Vertebrate Life, 9th Edition PDF - Chapter 2

Figure 2–8 Diagrammatic view of the form and early evolution of the cranium of vertebrates. The ancestral condition (a) was to have a chondrocranium formed from the paired sensory capsules, one pair for each part of the tripartite brain, with the underlying support provided by paired anterior trabec...

Figure 2–8 Diagrammatic view of the form and early evolution of the cranium of vertebrates. The ancestral condition (a) was to have a chondrocranium formed from the paired sensory capsules, one pair for each part of the tripartite brain, with the underlying support provided by paired anterior trabeculae (at least in jawed vertebrates) and parachordals flanking the notochord posteriorly. The splanchnocranium was probably ancestrally made up of seven pairs of pharyngeal arches supporting six gill openings, without any anterior specializations. In the lamprey (b), the mandibular (second segment arch, but termed arch 1 because there is no arch in the first segment) pharyngeal arch becomes the velum and other supporting structures in the head, and the remainder of the splanchnocranium forms a complex branchial basket on the outside of the gills (possibly in association with the unique mode of tidal gill ventilation). Above the level of the lamprey, the chondrocranium and splanchnocranium are surrounded with a dermatocranium of dermal bone, as first seen in ostracoderms (c). In gnathostomes (d,e) the pharyngeal arches of the mandibular (second) and hyoid (third) head segments become modified to form the jaws and jaw supports. The dermatocranium is lost in chondrichthyans (d). In osteichthyans (e) the dermatocranium forms in a characteristic pattern, including a bony operculum covering the gills and aiding in ventilation in bony fishes. The basics of this pattern are still seen in us. of large, closely spaced cells packed with incompressible fluid-filled vacuoles wrapped in a complex fibrous sheath that is the site of attachment for segmental muscles and connective tissues. The notochord ends anteriorly just posterior to the pituitary gland and continues posteriorly to the tip of the fleshy portion of the tail. The original form of the notochord is lost in adult tetrapods, but portions remain as components of the intervertebral discs between the vertebrae. The axial muscles are composed of myomeres that are complexly folded in three dimensions so that each one extends anteriorly and posteriorly over several body segments (Figure 2–10 on page 37). Sequential muscle blocks overlap and produce undulation of the body when they contract. In amphioxus, myomeres have a simple V shape, whereas in vertebrates they have a W shape. The myomeres of jawed vertebrates are divided into epaxial (dorsal) and hypaxial (ventral) portions by a sheet of fibrous tissue called the horizontal septum. The segmental pattern of the axial muscles is clearly visible in fishes. It is easily seen in a piece of raw or cooked fish where the flesh flakes apart in zigzag blocks, each block representing a myomere. (This pattern is similar to the fabric pattern of interlocking V shapes known as herringbone, although “herring muscle” would be a more accurate description.) In tetrapods, the pattern is less obvious, but the segmental pattern can be observed on the six-pack stomach of body builders, where each ridge represents a segment of the rectus abdominis muscle (a hypaxial muscle of tetrapods). Locomotion Many small aquatic animals, especially lar- vae, move by using cilia to beat against the water. However, ciliary propulsion works only at very small body sizes. Adult chordates use the serial contraction of segmental muscle bands in the trunk and tail for locomo- tion, a feature that possibly first appeared as a startle response in larvae. The notochord stiffens the body so it bends from side to side as the muscles contract. (Without the notochord, contraction of these muscles would merely compress the body like an accordion.) Most fishes still use this basic type of locomotion. The paired fins of jawed fishes are generally used for steering, braking, and providing lift–not for propulsion except in some specialized fishes such as skates and rays that have winglike pectoral fins and in some derived bony fishes (teleosts) such as seahorses and coral reef fishes. Energy Acquisition and Support of Metabolism Food must be processed by the digestive system into molecules small enough to pass through the walls of the intestine, then transported by the circulatory system to the body tissues. Oxygen is required for this process; the respiratory and the circulatory systems are closely intertwined with those of the digestive system. Feeding and Digestion Feeding includes getting food into the mouth, mechanical processing (“chewing” in the broad sense—although today only mammals truly chew their food), and swallowing. Digestion includes the breakdown of complex compounds into small molecules that are absorbed across the wall of the gut and transported to the tissues. Vertebrate ancestors probably filtered small particles of food from the water, as amphioxus and larval lampreys still do. Most vertebrates are particulate feeders; that is, they take in their food as bite-sized pieces rather than as tiny particles. Vertebrates move the food through the gut by rhythmical muscular contractions (peristalsis), and digest it by secreting digestive enzymes produced by the liver and the pancreas Basic Vertebrate Structure 35 (a) Chondrocranium and splanchnocranium of a lamprey Nasal capsule Position of first gill pouch Arcualia (vertebral rudiment) Gill slit Optic capsule Otic capsule Cartilages around buccal funnel Notochord Lingual cartilage Pericardial cartilage Branchial arch Hypobranchial rod Horizontal bar (b) Chondrocranium and splanchnocranium of a shark Position of spiracle Otic capsule Hyomandibular Position of first gill pouch Pharyngobranchial Optic capsule Rostrum Nasal capsule Epibranchial Position of fifth gill pouch Ceratobranchial Palatoquadrate Mandibular arch (upper and lower jaw) Hyoid arch Mandibular cartilage Hypobranchial Basibranchial (c) Dermatocranium of a primitive generalized (basal) bony fish Dermal roof bones Orbit Dermal bones of the supracleithral and opercular series Naris Palatoquadrate cartilage Mandibular cartilage (articular) Figure 2–9 The crania of three vertebrates. (a) the chondrocranium and splanchnocranium of a lamprey compared to (b) the chondrocranium and splanchnocranium of a living cartilaginous vertebrate (a shark), and (c) the dermatocranium of a generalized bony fish. 36 CHAPTER 2 Dermal lower jaw bones Dermal gular bones Splanchnocranium Chondrocranium Dermatocranium Vertebrate Relationships and Basic Structure © 1994 Cengage Learning, Inc. Myomeres V-shaped (a) Amphioxus Myomeres W-shaped Lamprey (b) Horizontal septum Shark (dogfish) Myomeres more complexly folded (c) Epaxial muscle Red muscle Hypaxial muscle (d) Bony fish (perch) Figure 2–10 Chordate body muscles (myomeres). (a) amphioxus (nonvertebrate chordate), (b) lamprey (jawless vertebrate), and jawed vertebrates, (c) shark, and (d) bony fish. into the gut. The pancreas also secretes the hormones insulin and glucagon, which are involved in the regulation of glucose metabolism and blood-sugar levels. In the primitive vertebrate condition, there is no stomach, no division of the intestine into small and large portions, and no distinct rectum. The intestine empties to the cloaca, which is the shared exit for the urinary, reproductive, and digestive systems in all vertebrates except therian mammals. Respiration and Ventilation Ancestral chordates probably relied on oxygen absorption and carbon dioxide loss by diffusion across a thin skin (cutaneous respiration). This is the mode of respiration of amphioxus, which is small and sluggish. Cutaneous respiration is important for many vertebrates (especially modern amphibians), but the combination of large body size and high levels of activity make specialized gas-exchange structures essential for most vertebrates. Gills are effective in water, whereas lungs work better in air. Both gills and lungs have large surface areas that allow oxygen to diffuse from the surrounding medium (water or air) into the blood. Cardiovascular System Blood carries oxygen and nutrients through the vessels to the cells of the body, removes carbon dioxide and other metabolic waste products, and stabilizes the internal environment. Blood also carries hormones from their sites of release to their target tissues. Blood is a fluid tissue composed of liquid plasma, red blood cells (erythrocytes) that contain the ironrich protein hemoglobin, and several different types of white blood cells (leukocytes) that are part of the Basic Vertebrate Structure 37 Carotid arteries Mesenteric arteries Dorsal aorta Efferent gill arteries Heart Ventricle Sinus venosus HEAD Segmental arteries GUT TRUNK GILLS Ventral aorta LIVER Atrium Hepatic vein Common cardinal vein Anterior cardinal veins KIDNEY Hepatic portal vein Renal portal vein Posterior cardinal veins Oxygenated blood Deoxygenated blood Figure 2–11 Diagrammatic plan of vertebrate cardiovascular circuit. All vessels are paired on the left and right sides of the body except for the midline ventral aorta and dorsal aorta. Note that the cardinal veins actually run dorsally in the real animal, flanking the carotid arteries (anterior cardinals) or the dorsal aorta (posterior cardinals). immune system. Cells specialized to promote clotting of blood (called platelets or thrombocytes) are present in all vertebrates except mammals, in which they are replaced by noncellular platelets. Vertebrates have closed circulatory systems; that is, the arteries and veins are connected by capillaries. Arteries carry blood away from the heart, and veins return blood to the heart (Figure 2–11). Blood pressure is higher in the arterial system than in the venous system, and the walls of arteries have a layer of smooth muscle that is absent from veins. The following features are typical of vertebrate circulatory systems: Capillary beds. Interposed between the smallest arteries (arterioles) and the smallest veins (venules) are the capillaries, which are the sites of exchange between blood and tissues. Their walls are only one cell layer thick; so diffusion is rapid, and capillaries pass close to every cell. Collectively the capillaries provide an enormous surface area for the exchange of gases, nutrients, and waste products. Arteriovenous anastomoses connect some arterioles directly to venules, allowing blood to bypass a capillary bed, and normally only a fraction of the capillaries in a tissue have blood flowing through them. Portal vessels. Blood vessels that lie between two capillary beds are called portal vessels. The hepatic 38 CHAPTER 2 portal vein, seen in all vertebrates, lies between the capillary beds of the gut and the liver (see Figure 2–11). Substances absorbed from the gut are transported directly to the liver, where toxins are rendered harmless and some nutrients are processed or removed for storage. Most vertebrates also have a renal portal vein between the veins returning from the tail and posterior trunk and the kidneys (see Figure 2–11). The renal portal system is not well developed in jawless vertebrates and has been lost in mammals. The heart. The vertebrate heart is a muscular tube folded on itself and is constricted into three sequential chambers: the sinus venosus, the atrium, and the ventricle. Our so-called four-chambered heart lacks a distinct sinus venosus, and the original atrium and ventricle have been divided into left and right chambers. The sinus venosus is a thin-walled sac with few cardiac muscle fibers. Suction produced by muscular contraction draws blood anteriorly into the atrium, which has valves at each end that prevent backflow. The ventricle is thick-walled, and the muscular walls have an intrinsic pulsatile rhythm, which can be speeded up or slowed down by the nervous system. Contraction of the ventricle forces the blood into the ventral aorta. Mammals no longer have a distinct structure identifiable as the sinus venosus; Vertebrate Relationships and Basic Structure rather, it is incorporated into the wall of the right atrium as the sinoatrial node, which controls the basic pulse of the heartbeat. The aorta. The basic vertebrate circulatory plan consists of a heart that pumps blood into the single midline ventral aorta. Paired sets of aortic arches (originally six pairs) branch from the ventral aorta (Figure 2–12). One member of each pair supplies the left side and the other the right side. In the original vertebrate circulatory pattern, which is retained in fishes, the aortic arches lead to the gills, where the blood is oxygenated and returns to the dorsal aorta. The dorsal aorta is paired above the gills, and the vessels from the most anterior arch run forward to the head as the carotid arteries. Behind the gill region, the two vessels unite into a single dorsal aorta that carries blood posteriorly. is known as an opisthonephric kidney. The compact bean-shaped kidney seen in adult amniotes (the metanephric kidney) includes only the metanephros, drained by a new tube, the ureter, derived from the basal portion of the archinephric duct. The basic units of the kidney are microscopic structures called nephrons. Vertebrate kidneys work by ultrafiltration: high blood pressure forces water, ions, and small molecules through tiny gaps in the capillary walls. Nonvertebrate chordates lack true kidneys. Amphioxus has excretory cells called solenocytes associated with the pharyngeal blood vessels that empty individually into the false body cavity (the atrium). The effluent is discharged to the outside via the atriopore. The solenocytes of amphioxus are thought to be homologous with the podocytes of the vertebrate nephron, which are the cells that form the wall of the renal capsule. The dorsal aorta is flanked by paired cardinal veins that return blood to the heart (see Figure 2–11). Anterior cardinal veins (the jugular veins) draining the head and posterior cardinal veins draining the body unite on each side in a common cardinal vein that enters the atrium of the heart. In lungfishes and tetrapods, the posterior cardinal veins are essentially replaced by a single midline vessel, the posterior vena cava. Blood is also returned separately to the heart from the gut and liver via the hepatic portal system. The Gonads—Ovaries and Testes Although the gonads are derived from the mesoderm, the gametes (eggs and sperm) are formed in the endoderm and then migrate up through the dorsal mesentery (see Figure 2–5) to enter the gonads. The archinephric duct drains urine from the kidney to the cloaca and from there to the outside world. In jawed vertebrates, this duct is also used for the release of sperm by the testes. Reproduction is the means by which gametes are produced, released, and combined with gametes from a member of the opposite sex to produce a fertilized zygote. Vertebrates usually have two sexes, and sexual reproduction is the norm—although unisexual species occur among fishes, amphibians, and lizards. The gonads are paired in jawed vertebrates but are single in the jawless ones: it is not clear which represents the ancestral vertebrate condition. The gonads usually lie on the posterior body wall behind the peritoneum (the lining of the body cavity); it is only among mammals that the testes are found outside the body in a scrotum. The gonads (ovaries in females, testes in males) also produce hormones, such as estrogen and testosterone. In living jawless vertebrates, which probably represent the ancestral vertebrate condition, there is no special tube or duct for the passage of the gametes. Rather, the sperm or eggs erupt from the gonad and move through the coelom to pores that open to the base of the archinephric ducts. In jawed vertebrates, however, the gametes are always transported to the cloaca via specialized paired ducts (one for each gonad). In males, sperm are released directly into the archinephric ducts that drain the kidneys in non-amniotes and embryonic amniotes. In females, the egg is still released into the coelom but is then transported via a new structure, the oviduct. The oviducts produce Excretory and Reproductive Systems Although the func- tions of the excretory and reproductive systems are entirely different, both systems are formed from the nephrotome or intermediate mesoderm, which forms the embryonic nephric ridge (Figure 2–13 on page 41). The kidneys are segmental, whereas the gonads (ovaries in females and testes in males) are unsegmented. The Kidneys The kidneys dispose of waste products, pri- marily nitrogenous waste from protein metabolism, and regulate the body’s water and minerals—especially sodium, chloride, calcium, magnesium, potassium, bicarbonate, and phosphate. In tetrapods the kidneys are responsible for almost all these functions, but in fishes and amphibians the gills and skin also play important roles (see Chapter 4). The kidney of fishes is a long, segmental structure extending the entire length of the dorsal body wall. In all vertebrate embryos, the kidney is composed of three portions: pronephros, mesonephros, and metanephros (see Figure 2–13). The pronephros is functional only in the embryos of living vertebrates and possibly in adult hagfishes. The kidney of adult fishes and amphibians includes the mesonephric and metanephric portions and Basic Vertebrate Structure 39 (d) Generalized gnathostome Arch 1 lost L R Gill slit 1 turned into spiracle 1 Gill slit 3 2 Ventral aorta Subclavian artery (to forelimb) 4 Aortic arch (c) Lamprey R Arch 1 lost L Carotid artery 5 6 Conus arteriosus (fourth heart chamber) 1 2 Gill slit 1 lost Ventricle 3 Carotid artery Atrium 4 Ventral aorta 5 Gill slit Sinus venosus 6 Aortic arch 7 3 gill slits and aortic arches added posteriorly 8 9 Dorsal aorta Iliac artery (to hindlimb) End of body in all gnathostomes Caudal artery (to tail) Ventricle Atrium Sinus venosus Dorsal aorta (b) Hypothetical first vertebrate R L Carotid artery 1 Gill slit 2 3 Aortic arch 4 Ventral aorta 5 6 (a) Protovertebrate R 1 Gill slit 2 L Carotid artery (takes blood to head) 3 Aortic arch Atrium Ventricle Sinus venosus Dorsal aorta 4 5 6 Ventral aorta (under gills, blood runs forward) Single heart chamber (= sinus venosus) 40 CHAPTER 2 Paired dorsal aortae (above gills, blood runs back toward body) Cardinal veins returning blood from body and head Dorsal aorta (to body) Vertebrate Relationships and Basic Structure Oxygenated blood Deoxygenated blood