Invertebrate Chordates, Parts of a Chordate

SECTION 2 SECTION 2 OBJECTIVES INVERTEBRATE Focus ● List the major characteristics of chordates. CHORDATES Overview ● Describe the evolution and classification of invertebrate Before beginning this section, chordates. The phylum Chordata (kawr-DAY-tuh) includes all of the review with students the ● Describe the structure of lancelets. vertebrates, or animals with backbones. It also includes two Objectives listed in the Student ● Describe the structure of tunicates. Edition. This section describes groups of invertebrates—animals that lack backbones. two chordate subphyla. One of VOCABULARY these subphyla, Cephalochordata, atriopore contains animals that are thought CHARACTERISTICS to resemble the ancestral chor- dates from which vertebrates All animals with a backbone are vertebrates, and they make up one evolved. The structural features of the subphyla in the phylum Chordata, whose members are that contribute to the organiza- called chordates. Chordates are so named because they have a tion of the chordate body are notochord, a stiff but flexible rod of cells that runs the length of the presented. body near the dorsal surface. Figure 38-10 illustrates the noto- chord. The stiffness of the notochord provides a resistance against www.scilinks.org which the body muscles can exert force when they contract. The Topic: Chordates flexibility of the notochord allows the body to bend from side to GENERAL Bellringer Keyword: HM60285 side as well as up and down. Put the Transparency, Lancelet Some kinds of chordates retain the notochord throughout their Interior, on the overhead. Have life. In most vertebrates, however, the notochord is present in students list the characteristics embryos but becomes greatly reduced when the vertebral col- not found on other animals they umn, or backbone, develops. In adult mammals, the notochord have studied. (notochord, dorsal nerve persists only as small patches of tissue between the bones of the cord, postanal tail, and pharyngeal vertebral column. pouches) LS Visual Recall that in addition to having a notochord, all chordates have the following three characteristics during some stage of their life: (1) a dorsal nerve cord, (2) pharyngeal pouches, and (3) a FIGURE 38-10 postanal tail. These characteristics are also illustrated in Figure Motivate All chordates have a notochord, a dorsal 38-10. Unlike the ventral nerve cords of invertebrates such as nerve cord, pharyngeal pouches, and a annelids and arthropods, the dorsal nerve cord of a chordate is a postanal tail during at least some stage Demonstration of their life. hollow tube. Lancelet Differences Have stu- dents look at the lancelets in Dorsal nerve cord Notochord Muscles Figure 38-11. Ask them how a lancelet is similar to a fish. (Both have streamlined bodies divided into muscular segments.) Ask them how a lancelet differs from a fish. (A lancelet lacks a head, eyes, a vertebral column, and fins.) LS Visual

Mouth Pharyngeal pouch Anus Postanal with slits tail

Chordates Have students 768 research chordates by using the Internet Connect box on this TEACHER RESOURCES page. LS Intrapersonal Workbooks Active Reading Guide (Section 2) Quick, Data, and Math Labs • Comparing the Structure of the Notochord and Nerve Cord Technology Visual Concepts CD-ROM • Characteristics of Invertebrate Chordates, Parts of a Chordate

768 CHAPTER 38 In vertebrates, the anterior end of the nerve cord enlarges dur- Quick Lab ing development to form the brain, and the posterior end forms the spinal cord. The brain receives information from a variety of com- Modeling Chordate Quick Lab Answers plex sensory organs, many of which are concentrated at the ante- Characteristics rior end of the body. Modeling Chordate The pharyngeal pouches are outpockets in the pharynx, the por- Materials several colors of clay, toothpicks, masking tape Characteristics tion of the digestive tract between the mouth and the esophagus. In aquatic chordates, the pharyngeal pouches have slits and Procedure Build clay models of Time Required 20 minutes a lancelet and an adult tunicate by evolved first into filter-feeding structures and later into gill cham- using different colors of clay for the Safety Tell students that the bers. In terrestrial chordates, the pouches evolved into a variety of structures shown in Figures 38-11 toothpicks are sharp and to structures, including the jaws and inner ear. and 38-12. Make flags using mask- be careful with them. The notochord or backbone extends into the postanal tail, and ing tape attached to toothpicks, and muscles in the tail can cause it to bend. The postanal tail provides use them to identify any of the four Procedural Tips Have students much of the propulsion in many aquatic chordates. Invertebrates major characteristics of chordates work in pairs. that are found in your models. in other phyla lack this form of propulsion, and the anus, if present, Answers to Analysis All four chordate is located at the end of the body. Analysis Which of the major characteristics of chordates are characteristics are present in the lancelet. found in the lancelet? the adult Adult tunicates only have pharyngeal tunicate? Which of the four pouches. Both lancelets and adult tuni- characteristics is shared by both? cates have pharyngeal pouches. Tunicates EVOLUTION AND Why is the tunicate classified as have all four chordate characteristics CLASSIFICATION a chordate despite the lack of all four chordate characteristics? during their larval stage. Like echinoderms, chordates are deuterostomes. This similarity provides evidence that echinoderms and chordates likely evolved READING from a common ancestor. The phylum Chordata is divided into SKILL three subphyla: Vertebrata, Cephalochordata (SEF-uh-loh-kawr-DAY-tuh), BUILDER and Urochordata (YOOR-uh-kawr-DAY-tuh). Members of the subphylum K-W-L Tell students to read Vertebrata, the vertebrates, constitute more than 95 percent of all over the lists that they made for chordate species. Members of the other two subphyla live only in Section 1. Have students list what the ocean. They are the closest living relatives of the early animals they have Learned. Conclude by from which all chordates evolved. asking students which questions Subphylum Cephalochordata are still unanswered and what The subphylum Cephalochordata contains about two dozen FIGURE 38-11 new questions they have. Branchiostoma species of blade-shaped animals known as lancelets. Figure 38-11 (a) The lancelet LS Verbal lanceolatum lives with most of its body shows that lancelets look much like the idealized chordate drawn buried in the sand. (b) Even as adults, in Figure 38-10. They retain their notochord, dorsal nerve cord, lancelets clearly show all four chordate GENERAL pharyngeal pouches, and postanal tail throughout their life. characteristics. Activity Tunicate Features Purchase some live tunicates from a biolog- Mouth ical supply company, and have Notochord the students examine them. Ask Dorsal Tentacles nerve cord the students if they can distin- Pharynx guish the incurrent siphon from Anus Tail with slits the excurrent siphon. (Look for the

Intestine direction of water currents near the siphons.) Ask them if they can see

Atriopore any similarities between these Segmented muscles (a) (b) animals and vertebrates. (No similarities should be visible.) Tell stu- 769 dents that despite their appear- ance, tunicates are members of the phylum Chordata, which MEDICINE TEACHER RESOURCES includes all vertebrates. Explain CONNECTION Workbooks that phylogenetic relationships Sea Squirt “Kidney Stones” Sea squirts are not always obvious from the have a structure called a renal sac that Datasheets for In-Text Labs GENERAL external appearance of adult ani- excretes wastes, as a human kidney does. • Modeling Chordate Characteristics mals; sometimes such relation- Also like the human kidney, the renal sac Technology ships can be revealed only when develops deposits of uric acid and calcium Transparencies development of the animals oxalate—the compounds that make up • H45 Lancelet Interior is studied. LS Visual kidney stones. Living inside the sea squirt • H46 Exploration of a Lancelet are symbiotic fungi that break down the uric acid in the “kidney stone” deposits. Visual Concepts CD-ROM • Anatomy of a Lancelet The sea squirt itself can’t break down its own stones, nor can a human kidney.

ECHINODERMS & INVERTEBRATE CHORDATES 769 Lancelets live in warm, shallow waters. They wriggle backward into the sand with a muscular tail. Only their anterior end pro- trudes from the sand. Lancelets have cilia that draw water into the Close pharynx through the mouth. Food particles in the water are trapped as the water passes through the slits in the pharynx. The Reteaching food enters the intestine to be digested. The water leaves the body Comparing Lancelets and through an opening called the atriopore (AY-tree-oh-POHR). Tunicates Have students make Lancelets can swim weakly, powered by the coordinated con- tables comparing lancelets and traction of muscles that run the length of their body. If you look (a) tunicates. Tables should include closely at Figure 38-11, you can see that these muscles are arranged as a series of repeating segments. Body segmentation is another chordate characteristics in larvae Incurrent siphon common feature of chordates. Recall that annelids and arthropods and adults, anatomical features, also have segmented bodies. However, animals in those phyla prob- Pharynx with slits and behavioral adaptations. ably evolved body segmentation independently of chordates. (Lancelets: show all four chordate Tunic Excurrent siphon characteristics; mouth, tentacles, intes- Subphylum Urochordata tine, atriopore, anus, segmented mus- The 2,000 species in the subphylum Urochordata are commonly cles; swim weakly, burrow into sand called tunicates because their bodies are covered by a tough cover- and leave mouth exposed for feeding. ing, or tunic. Tunicates are also called sea squirts because they squirt Anus Tunicates: show four chordate charac- out a stream of water when touched. As adults, most tunicates are sessile, barrel-shaped animals that live on the sea bottom. They may teristics in larvae, only pharyngeal be solitary or colonial. pouches in adult; incurrent siphon, Figure 38-12 shows that tunicates are adapted for filter feeding. excurrent siphon, tunic, anus, stomach, Stomach Propelled by the beating of cilia, water enters the body through an hermaphroditic; water enters incurrent (b) incurrent siphon, passes through slits in the pharynx, and exits siphon during filter feeding, solitary or through an excurrent siphon. Food that is filtered by the pharynx colonial) LS Verbal FIGURE 38-12 moves into the stomach. Undigested material leaves via the anus, Most adult tunicates, such as Polycarpa which empties into the excurrent siphon. aurata (a), are sessile filter feeders. A Tunicates are hermaphrodites. Sperm and eggs are released Quiz GENERAL drawing of a tunicate’s internal structure through the excurrent siphon into the surrounding water, where (b) shows its enlarged pharynx with slits, fertilization occurs. 1. Name two chordate subphyla. the only chordate characteristic retained (Cephalochordata, Urochordata) by adult tunicates. Adult tunicates bear little resemblance to the idealized chordate shown in Figure 38-10. Although they do have a pouchlike pharynx 2. List one representative from with slits, they have no notochord, dorsal nerve cord, or postanal each of the two subphyla in tail. Larval tunicates, however, possess all four chordate charac- Question 1. (lancelet: Branchiostoma teristics, but they lose most of them during metamorphosis. lanceolatum; tunicate: Polycarpa aurata) 3. How do lancelets obtain food? SECTION 2 REVIEW (They burrow backward into the 1. What are the four major characteristics of CRITICAL THINKING sand with their mouths exposed for chordates? 5. Making Comparisons Compare the function filter feeding.) 2. What is the function of pharyngeal pouches in of pharyngeal gill slits with that of gills in a aquatic chordates? crayfish. TEACHER RESOURCES 3. What evidence of body segmentation do 6. Inferring Relationships How is burrowing back- lancelets display? ward, with the head out, adaptive in lancelets? Technology 4. Which of the chordate characteristics do 7. Relating Concepts In tunicates, the anus emp- Transparencies tunicates retain as adults? ties into the excurrent siphon. What is the • H47 Adult Tunicate advantage of this anatomical arrangement? Visual Concepts CD-ROM • Anatomy of a Tunicate 770

Answers to Section Review 5. In adult lancelets and tunicates, pharyngeal slits 1. The four major characteristics are a notochord, a are involved in filter feeding. In crayfish, gills are dorsal nerve cord, pharyngeal pouches, and a involved in gas exchange. postanal tail. 6. Burrowing with the head out enables the lancelet 2. The pharyngeal pouches were first involved in to filter feed while protecting the rest of its body filter feeding, and then evolved into gill chambers. from predators. 3. Lancelets have segmented muscles that run the 7. Undigested wastes that are eliminated by the anus length of the body. are carried away from the body by the water that 4. Adult tunicates retain an enlarged pharynx leaves through the excurrent siphon. with slits.

770 CHAPTER 38