The Chordates: Putting a Backbone Into Spineless Animals Note: These links do not work. Use the links within the outline to access the mages in the popup windows. This text is the same as the scrolling text in the popup windows.. What is a chordate animal? (Page 1) Phylum Chordata: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/phylum_chordata.html The chordate animals form the second major group of the deuterostome line. In this topic you will learn about the nvertebrate chordates and receive a brief introduction to the vertebrates. Characteristics: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/characteristics.html This diagram of a generalized chordate animal illustrates the four chordate characteristics. These are the notochord, dorsal, hollow nerve cord, pharyngeal slits, and a post-anal tail. Note that the nerve cord is expanded to form a brain at the anterior end of the animal, and that the pharynx is the first part of the digestive tract. The tail is defined as post anal because it extends beyond the anus. Notochord: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/notochord.html The notochord is a skeletal structure located above the gut. It is stiff enough to provide support for the body, yet lexible enough to bend. The notochord is composed of large, fluid-filled cells that bulge due to internal water pressure. They are held in place by two sheaths of tissue. Thus, the stiffness of the notochord results from hydrostatic pressure, not from the presence of hard minerals as are found in shells or bone. During Development: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/during_development.html You have recently learned the basics of early development in deuterostome embryos. The process of gastrulation is hown in this diagram. The region colored green is the developing notochord. It begins to form during gastrulation and is in place on the dorsal side of the embryo by the end of the gastrulation process. Note that at this point, before a coelom has formed, the notochord is within the roof of the developing gut. It will detach from the gut tube during coelom formation. Swimming Motions: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/swimming_motions.html The function of the notochord is to stiffen the body as the animal swims. Muscles along the sides of the body contract alternately on right and left sides, resulting in a sinuous body motion. Forward locomotion is much faster and more efficient than it would be in a flabby body, lacking a notochord. The ability to swim efficiently is probably one reason for the notable success of Chordate animals as early as the Cambrian period. Nerve Cord: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/nerve_cord.html n this cross section through the body of a simple, chordate animal, the location of the notochord and nerve cord are clearly visible. The nerve cord lies just above the notochord on the dorsal side of the body. Annelids and Arthropods: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/annelids_and_arthropods.html
These illustrations should serve as a reminder that the nerve cords of annelids and arthropods are located along the ventral side of the body. Also, these nerve cords are solid, whereas the nerve cord of chordate animals has a hollow center.
Soft-bodied Animals: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/softbodied_animals.html Water Flow: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/water_flow.html This diagram illustrates water flow through the pharynx of a simple chordate animal. Water enters through the mouth and exits from slits within both sides of the pharynx. Without slits, water would continue through the digestive tract and emerge from the anus. I. What are the different kinds of chordates? (Page 2-4) Phylum Chordata: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_bigimages/phylum_chordata.html This chart is very helpful in reviewing how the deuterostome line fits into the large division of bilaterally ymmetrical animals. It also illustrates the relationship between the echinoderms and chordates. Phylum Chordata contains all of the groups beneath the Chordates heading. The urochordates and cephalochordates are two ubphyla of chordate animals. The vertebrates are the third subphylum, consisting of the jawless lampreys and the awed vertebrates (fish, amphibians, reptiles, birds and mammals). We will return to this chart later to clarify the mportant changes that occurred during vertebrate evolution. Tunicate Larvae: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/tunicate_larvae.html Here we see a living tunicate larva and a diagram that illustrates the major chordate characteristics. Note that the notochord stiffens the tail, but does not extend into the head region. Water flows into the pharynx through the mouth, then exits through slits into an excurrent siphon that expels water from the body. Metamorphosis: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/metamorphosis.html Most tunicates are sessile as adults. When ready to change to the adult body form, the free-swimming larval stage attaches to a solid object by means of adhesive glands at the tip of its head. During metamorphosis, the notochord degenerates and the nerve cord is reduced to a few small ganglia. At the same time, the pharynx grows until it occupies most of the body cavity. The soft body is supported by a thick covering called a tunic, from which the animal s common name is derived. The structure of the tunic is unusual in that it contains cellulose, a substance ound in plant cell walls, but rarely in animal tissues. Pharynx: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/pharynx.html The large pharynx,with its many small slits, fills most of the space within the tunicate s body as can be seen in the pecimen on the left. Water flowing through the pharynx carries tiny food particles into the pharyngeal cavity. Water then passes through the pharyngeal slits and exits via an excurrent siphon. Food particles are too large to pass hrough to slits, so are trapped within the pharynx and pass through the gut where digestion and absorption occur. Undigested particles are expelled through the anus into the excurrent siphon and leave the body in the outgoing water current. Thus the pharynx of tunicates provides a sophisticated method of filter-feeding. Sea Squirts: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/sea_squirts.html Most species of tunicates are sea squirts. These, small sac-like animals are common in coastal waters and often wash up on the beach. The two siphons (for water inflow and outflow) are clearly visible in this sea squirt. If you were to pick up the animal, its body would strongly contract, squirting sea water from both siphons. Hence the name sea squirt.
These sea squirts appear fragile and defenseless, but few predators eat them. Presumably they contain foul-tasting chemicals and some species are poisonous. In fact, it is said that the wife of Julius Caesar eliminated some of his enemies by feeding them poisonous sea squirts! Hermaphroditic: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/hermaphroditic.html This diagram shows the location of gonads within the sea squirt s body. Both eggs and sperm are shed from long ubes that enter the excurrent siphon. Although sea squirts are hermaphroditic, eggs are usually fertilized by sperm rom another individual that is spawning nearby. Colonies of Sea Squirts: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/colonies.html Many species of sea squirt form colonies. Here are several examples. Excurrent Siphon: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/excurrent_siphon.html n this sea squirt colony, the individual bodies are arranged around a central excurrent siphon that is shared by all of he sea squirts (seven in this colony). Each sea squirt retains its incurrent siphon, which serves as a mouth to bring water into the pharynx. Nuisance: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/nuisance.html These colonial sea squirts have created a massive growth on an underwater rope. Salps: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/salps.html This salp was photographed in Monterey Bay. It is one of the larger species, reaching a length of 30 cm. Like the ea squirts, it filter-feeds by pumping water through a large pharynx. Salp populations are quite dense in some parts of the ocean. The fecal pellets that they produce are rich in the nutrients needed by marine algae. Thus, salps play an important role in the marine ecosystem. Transparent Body: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/transparent_body.html The body wall of salps is composed of a clear, gelatinous material similar to that of jellyfish. Thus the body is ransparent, which may provide the salp s main line of defense. The dark structure seen within this salp is the gonad Chain-like Colonies: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/chainike.html Many salps form colonies. In some species the chain of salps can reach a length of 20 meters. The salps, in conjunction with other gelatinous animals such as jellyfish, siphonophores and comb jellies, form what is called a jelly web in deeper ocean waters. The significance of this web of life in the ecology of oceans has only recently been recognized and is not yet well understood. Larvaceans: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/larvaceans.html These small tunicates are literally larvae that lost the ability to undergo metamorphosis. At the same time, they developed mature gonads and thus were able to reproduce. So the adult larvacean lives its entire life in a larval body orm. Since they lack a large pharynx, the larvaceans have developed a different style of filter-feeding. They secrete hin strands of mucus to form what is called a house in which they live. An empty, football-sized house is shown
n this image. The larvacean beats its tail to generate a water current through the house. Food organisms stick to the mucus and are consumed. When the house becomes cluttered with inedible debris, as the one shown here, the arvacean leaves it and constructs a new house. These tunicates are difficult to see, since they are small as well as ransparent. Some marine biologists think that they may more abundant than realized and constitute one of the major planktonic animals of the sea. Head of a Cephalochordate: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/head.html n this diagram of a cephalochordate animal, the main chordate characteristics are well illustrated. Note that the notochord extends all the way to the tip of the head. This feature is not found in any other group of chordates. Lancelet: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/lancelet.html Lancelets are small animals, rarely exceeding 3 cm in length. Their stiff notochord and strong body muscles provide an efficient swimming locomotion, while the thin, dorsal fin confers stability in the water. n the Sand: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/in_the_sand.html Lancelets feed while buried in the sand, with only the tip of their head extending into the water. They pull water hrough the pharynx and extract tiny food organisms, similar to the feeding style of the tunicates. Also like the unicates, lancelets absorb oxygen from the water as it passes through the pharyngeal slits. Features Similar to Vertebrates: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/features.html n addition to the four characteristics common to all chordates, the lancelets have several structures similar to those ound in vertebrate animals. Both lancelets and vertebrates have segmental body wall muscles which are controlled by nerve roots from the nerve cord. Lancelets have an outgrowth of the digestive tract that resembles a rudimentary vertebrate liver. They also have a pulsating ventral blood vessel that may be the forerunner of the vertebrate heart, and their entire circulatory system is much like that of young vertebrate embryos. Thus the lancelet, often called amphioxus, is routinely used in biology classes to illustrate the essential features of vertebrate form and function. Backbone: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/backbone.html The vertebrate animals are defined as possessing a vertebral column, or backbone. This structure is composed of many small vertebrae that are connected by joints along the dorsal side of the body. Unlike the notochord, the vertebral column is hardened by minerals and composed of cartilage or bone. The initial function of the vertebral column was to protect the nerve cord by providing solid arches above it. In the single vertebra shown here, you can ee where the nerve cord was located in life. The bases of vertebrae replace the notochord during the embryonic development of land-dwelling vertebrates, thus providing a stronger support and a place to anchor the limbs. Vertebrates: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/vertebrates.htm Here you can see representatives of the 7 groups of living vertebrates. You are not required to memorize the class names, but you will need to know the common names of each group (such as cartilaginous fish). Embryo: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/embryos.html n vertebrate embryos, a prominent notochord is present. Likewise, there is a pharynx with slits, although the slits do not completely open in mammalian embryos. As development proceeds in land-dwelling vertebrates, the notochord is lost, and the pharyngeal region changes, giving rise to other structures as the slits close. In fish, the pharyngeal slits become part of the gill apparatus, whereas pieces of the notochord are retained and incorporated nto the vertebral column. In all vertebrates, the anterior end of the nerve cord becomes greatly expanded to from a
brain. First Vertebrates: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/first_vertebrates.html Here is a drawing based on fossils from 500 million years ago. This animal looks much like a lancelet, but had a brain and large eyes, in addition to a vertebral column. As far as we know, this is the first vertebrate animal. Ancestor: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/ancestor.html The likely ancestor of the first vertebrates is this ancient cephalochordate, shown both as the fossil imprint and an artist s reconstruction. Another drawing of the same cephalochordate was used earlier to illustrate the swimming motion of early chordate animals. Ostracoderms: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/ostracoderms.html These drawings are based on fossils of the first known fish, named Ostracoderms, that inhabited the oceans prior to he evolution of land-dwelling vertebrates. These fish were small, 3-10 cm long. Their bodies were covered with bony plates and usually flattened in the dorsal-ventral plane, suggesting that they spent most of their time on the sea bottom. They lacked jaws, and were probably filter-feeders like the cephalochordate ancestors. Midway through heir history, most Ostracoderm species moved into fresh water habitats. Before becoming extinct, they gave rise to other groups of fish that had jaws and paired appendages. Lamprey: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/lamprey.html Almost all modern fish have jaws. However two small groups, the hagfish and the lampreys are jawless and apparently evolved from a line of Ostracoderms that retained the primitive, jawless condition. Like their ancestors, hey also lack paired appendages. You will learn more about these jawless fish later in the course. Ancient Fish: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/ancient_fish.html Here is an artist s reconstruction of one of the early jawed fish. Hinged jaws allowed these fish to eat a much wider variety of food, including tough plant material and animal prey. The presence of paired fins gave them greater tability in the water and more ability to maneuver. Thus, these fish were stronger swimmers than the Ostracoderms, and many become predators of other aquatic animals. Evolution of the Vertebrates: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_bigimages/vertebrate_evolution.html This chart serves to bring together several concepts of vertebrate evolution, as well as indicating the relationship between groups of vertebrates. You do not need to know the names written in black at the top of the chart, but you hould remember the types of vertebrates that have jaws, four limbs, and embryos with an amnion. The orange boxes indicate points in evolutionary history were key structures appeared. You have already learned about the mportance of the vertebral column, jaws and paired appendages. As the course progresses, the other important events depicted here will be described.