate larval evolutionlarval evolution

In the 1920’s it was proposed that the In the 1920’s it was proposed that the vertebrates were derived from an vertebrates were derived from an ancestral ascidian that retained its ancestral ascidian that retained its characteristics into adulthood (the process characteristics into adulthood (the process by which juvenile characteristics are by which juvenile characteristics are retained into adulthood is referred to as retained into adulthood is referred to as paedomorphosis).paedomorphosis).

Figure 23.12

Garstang’s hypothesis of chordate Garstang’s hypothesis of chordate larval evolutionlarval evolution

Garstang’s hypothesis is supported by Garstang’s hypothesis is supported by embryological evidence, but more recently embryological evidence, but more recently molecular analyses have suggested that molecular analyses have suggested that sessile ascidians are a derived form and sessile ascidians are a derived form and that the free-living larvaceans are more that the free-living larvaceans are more likely to be the closest relatives of the likely to be the closest relatives of the chordates.chordates.

Subphylum CephalochordataSubphylum Cephalochordata

The cephalochordates are the lancelets, which The cephalochordates are the lancelets, which are small (3-7 cm long) laterally compressed are small (3-7 cm long) laterally compressed fishlike animals that inhabit sandy sediments of fishlike animals that inhabit sandy sediments of coastal waters. They lack a distinct head and coastal waters. They lack a distinct head and have no cranium.have no cranium.

They are commonly referred to as Amphioxus as They are commonly referred to as Amphioxus as this was the original genus name. There are 29 this was the original genus name. There are 29 species, five of which occur in North American species, five of which occur in North American coastal waters.coastal waters.

Figure 23.09b

AmphioxusAmphioxus

Amphioxus is a filter feeder.Amphioxus is a filter feeder.

Water enters the mouth and then is moved Water enters the mouth and then is moved by beating cilia through the pharyngeal by beating cilia through the pharyngeal slits, where food is trapped in mucus. Cilia slits, where food is trapped in mucus. Cilia then move the food to the gut. then move the food to the gut.

Figure 23.09a

Amphioxus

AmphioxusAmphioxus

Amphioxus is interesting because it displays the Amphioxus is interesting because it displays the basic chordate characteristics in a simple and basic chordate characteristics in a simple and obvious form because of its transparency.obvious form because of its transparency.

Amphioxus is considered to be the closest living Amphioxus is considered to be the closest living relative of the vertebrates because it shares relative of the vertebrates because it shares several characteristics with vertebrates that several characteristics with vertebrates that Urochordates do not possess.Urochordates do not possess.

Amphioxus characteristics Amphioxus characteristics shared with vertebratesshared with vertebrates

Characteristics Amphioxus shares with Characteristics Amphioxus shares with vertebrates include: vertebrates include: Segmented myomeres (blocks of striated Segmented myomeres (blocks of striated

muscle separated by connective tissue)muscle separated by connective tissue) Dorsal and ventral aortasDorsal and ventral aortas Branchial (gill) arches (blood vessels running Branchial (gill) arches (blood vessels running

over the gills).over the gills).

Amphioxus characteristics not Amphioxus characteristics not shared with vertebratesshared with vertebrates

Amphioxus however Amphioxus however lackslacks several several characteristics that biologists think the ancestor characteristics that biologists think the ancestor of vertebrates possessed. These include:of vertebrates possessed. These include: Tripartite brainTripartite brain (with forebrain, midbrain and (with forebrain, midbrain and

hindbrain) protected by a hindbrain) protected by a cranium (skull)cranium (skull) Chambered heartChambered heart Closed circulatory systemClosed circulatory system Muscular gut and pharynx (food moved through Muscular gut and pharynx (food moved through

gut by ciliary action not peristalsis)gut by ciliary action not peristalsis) List continues on next slideList continues on next slide

Amphioxus characteristics not Amphioxus characteristics not shared with vertebratesshared with vertebrates

Various special sensory organsVarious special sensory organs (eyes, (eyes, chemical and pressure receptors, nose, inner chemical and pressure receptors, nose, inner ear)ear)

Neural crestNeural crest (ectodermal cells that are found (ectodermal cells that are found on the embryonic neural tube and are on the embryonic neural tube and are engaged in the formation of the cranium, tooth engaged in the formation of the cranium, tooth dentine, some endocrine glands and Schwann dentine, some endocrine glands and Schwann cells, which provide myelin insulation to nerve cells, which provide myelin insulation to nerve cells). cells).

Significance of differences between Significance of differences between amphioxus and vertebrates amphioxus and vertebrates

The differences between non-vertebrate The differences between non-vertebrate chordates such as Amphioxus and early chordates such as Amphioxus and early (and modern) vertebrates are a result of (and modern) vertebrates are a result of the increased the increased sizesize and and activityactivity of of vertebrates.vertebrates.


Because vertebrates are large they cannot Because vertebrates are large they cannot depend on slow processes such as depend on slow processes such as diffusion and ciliary action to support them.diffusion and ciliary action to support them.

As a result, specialized organ systems are As a result, specialized organ systems are needed to carry out physiological tasks at needed to carry out physiological tasks at a much faster rate. a much faster rate.


Vertebrates are also much more active than Vertebrates are also much more active than nonvertebrate chordates and are predators or at least nonvertebrate chordates and are predators or at least grazers (which is “predation” on plants). grazers (which is “predation” on plants).

Activity requires a suite of traits to support it. Activity requires a suite of traits to support it. sensory systems so you can seek things out;sensory systems so you can seek things out; complex nervous system to coordinate activity;complex nervous system to coordinate activity; more efficient circulatory, respiratory and digestive systems to more efficient circulatory, respiratory and digestive systems to

fuel the activity; fuel the activity; muscles and skeleton to facilitate movement.muscles and skeleton to facilitate movement.

The transition from nonvertebrate chordate to vertebrate The transition from nonvertebrate chordate to vertebrate thus is closely related to the adoption of a more active thus is closely related to the adoption of a more active lifestyle. lifestyle.

HaikouellaHaikouella

An early stage in the transition to vertebrates is An early stage in the transition to vertebrates is marked by the Cambrian era (520mya) fossil marked by the Cambrian era (520mya) fossil HaikouellaHaikouella

HaikouellaHaikouella is likely similar to the common is likely similar to the common ancestor of the vertebrates and it possesses a ancestor of the vertebrates and it possesses a muscular pharynx, which implies it pumped muscular pharynx, which implies it pumped water across its gills, which implies in turn a water across its gills, which implies in turn a more active lifestyle, although it also possesses more active lifestyle, although it also possesses a filter-feeding apparatus.a filter-feeding apparatus.

Once physiological systems began to Once physiological systems began to develop that enabled vertebrates to be develop that enabled vertebrates to be more active and sense their environments more active and sense their environments we would expect ecological competition we would expect ecological competition and selection to have rapidly driven the and selection to have rapidly driven the evolution of vertebrate diversity.evolution of vertebrate diversity.

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Read chapter 2 of the text. Phylum Chordata The chordates are a group of particular interest to us as we belong to it, being members of the subphylum