The  Hemichordates and Chordates

Phylum Hemichordata

Why does this group have a name that means half – chordate? All members of the phylum Chordata have a

notochord and pharyngeal gill slits at some time in their lifecycle.

Hemichordata have gill slits and what was once believed to be a notochord. It is now known that they do not have a notochord, so are chordate – like only in the possession of pharyngeal gills slits.

Phylum Hemichordata Two groups =

Enteropneusta (acorn worms) and the seldom encountered Pterobranchia

Enteropneusta are burrowing worms that feed on particles strained out of the water by their gill slits..

http://tolweb.org/tree?group=Hemichordata&contgroup=Metazoa

Phylum Hemichordata

Pterobranchs are minute tube dwellers, filter-feeding with their tentacles.

Both are important phylogenetically.

 Echinoderm affinities Formation of the gastrula and three-part coelom

very similar to same stages in echinoderms. As is typical with deuteroscomes, the body is

divided into three regions. In hemichordates these regions are an anterior proboscis region, a short collar region and a long trunk.

In echinoderms it is seen in the three layers of the nervous system and in embryonic development of water vascular system and coelom.

 Echinoderm affinities Tornaria larva is virtually identical to the

bipinnaria of the asteroids. Have subepidermal nerve network. Pterobranchs may be similar to common

ancestor of chordates and echinoderms. Have arms and tentacles on dorsal side of collar. Echinoderm water vascular system is believed to

have been originally a food-catching device and may have come from such tentaculated arms.

Chordate affinities Pharyngeal lateral gill clefts. Only chordates and

hemichordates have them. In hemichordates the primary function of the gill slits

(there are no gills) is for feeding (probably was primary function of gill slits in primitive chordates as well). Food particles are caught in mucus and brought to mouth by ciliary action on proboscis and collar and are strained out of the branchial water that leaves through the gill slits and then directed into the digestive system.

Chordate affinities Thickenings of the nerve network form

longitudinal nerve cords. Ventral trunk ends at collar, but dorsal cord continues into the collar as the collar cord. It becomes internal, separated from the epidermal network, and is hollow in some acorn worms so may be homologous to dorsal hollow nerve cord of chordates.

Phylum Chordata– major characteristics Notochord. = turgid, flexible, rodlike

structure, extending the length of the body beneath the nerve cord.

Serves as a skeletal support in protochorcates and in primitive vertebrates. 

In vertebrates a series of cartilaginous or bony vertebrae is formed from the connective tissue sheath around the notochord and usually replace it as the chief mechanical axis of the body.

Phylum Chordata– major characteristics Dorsal, tubular nerve cord. The hollow cord

is formed by the infolding of ectodermal cells on the dorsal side onf the body above the notochord.

Among the vertebrates, the nerve cord lies in the neural arches of the vertebrae, and the anterior brain (= thickening and enlargement of cord at anterior end) is surrounded by a bony or cartilaginous cranium (= skull).

Phylum Chordata– other characteristics Postanal tail. Projects beyond anus at some

stage, but may or may not persist. In conjunction with musculature of body, and

stiffening notochord, tail gives motility to larval and adult free-swimming protochordates.

Evolved for propulsion in water. Efficiency enhanced in fishes with addition of fins.

Phylum Chordata– major characteristics Typical circulatory system consists of ventral

heart, with dorsal and ventral blood vessels; closed circulatory system.

Endoskeleton (cartilaginous or bony) present in most (i.e. the vertebrates).

Deuterostome embryology.

Adaptations that have guided chordate evolution Living endoskeleton.

Permits continuous growth without molting. Provide ample surface for muscle attachment so they can

grow large. Still keep exoskeleton in many. Most fishes have dermal

scales, and many higher vertebrates developed epidermal scales, beaks, claws and feathers from keratin (=tough, fibrous protein).

Endoskeleton was probably composed of cartilage that later was replaced by bone. Cart. is first skeleton in all larval and embryonic vertebrates In prim. fishes cart. is only skeleton.

Living endoskeleton (continued) Bone has two major advantages over

cartilage. Serves as reservoir for phosphate, which is an

indispensable component of high energy compounds (such as ATP), of membranes and of DNA.

Provides the structural strength required for terrestrial life. Mechanical stresses on the skeleton far greater than in water.

Adaptations that have guided chordate evolution  Paired limbs. Pectoral and pelvic

appendages are present in most vertebrates in the form of paired fins or legs. Originated as swimming stabilizers and later became developed into legs for terrestrial locomotion.

Adaptations that have guided chordate evolution Pharynx and efficient respiration Gill slits evolved as a ciliated filter-feeding apparatus. Still used

this way by hemichordates and protochordates. Cilia were replaced by muscular feeding apparatus that pumps

water through the pharynx by expanding and contracting the pharyngeal cav. --> internal gills.

Different from invertebrate gills. Invertebrate gills are typically feathery projections from the body surface without the pharyngeal pump which makes the vertebrate gill so efficient.

Some fish gills can remove 85% of the oxygen from water in a single pass over gill surface. Only arthropod tracheal system can approach this efficiency, but good only for small animal.

Adaptations that have guided chordate evolution Advanced nervous system Only the chordates have a single, hollow,

dorsal nerve cord, with clear separation of the brain (in which nerve cells are concentrated) and spinal cord (containing mostly fibers).

This system permits the greatest possible utilization of space for well-integrated nervous patterns. Brain has chance to develop and size is no problem.

Subphylum Urochordata (the tunicates) Tunicates are widely distributed in all seas from near the shoreline to great depths. Most of them are sessile, at least as adults.

http://www.auburn.edu/academic/classes/zy/0301/Topic3/Topic3.html

Subphylum Urochordata (the tunicates) They have a tunic which is usually quite thick

and functions as an external supportive and protective skeleton, but is a living, not a dead, skeletal envelope and is rather like connective tissue. Contains cellulose, amoebocytes and blood vessels (in many) although lies outside epidermis. Unique.

Tunic, pharyngeal basket and hermaphroditism all correlated with being sessile filter feeders.

Subphylum Urochordata (the tunicates) Pharynx highly specialized for filter feeding. A

mucus film is produced by food groove (endostyle) and carried across the inner surface of the pharynx by frontal cilia and traps food.

Tunic, pharyngeal basket and hermaphroditism all correlated with being sessile filter feeders.

Subphylum Urochordata (the tunicates) Development leads to a

tadpole larva, which possesses all of the chordate characters. Adults lack notochord, tubular nerve cord, postanal tail, sense organs and segmented musculature. Superficially look like sponges.  This larva is one of the best candidates for the ancestor of vertebrates.

http://www.auburn.edu/academic/classes/zy/0301/Topic3/Topic3.html

Urochordate tadpole larva Larva is tiny, active, site-seeking form with all

the characteristics missing in the adult. Such a larva with its propulsive tail, stiffening

notochord and dorsal nerve cord, which integrates sensory information and motor activity is an excellent candidate for the vertebrate ancestor. Requires neoteny.

Subphylum Cephalochordata

http://bioweb.uwlax.edu/zoolab/Table_of_Contents/Lab-9a/Lancelets/lancelets.htm

Subphylum Cephalochordata: Amphioxus You may have heard of Amphioxus because

this is one of the basic animals in zoology (along with Hydra and frogs). The genus has now been renamed Branchiostoma (gills + mouth). Common name is lancelet, but this is probably less familiar than Amphioxus.

Small animals, but are probably the direct ancestors of the vertebrates, so they are very important.

Subphylum Cephalochordata: Amphioxus They are found buried in the sand of southern

marine beaches with their anterior ends protruding into the water.

Lancelets are small (rarely exceed 7 cm, 2.5 inches). Can dive with amazing speed into the sand. When disturbed, it darts out of the sand, gyrates wildly about in zig-zag fashion for a few seconds, and then plunges head-first into sand again and soon projects snout out into the water. Except for these brief moments of activity, its habits are as sedentary as those of a fixed tunicate. Asymmetron (a related genus) has similar habits but is reported to be actively pelagic at night.

Chordate characters Filter feed by passing water through the mouth (via

ciliary currents) and out the pharyngeal gill slits to an atrium which surrounds the body. Out ventral atrial opening.

Mucus from endostyle (ciliated food groove) traps food as it goes through gill slits. Food goes from food groove to intestine where it is separated from the mucus and passed on to the hepatic cecum, off the intestine, where it is digested. Out through the anus.

No liver or pancreas, but have the cecum. In vertebrates the liver and pancreas arise as outgrowths of the intestine in this region.

Chordate characters Notochord runs from end to end of the laterally

compressed, tapered body, just above the digestive system. Plays an important part in the animal's movement.

Nerve cord lies just above the notochord (corresponds to spinal cord of vertebrates) and gives rise to alternate pairs of sensory and motor nerves (as in vertebrates). These remain separate and do not unite to form complex structures like the spinal nerves with their double roots of the vertebrates. No centralized brain and no trace of olfactory, optic or auditory sense-organs and nerves.

Chordate characters Postanal tail. Myotomes. On either side of notochord run

powerful longitudinal muscles, which are divided up into chevron-like segments = myotomes.

Such a metameric arrangement of muscles is similar to the somites of vertebrate embryos. Segmentation here is a function of muscles not of the coelom as in protostomes.

Chordate characters Circulatory system. Closed, but no heart. Blood

flows posteriorly in a dorsal aorta, to small vessels, to veins, to ventral aorta that pumps the blood forward by peristaltic-like contractions of the vessel wall, to branchial vessels in the gill bars where it is oxygenated and finally back into the dorsal aorta. Similar to fish.

Excretion is primitive. Essentially a protonephridial system with numerous nephridia called solenocytes (resemble modified flame cells). Empty into atrium.

Evolutionary position Clearly a chordate. Probably derived from sessile ancestors of a

primitive tunicate stock which retained its larval habits and form rather than its attached adult habits. Then it became specialized secondarily as an animal which is sedentary and filter-feeds.

Evolutionary position Main line of chordate evolution then becomes

a line of sedentary plankton-feeding organisms which began with an external apparatus of ciliated tentacles and food grooves (hemichordates), and ended with pharyngeal gill slits and an endostyle (protochordates).

 Evolutionary position The lancelets (cephalochordates) can be derived

from neotenic tunicate (urochordate) larvae. Many of their features clearly foreshadow the early fish (but remember that the living lancelets are highly specialized for their particular life style).

Probably a sister group to the vertebrates because lack a head, brain and complex sensory apparatus of vertebrates, pharyngeal gills and doesn't have the pumping mechanism of forcing water over the gills that is characteristic of fishes.

Subphylum Vertebrata (At last!) Cladistics versus Traditional

Classification of the vertebrates I will use the old fashioned traditional way of

classifying the vertebrates, as do your authors.  Realize that this is subject to change based on a different approach to phylogenetic trees, called cladistics.  

Brief review of cladistics Cladistic analyses are based on the

identification of  monophyletic groups, taxa which include the common ancestor of the group and all of its descendents.  To determine clades, extensive analysis using computers is necessary to find the groupings that have shared derived characters.

Brief review of cladistics Problems arise when we look at groups that

are paraphyletic.  These groups do not include all of the descendents of their most recent common ancestor.  Your text shows both systems of classification.  I bring it up again now because there is a strong divergence when we examine the cladistic version of the evolutionary trees of the vertebrates and compare them with the classical view.  You should be aware of such differences.

According to the cladistic view the reptiles, birds and mammals are a single clade, the Amniota because of the type of egg and extraembryonic membranes they all have.  (We will discuss this in detail later.) 

To separate the reptiles out as an independent taxon violates the principles of cladistics because the group does not contain all of the known descendents of the ancestor that gave rise to it, and is therefore paraphyletic. 

In fact, it turns out that according to cladistic analyses the birds are more closely related to crocodiles (considered reptiles) than are lizards and snakes (also reptiles).  So are birds really reptiles? 

Mammals also descended from a reptilian ancestor, although it was quite different from the ancestor of birds.  

The classical approach to systematics examines the amount of divergence between groups.  Birds are separated from their dinosaur reptilian ancestors because they have feathers, wings and can fly.  All of the adaptations for flight that accompany these morphological and behavioral differences from reptiles place them in a separate taxonomic category.  They have diverged so far from their ancestor that they are now their own class of organisms.   

The cladistic approach is more objective and based on testable hypotheses, so believed to be more "scientific".  It is believed to reflect evolutionary history better than the classical approach. 

The classical approach is based on our decisions regarding the importance of degrees of divergence. 

Arguments about which is "best" are vigorous among scientists although the cladistic approach seems to be winning at the moment. 

I use the classical version because I am old-fashioned, because you are familiar with it and because your text keeps that system.

Characteristics of vertebrates The chief diagnostic features of the chordates (notochord, pharyngeal gill slits, dorsal hollow nerve cord and post anal tail) are all present at some stage of the life cycle.

Body is covered by skin. = organ of outer ectodermal and inner mesodermal origin. It usually develops protective structures such as scales, feathers, hair, claws and horns.

Characteristics of vertebrates The primitive notochord is replaced in higher

vertebrates by the vertebral column of cartilage, bone or both.

Provides increased flexibility and more efficient support.

In the head, the notochord is replaced by the skull which encloses the brain.

Characteristics of vertebrates Usually provided with two pairs of

appendages, anterior and posterior, in the form of fins in fishes, limbs in terrestrial vertebrates. Both can be lost and greatly modified in adapting to specific habits (e.g. wings in birds and bats and limbless snakes). Probably first vertebrates lacked limbs. Extinct ostracoderms (earliest known jawless fish) had none.

Characteristics of vertebrates Usually provided with two pairs of

appendages, anterior and posterior, in the form of fins in fishes, limbs in terrestrial vertebrates.

Both can be lost and greatly modified in adapting to specific habits (e.g. wings in birds and bats and limbless snakes). Probably first vertebrates lacked limbs. Extinct ostracoderms (earliest known jawless fish) had none.

Characteristics of vertebrates Muscular system. Fundamentally = paired series of

muscle blocks or myotomes along the body axis. Obvious in lower vertebrates and the embryos of higher ones, but obscured in adults of higher vertebrates by spreading of the limb musculature.

The segmental condition of the musculature imposes a segmental structure upon some other systems, especially the vertebral column and nervous, circulatory and urogenital systems. Have to study embryology to see this.

Clearly analogous, not homologous, to segmentation of arthropods and annelids.

Characteristics of vertebrates Nervous system follows basic plan. Highly cephalized. They have a well

developed brain along with the three pairs of sense organs characteristic of the vertebrate head - the olfactory bulbs, the eyes (peculiar because their nervous tissue is part of the brain wall) and the ears (which combine the fns. of equilibration and hearing).

Thick-walled tube (= spinal cord) in trunk with paired segmental nerves coming from it.

Characteristics of vertebrates Coelom. In lower vertebrates coelom is

divided into pericardial cavity containing the heart and pleuro-visceral cavity with other viscera.

In mammals the pleuro-visceral cavity is subdivided by the diaphragm into pleural cavities containing the lungs and an abdominal cavity for the remaining organs.

Characteristics of vertebrates (digestive system) Distinctive feature is the pharynx. In lower vertebrates and the embryos of higher ones

find pharyngeal gill slits. There are gills in the walls of the slits in lower aquatic vertebrates.

Gill arches (= skeletal components) which are found in tissue between gill slits originally served to anchor respiratory muscles. First vertebrates lacked jaws, but early in vertebrate history the first pair of gill arches became modified to form the jaws. Remaining gill arches persist in terrestrial vertebrates, but extremely modified and reduced. Contribute to the cartilages of the larynx and to the middle ear bones.

Characteristics of vertebrates (digestive system) Large digestive glands = liver and pancreas. In most vertebrates digestive tract terminates

in a cloaca. Anus and excretory ducts inter into it.

Characteristics of vertebrates (respiratory system) Gills in lower aquatic vertebrates. Swim bladders of fish give rise to lungs of

higher terrestrial ones, including those secondarily aquatic such as whales and seals.

Characteristics of vertebrates (circulatory system) Large ventral heart with 2 to 4 chambers Curved aortic arches connect ventral and

dorsal aortas and pass from heart between gill slits to, gills. Associated with aquatic resp. Modified in air breathers into pulmonary and systemic systems

Closed system of arteries, capillaries and veins.

Characteristics of vertebrates (circulatory system) Main dorsal artery (aorta) runs body length

and 2 portal circulations (capillary systems) are interposed in the venous system, one in the kidneys, the other in the liver.

Blood contains red cells with Hb. and white cells, etc.

Characteristics of vertebrates (urogenital system) Close assoc. of two. Genital ducts derived

from kidney ducts. Pair of kidneys composed of urine producing

tubules, originally segmentally arranged. Drain to cloaca or anal region.

Almost always dioecious. In aquatic forms eggs laid in water. Better protection of embryo necessary on land. Egg enclosed in shell in reptiles and birds and internal incubation in most mammals.