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With over 50,000 described species,  the Phylum Chordata ("back cord") is  one of the largest animal groups on earth.  Furthermore,  many of the chordate species are those that are most conspicuous and familiar to us.  All chordates possess (at least at some time in their development) the following unique characteristics...

(1) A notochord,  a stiff rod-like structure that extends the length of the body.  The notochord provides a rigid axis for muscle attachment and is the first part of the endoskeleton to appear in the embryo.  In most vertebrates, a series of cartilaginous or bony vertebrae replace the notochord as the primary support structure.

(2) A dorsal, tubular nerve cord.  In the invertebrate phyla, the nerve cord (often paired) is ventral to the alimentary canal and is solid.  In chordates, a single nerve cord is found dorsal to the alimentary canal.  Although the hollow center may be obliterated during growth, it is always hollow at some point.  The anterior end of the nerve cord is enlarged to form a brain.




Introduction (Part 2)


(3) Pharyngeal gill slits.  These consist of perforated, slit-like openings that lead from the pharyngeal cavity to the outside.  The slits have a supporting framework of gill bars, which in aquatic vertebrates become the gills.  Note: The possession of pharyngeal gill slits is not actually a unique chordate character.  Pharyngeal gill slits are also found in some hemichordates (acorn worms), a closely related sister group previously considered to be a subphylum within the Phylum Chordata.

(4) A postanal tail.  As a structure added to the body behind the end of the digestive tract, the tail has evolved specifically for propulsion in water.

The Origin of the Chordates.  Despite a great deal of work, it has been impossible to locate the direct ancestor of modern chordates since the earliest forms were most likely soft-bodied creatures that stood little chance of being preserved as fossils.  Almost certainly, however,  the ancestor of the chordates was a deuterostome, possibly an echinoderm or something that gave rise to both groups.

  The taxonomy of the chordates


Depending on the system of classification used, the chordates may be placed into three subphyla, the first two of which are sometimes called the "protochordates" because they share a number of primitive features in common, including the lack of an internal skeleton.  The third subphylum (the vertebrates) contains species, most of which have a spinal column made of interlocking vertebrae. 

   Subphylum Tunicata (Urochordata):  sea squirts, salps, larvaceans

   Subphylum Cephalochordata: lancelets

   Subphylum Vertebrata: vertebrates

       Class Myxini: hagfishes

       Class Cephalaspidomorphi: lampreys

       Class Chondrichthyes: cartilaginous fishes (sharks, skates, rays)

       Class Osteichthyes: bony fishes

       Class Amphibia: amphibians (frogs, toads, salamanders, caecilians)

       Class Reptilia: reptiles (snakes, lizards, turtles, crocodilians, etc.)

       Class Aves: birds

       Class Mammalia: mammals

Sea squirts (Subphylum Tunicata)


The Subphylum Tunicata (Urochordata) contains about  1,600 species of marine animals called tunicates, or sea squirts (plus some other strange forms).  The name tunicate comes from the cellulose-containing tunic that surrounds the animal.  In terms of form and function, tunicates have either spherical or cylindrical bodies that are attached to the substrate by a base or stalk.  On the outside are two projections, an incurrent siphon that brings water into a pharyngeal chamber and an excurrent siphon through which the water is expelled.  Feeding depends on the formation of a mucous net that traps plankton which are transported to the stomach for digestion.  Although adult sessile tunicates lack most of the chordate characteristics, the tunicate larva (sometimes called "tadpole larva") reveals its chordate heritage with its well-developed notochord, propulsive tail, dorsal-tubular nerve cord plus a brain, balancing organ and an eye, complete with lens!

Subphylum Cephalochordata




The Subphylum Cephalochordata contains about 30 species of animals called lancelets.  Although they can swim fairly well, most animals are found with their tails buried in the sand with only the anterior end protruding.  Like the tunicates, lancelets are filter feeders.  They take in food and water through an oral hood whose edges bear delicate projections called cirri that act as a strainer to exclude larger objects.  Inside of the oral hood are bands of cilia that sweep food into the mouth. Cilia on the pharyngeal gill slits drive a current of water through the pharynx, which then enters a chamber called an atrium to exit out an opening called the atriopore. Although a small and relatively obscure group by some standards, current evidence suggests that the cephalochordates were most likely the direct ancestors of the vertebrates, which certainly makes them worthwhile subjects for study!

Hagfishes (Class Myxini)


Hagfishes are an entirely marine group of jawless, scavengers that feed on dead or dying fishes, annelids, molluscs and crustaceans.  Although almost completely blind, the hagfish quickly finds food with it keenly developed sense of smell and touch (note the sensory barbules that surround the mouth).  Using their rasping tongues, they burrow into carcasses and feed from within.

Lampreys (Cephalaspidomorphi)


Lampreys are jawless vertebrates that feed on other fish by attaching to them with a suctorial mouth, rasping a hole in the body wall and sucking out the contents. The  Atlantic sea lamprey is a species particularly despised by those that fish the Great Lakes.  The construction of the Welland Canal around Niagara Falls in 1829 provided the sea lamprey with access to Lake Erie and from there, Lakes Huron, Michigan and Superior.  In some lakes, virtually every lake trout, whitefish, salmon, carp or other game fish has at least one lamprey on it or the scar of a past encounter with this ectoparasite.  Efforts to control lamprey populations by killing their ammocoetes larvae in the headwaters of streams and rivers where the adults breed have reduced Great Lake populations considerably. 

Sharks (Class Chondrichthyes


The Class Chondrichthyes contains over 700 species of sharks, skates, rays and a primitive form called a ratfish.  Although they are mostly marine, a few species have secondarily invaded freshwater habitats.  Of these animals, it is the sharks that have captured the greatest amount of public attention.  Indeed, a number of aspects of their anatomy and physiology have  contributed to their widespread success as some of the oceanīs most formidable predators.  Moveable upper and lower jaws studded with sharp,  recurved teeth can open wide to firmly grasp prey, which can be located with a variety of sophisticated sense organs including those that detect chemical odors, water currents, bioelectric fields emitted by their prey and even the earthīs magnetic fields.  Interestingly, the whale shark (which is the largest fish in the sea with a length that can reach 10 meters) feeds on some of the smallest prey (plankton) that is filtered from the water column with its gills!  Although many people who enter the water fear sharks, attacks are relatively rare and almost always represent a case of mistaken identity in which a swimmer is mistaken for more normal prey (marine mammals or large fish).  Indeed, it is usually humans that eat (or just kill) sharks, and many species are now threatened or endangered

Skates & rays (C. Chondrichthyes)


In addition to sharks, the Class Chondrichthyes also contains a number of skates and rays.  In these animals, the pectoral fins are greatly enlarged to form "wings" that undulate gracefully during swimming, and the tail fin is reduced or absent.   Skates and rays differ from sharks in having few scales and being generally adapted for feeding on bottom-dwelling animals such as molluscs and crustaceans.  Instead of sharp, recurved teeth, skates and rays have flat, plate-like teeth that are used for crushing prey.  Their bodies are flattened dorsoventrally,  enabling them to glide slowly over the bottom in search of prey.  The eyes and spiracles (openings for taking in water) of skates and rays are located on top of the head, allowing them to take in water for gill ventilation while being partially buried in sand.  Another difference between sharks and skates and rays is the fact that the former are normally viviparous (live bearers) whereas skates and rays are oviparous, releasing their eggs in rectangular cases sometimes called "mermaidīs purses".  Although a few electric rays are capable of delivering a powerful electric shock and some sting rays have large, barbed stingers, in general,

The coelacanth (C. Osteichthyes)


The bony fishes (Class Osteichthyes) contain more species than all other vertebrate groups combined.  They show a fantastic adaptive radiation into almost every conceivable aquatic niche and come in a variety of sizes, shapes, colors, etc.  Among the most primitive forms are those found in the Subclass Crossopterygii, which contains some of the lobe-finned fishes, a name that refers the fact that their fins are supported by fleshy lobes (rather than by the bony rays seen in modern fishes). Until 1938, this group (which is well represented in the fossil record) was thought to have been extinct for over 60 million years.  At that time, a living specimen was dredged up by fisherman from the depths of the ocean off the coast of South Africa.  Scientists had to wait until 1952 before another specimen was found, and even today, very little is known about the fish known as the coelacanth.  More recently, a few specimens (of what is apparently a different species) have been discovered in the seas off the island nation of Indonesia.   With sturdy skeletons of the basic tetrapod (four legged) type and fleshy lobe-fins that can be used like legs to crawl across the bottom,  a lobe-finned fish (perhaps from the group containing the coelacanth or from another group) almost certainly gave rise to the limbed vertebrates that roam the land today!

Bony fishes (Class Osteichthyes)


In their evolution from the early lobe-finned fishes to modern ray-finned fishes, the bony fishes have passed through several stages making the following changes:

(1) A completely ossified internal skeleton with a stronger cranium.

(2) The transformation of the air-breathing lungs into a buoyancy-compensating device called a swim bladder.

(3) The transformation of the asymmetrical heterocercal tail (seen in sharks and some primitive bony fishes) into the symmetrical and more efficient homocercal tail.

(4) The development of thin cycloid scales and ctenoid scales from the thicker ganoid scales.

(5) The loss of spiracles, development of stout spines in the fins and more efficient jaws.

All of these developments culminate in the appearance of the teleosts, a Superorder which contains over 21,000 species representing about 96% of all living fishes.

The amphibians (Class Amphibia)


 Salmanders and newts


 Frogs and toads


The Class Amphibia ("double life") contains over 4,000 species of animals that are somewhat transitional between fish and reptiles.  The amphibians make their first appearance almost 400 million years ago, and the surviving species represent a small fraction of the total number of amphibians that once existed on earth.  General characteristics of amphibians include a bony skeleton and usually four limbs. The skin of amphibians is moist and thin with no scales.  Since it is too thin to provide much protection against dehydration or predators, they must employ other means of defense.  For example,  amphibian skin contains many mucous glands that keep them moist and make them slippery, which helps them escape from predators.  The skin of all amphibians also contain poison (serous) glands that produce toxins that range from mildly noxious to deadly poison (some are among the most potent toxins produced by any vertebrates).   In keeping with their toxic nature, many amphibians are brightly marked with aposematic (warning) colors that advertise their toxicity.  These colors are due to pigment-containing cells call chromatophores that come in several varieties.  Living amphibians are placed in three orders, each of which has two commonly-used names...                                                                               

     Order Urodela (Caudata): salamander, newts, mudpuppies, etc.  

     Order Gymnophiona (Apoda): caecilians

     Order Anura (Salientia): frogs and toads

Salamander and newts (Order Urodela)


Salamanders and newts are tailed amphibians with short, stubby legs (some reduced or absent). More than 400 species are known, most of which are small and found in temperate regions.  Fertilization in salamanders is usually internal.  It occurs when a female picks up a packet of sperm called a spermatophore that is deposited by a male on a leaf, stick, etc. and inserts it into her cloaca.  Some species have direct development where the young hatch as miniature adults instead of going through a larval (tadpole)stage with metamorphosis.  All urodeles hatch with gills, but these are usually lost in all but aquatic forms or those that fail to metamorphose.  Lungs are the rule in adult forms, but some salamanders lack lungs or gills, using only their skin and mouth for gas exchange!

Caecilians (Order Gymnophiona)


The caecilians include about 165 limbless amphibians that inhabit tropical forests of South America, Africa and Southeast Asia.  Because they are entirely aquatic or burrowing organisms, they are seldom seen by humans.  Their food consists mostly of earthworms and other small invertebrates. Fertilization in caecilians is internal, and the male is equipped with a protrusible copulatory organ.  Viviparity (live birth) is also seen in some caecilians, with the embryos obtaining nourishment by eating the wall of the oviduct of the mother!

Frogs and toads (Order Anura)


Over 4,000 species of frogs and toads exist, ranging in size from 1 cm to over 30 cm!  Anurans lack tails and usually have enlarged hind feet that are used for jumping, swimming and climbing.  The typical pattern of reproduction in temperate forms begins when a male clasps a female in a process called amplexus and sheds sperm over the eggs as they are being extruded from the female into the water. The fertilized eggs hatch into larval tadpoles that later metamorphose into adults.  Although most anurans are normally solitary, they often come together in large breeding aggregations during the spring or  rainy season.  Males of many species produce distinctive, loud breeding calls that attract females of the same species.  In addition to their contributions to scientific research (particularly studies of developmental biology and medicine), anurans serve as important food source for many vertebrates (including humans) and as predators on insects.  Unfortunately, frogs and toads (amphibians in general) may be especially sensitive to environmental degradation, and many populations have declined, gone extinct, or are suffering from an increase in the incidence of developmental abnormalities.

The reptiles (Class Reptilia)


 Turtles and tortoises

 Alligators and crocodiles





The reptiles (Class Reptilia  make their appearance in the fossil record around 340 million years ago during the Paleozoic Era.  By the beginning of the Mesozoic Era (about 250 million years ago) the ruling reptiles had radiated into just about every conceivable niche (terrestrial, marine, aquatic, flying, etc.) ushering in the "Age of Reptiles".  Certainly the most spectacular forms were the dinosaurs ("terrible lizards") that ruled the land for over 100 million years and then disappeared abruptly around 65 million years ago.  No one is really sure why, but the most recent theory implicates an asteroid that struck the earth, causing so much smoke and dust, that it cooled the earth and interfered with primary productivity of plants.  In general, modern reptiles show a number of developments that make them better suited to life on land than the amphibians.  These include a waterproof skin, durable scales, a stronger skeleton, more efficient circulatory, respiratory and nervous systems and the ability to get rid of their nitrogenous wastes in a manner that requires the loss of very little water.  In addition, unlike amphibians that are always tied to water for their reproduction, reptiles engage in internal fertilization and produce eggs that can be deposited on land (some species are even capable of giving birth to live young). Modern reptiles are placed in the following four orders...

     Order Chelonia (Testudines): turtles, tortoises, terrapins    

     Order Crocodilia: alligators, crocodiles, caimans, etc.   

     Order Squamata: lizards and snakes                     

     Order Sphenodonta (Rhynchocephalia): tuataras    

Turtles and tortoises (Order Chelonia)


Turtles and tortoises are easily recognizable reptiles that show some unique features among vertebrates.  All species are encased in a pair of bony shells, a dorsal carapace and a ventral plastron (which is hinged in box turtles).  The shells (which are covered with a horny layer of keratin) are formed from sections of dermal bones that are fused to each other and to the ribs and thoracic vertebrae.  This arrangement presents some problems in terms of respiration since turtles cannot expand their chests to breathe.  They solve this problem by using certain abdominal and pectoral muscles as a "diaphragm" to change the size of the body cavity, which causes the lungs to inflate and deflate. While submerged, many freshwater turtles use pharyngeal (buccal) respiration by pumping water in and out of their highly vascularized mouth cavities.  Some species even have a pair of accessory air bladders connected to the cloaca that allow them to breathe while submerged, which is called cloacal respiration.  Although all turtles are oviparous and bury their eggs in the soil, in some species, the temperature at which the eggs are incubated determines the sex of the offspring (cooler temperatures produce males).

Alligators & crocodiles (Order Crocodilia)


The modern crocodilians (a small remnant of a once large and successful group) are the only living members of the archosaurian line that gave rise to the dinosaurs and birds. All crocodilians have well-reinforced skulls with massive jaw musculature that provides a wide gape and rapid, powerful closure over prey. Their many sharp teeth, which are set in sockets (thecodont), are designed for gasping but not for chewing.  A muscular gizzard (like that of birds) finishes the job of grinding up food with the help of swallowed stones, etc. Crocodilians (like mammals) have a complete secondary palate, which permits them to breathe when the mouth is full of water or food, as well as while completely submerged except for the nostrils. All species are oviparous and lay eggs in nests (which they guard) constructed of dead vegetation.  Like some turtles, crocodilians show temperature dependent sex determination in which the incubation temperature of the nest determines the sex of the offspring.  In crocodilians, however, high nest temperatures produce males (the reverse of the pattern seen in turtles)!   Although they have managed to survive almost unchanged for nearly 200 millions years, many of the 22 species of alligators, caimans and crocodiles found in tropical regions of the world are now facing the threat of extinction as a result of human activities.

Lizards (Order Squamata)


The more than 3,000 species of lizards belong to the most successful group of reptiles, the squamates.  Part of this success is certainly due to their highly mobile jaws that can be used to seize and swallow large prey.  Although many forms are found in the tropics, some species range well into temperate regions as well as the driest deserts of the world where their waterproof skin and efficient excretory physiology are well suited to water conservation.  Most lizards feed on insects and other arthropods, but the larger ones (such as iguanas) feed on plant material.  The largest lizards (the monitor lizards that can reach a length of up to 3 m and weigh up to 250 kg) can even feed on pigs, deer and other mammals. Fertilization in lizards is internal, with the male being equipped with two hemipenes (instead of one, as in crocodilians and turtles).  While many lizards are oviparous, depositing their eggs in the soil, others are live-bearing.  Only two species of lizards are venomous, the gila monster and Mexican beaded lizard, which are found in the deserts of Arizona and northern Mexico.  The venom in these lizards is not injected but rather flows from poison glands into the wound.  They offer no real threat to humans (unless messed with!) and are protected in the United States

Snakes (Order Squamata)


Like lizards, the more than 2,500 species of snakes owe much of their success to their highly mobile jaws that can be used to seize and swallow large prey (often larger than the snake itself).  This is possible because of the loose articulation of many of the skull bones and their ability to unhinge their jaws (the mandibles are not rigidly fused as they are in humans, but rather joined by muscles and skin).  Although it may take quite a while, powerful digestive enzymes eventually break down most of the materials in the swallowed prey.  For this reason (and their low metabolic demands), snakes may go for months without feeding on a large prey item.  Some snakes simply grab and swallow small prey, but larger prey must first be killed before being swallowed.  Venomous species (like rattlesnakes, copper heads, water moccasins and coral snakes) use venom produced in modified salivary glands for capturing prey and for defense.  Some snakes are constrictors (boas, pythons, milk snakes, anacondas, etc.) that kill by wrapping coils of the body around the chest cavity of the prey and preventing it from breathing.  In spite of the fact  that rarely do snakes pose any real threat to people, they have been unjustly persecuted and deserve to be protected,  not only for the beneficial role they play in the control of rodent populations but also for the contribution they make to the Earthīs biodiversity and biocomplexity.   It is against the law in most states to kill nonvenomous species and sometimes even venomous ones!  

The birds (Class Aves)


The birds (Class Aves)  make their appearance in the fossil record during the Jurassic period about 150 million years ago, having most likely evolved from a small, bipedal dinosaur.   With over 9,000 species in 27 Orders (more than any other vertebrate class except for the bony fish), birds are found in just about every habitat on earth.  In terms of their form and function, almost every characteristic of birds reflects their special needs of flight.  For example, the skeleton is strong but light weight, and the sternum (breast bone) is large and keeled in most forms, providing a large surface area for the attachment of the powerful flight muscles.  Birds are covered with feathers, which not only permit flight but also insulate, allowing them to maintain high, constant body temperatures.  The respiratory system of birds is the most efficient of any vertebrate, reflecting the high metabolic demands of flight.  The nervous system of birds is more sophisticated than that seen in reptiles,  with a large brain with well developed cerebral hemispheres, cerebellum and optic lobes, and keen sense organs, particularly the eyes, which have the greatest visual acuity (resolution) of any vertebrates.  

The mammals (Class Mammalia)


The Class Mammalia contains over 4,000 species that range in size from tiny bats and shrews weighing a few grams to giant whales weighing over 100 tons!  After the disappearance of the dinosaurs at the end of the Cretaceous period (around 65 million years ago), the mammals undergo a tremendous adaptive radiation filling practically all of the vacated niches.  The Cenozoic Era (the most recent one) is often referred to as the "Age of Mammals", reaching its greatest diversity and abundance from 30 to 55 million years ago. The most distinctive features of mammals are: a covering of hair (in most species) that provides insulation and protection from the elements, mammary glands that provide milk for nourishing the young, live birth with an intimate connection called a placenta between the mother and fetus (in most  but not all species),  a new jaw articulation (as well as other unique skeletal features) and an expanded brain with well developed cerebral hemispheres that are joined by a band of tissue called the corpus callosum.

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