The Chordates: Putting a Backbone Into Spineless Animals Note: These links do not work. Use the links within the outline to access the images in the popup windows. This text is the same as the scrolling text in the popup windows. I. 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 invertebrate 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 flexible 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 shown 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 In 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. 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. II. 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 symmetrical 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 subphyla of chordate animals. The vertebrates are the third subphylum, consisting of the jawless lampreys and the jawed vertebrates (fish, amphibians, reptiles, birds and mammals). We will return to this chart later to clarify the important 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 found 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 specimen 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 through 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. Soft-bodied Animals: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/soft- bodied_animals.html 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 tubes that enter the excurrent siphon. Although sea squirts are hermaphroditic, eggs are usually fertilized by sperm from 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 In this sea squirt colony, the individual bodies are arranged around a central excurrent siphon that is shared by all of the 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 sea 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 transparent, 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/chain- like.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.