AP Biology Chapter 23 Exercise #17: Chordates: Urochordata & Cephalochordata Lab Guide Chordates show remarkable diversity. Most are vertebrates. All animals that belong to this phylum MUST, at some point in their life cycle, show the following characteristics: Classification Phylum Chordata 1. Notochord: slender rod of cartilage like tissue lying near the dorsal side and extending most of the length of the animal. (in most vertebrates, it s only found in embryos) 2. Pharyngeal pouches/slits: series of paired slits in the pharynx, serving as passageways for water to the gills. (in some vertebrates, it s only found in embryos) 3. Dorsal nerve cord: with/modified portions that are identified as a brain, form the central nervous system. Normally lies dorsal to the digestive tract. 4. Endostyle or thyroid gland: found in all chordates, but not other animals. Normally secretes mucous and traps small food particles (for early vertebrates). 5. Postanal tail: usually projects past the anus at some stage and serves as a means of propulsion in water. May or may not persist in the adult form. Subphylum Urochordata Tunicates, only larval forms have all chordate characteristics, almost all are sessile, without notochord and dorsal nerve cord, body enclosed by a tunicate Subphylum Cephalochordata Lancelet, Notochord and nerve cord persist throughout life, lance shaped. Subphylum Vertebrata Vertebrates, enlarged brain enclosed in cranium, nerve cord surrounded by bony or cartilaginous vertebrae, notochord in all embryonic stages and persists in adults of some fishes, typical structures include two pairs of appendages and body plan of head, trunk, and postanal tail. Class Petromyzontidae Lampreys Class Myxini Hagfishes Superclass Gnathostomata Jawed fishes, all tetrapods, jaws present, usually paired limbs, notochord persistent or replaced by vertebral centra Class Chondrichthyes Sharks, skates, rays, and chimaeras Class Actinopterygii and Sarcopterygii (formally class Osteichthyes) Bondy fish Class Amphibia Amphibians; frogs, toads, and salamanders Class Reptilia Snakes, lizards, turtles, crocodiles, and others Subclass Aves Birds Class Mammalia Mammals
Exercise 17A: Subphylum Urochordata Ciona, Ascidian (Sea Squirts) Urochordata are commonly called tunicates, and at other times called sea squirts. Most adults are sessile. Respiratory system: The branchial sac or pharynx is the largest internal structure and the space between it and the mantle is the atrium. The pharyngeal wall is perforated with many pharyngeal slits. The wall of the pharynx is very vascular. Circulatory system: The heart will be located near the posterior end on the right side. The system is open. Blood cells are numerous and colorful, but there are no respiratory pigments. Digestive system: At the junction of the mouth and pharynx is a circlet of tentacles that form a grid, which screens the incurrent water. Food is propelled to the esophagus, then stomach. The anus empties into the atrium near the excurrent siphon. Excretion: located near the intestine, but not visible. Reproduction: hermaphroditic (both male and female reproductive structures), single ovary and testis. Open into the atrium. Nervous System: primitive with a pigmented, eye spot and a statocysts. Procedure 1. Now, examine a preserved Ciona (Sea Squirt). Use fine scissors to slit the tunic longitudinally, beginning at the incurrent siphon and continuing the cut to the base of the pharynx. Be VERY careful to cut only the tunic and NOT the mantle beneath it. Slip the animal out of its tunic. Then return the animal to the bowl of water. Study with a dissecting microscope and identify the following: excurrent siphon, incurrent siphon, tunic, pharynx, endostyle, pharyngeal slits, mantle, heart, stomach, intestine, atrium, gonads (ovary/testis), stolons SKETCH
Analysis 1. Explain why they are called tunicates. At other times they are called sea squirts. Why? 2. They are often described as sessile. What does this mean? 3. Where do you commonly find tunicates? 4. Describe the path water takes after entering the pharyngeal slits. The wall of the pharynx is vascular. Why? 5. The circulatory system of a tunicate is open. What does this mean? 6. The blood flow in tunicates is interesting. Explain. 7. Tunicates DO NOT have any respiratory pigments in their blood, so what ability are they lacking when compared to other chordates? 8. The endostyle of these animals is very important to their intake of food. Explain. 9. Explain how tunicates reproduce. 10. Their nervous system is primitive. Explain the function of the eye spot and statocysts.
Exercise 17B: Subphylum Cephalochordata Amphioxus The little lancelet, also called Amphioxus, demonstrates the basic chordate structure. Anatomy The animal has a dorsal fin which will broaden in the tail region and become the ventral fin. At the anterior tip is the rostrum. These animals have an oral hood which is fringed by tentacles (also called buccal cirri). On the ventral surface is the atriopore, which is anterior to the ventral fin. It is the opening to the atrium, which is a large cavity surrounding the pharynx. The anus will open slightly to the left of the posterior end of the ventral fin. In mature specimens, block like gonads will lie in the atrium, anterior to the atriopore. They can be seen through the body wall. Segmentally arranged muscles called myotomes can be seen in cross sections. Beginning right above the oral hood is the notochord, which may help spread the hood open. The mouth is difficult to see, but is a very small opening right behind the cavity created by the oral hood. It leads to the pharynx. Within the cavity is several, finger like ciliated patches that compose the wheel organ. The large pharynx will narrow into a straight intestine extending to the anus. The sidewalls of the pharynx are composed of a series of gill bars, between which are pharyngeal slits. Just posterior to the pharynx is the hepatic cecum (liver) which extends forward along one side of the pharynx. Surrounding the pharynx is the atrium which extends to the atriopore. These animals are filter feeders. Oxygen Carbon Dioxide exchange occurs in the epithelium covering the gills bars. The notochord helps to provide skeletal support and a point of attachment for muscles. Above the notochord is the dorsal tubular nerve cord. It contains pigmented photoreceptor cells. Chemoreceptors are scattered over the body. Touch receptors are located over the entire body. These animals do not have a heart to move blood through their body. Nephridial tubules will be used to carry and remove waste. Procedure 2. Now, examine with low power, a stained whole mount and cross section of Amphioxius. Identify the following: dorsal fin, tail region (caudal fin), ventral fin, rostrum, myotomes, oral hood, pharynx, wheel organ, intestine, gill bars, pharyngeal slits, intestine, anus, hepatic cecum, atriopore, notochord, dorsal nerve cord, endostyle SKETCH Analysis 1. Where will you find Amphioxus and what is unique about the way it will present itself in its environment? 2. What is the function of the oral hood with tentacles (buccal cirri)?
3. What is the purpose of the wheel organ in Amphioxus? 4. Describe the path water will take in Amphioxus. 5. Explain the COMPLETE process of Amphioxus feeding behavior. 6. How does oxygen-carbon dioxide exchange occur in Amphioxus? 7. Amphioxus have photoreceptors and chemoreceptors. Explain their function. 8. Amphioxus do not have a heart. Explain how blood is moved through this animal. Make sure to mention all applicable anatomical parts while referencing gas exchange and nutrient absorption.