Movimientos en el Campo

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 inter-twined 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 mol-ecules that are absorbed across the wall of the gut and transported to the tissues.

Vertebrate ancestors probably fi ltered small par-ticles of food from the water, as amphioxus and lar-val 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 con-tractions (peristalsis), and digest it by secreting diges-tive enzymes produced by the liver and the pancreas of large, closely spaced cells packed with

incompress-ible fl uid-fi lled vacuoles wrapped in a complex fi brous sheath that is the site of attachment for segmental muscles and connective tissues. Th e notochord ends anteriorly just posterior to the pituitary gland and continues posteriorly to the tip of the fl eshy portion of the tail. Th e original form of the notochord is lost in adult tetrapods, but portions remain as components of the intervertebral discs between the vertebrae.

Th e 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 mus-cle 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. Th e myomeres of jawed vertebrates are divided into epaxial (dorsal) and hypaxial (ventral) portions by a sheet of fi brous tissue called the horizontal septum.

Th e segmental pattern of the axial muscles is clearly visible in fi shes. It is easily seen in a piece of raw or cooked fi sh where the fl esh fl akes apart in zigzag blocks, each block representing a myomere. (Th is 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. How-ever, ciliary propulsion works only at very small body sizes. Adult chordates use the serial contraction of seg-mental muscle bands in the trunk and tail for

locomo-Figure 2–8 Diagrammatic view of the form and early evolution of the cranium of verte-brates. Th e 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 fl anking the notochord posteriorly. Th e 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 fi rst segment) pharyngeal arch becomes the velum and other supporting structures in the head, and the remain-der 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 fi rst seen in ostracoderms (c). In gnathostomes (d,e) the pharyngeal arches of the mandibular (second) and hyoid (third) head segments become modifi ed to form the jaws and jaw supports. Th e 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 fi shes. Th e basics of this pattern are still seen in us.

Rostrum Optic capsule Otic capsule

Position of first gill pouch Hyomandibular

Pharyngobranchial Position of spiracle

Ceratobranchial Position of fifth gill pouch

Epibranchial

Hypobranchial Basibranchial Hyoid arch Mandibular cartilage

Palatoquadrate Mandibular arch

(upper and lower jaw) Nasal capsule

Naris Orbit

Dermal roof bones

Dermal bones of the supracleithral and opercular series

Palatoquadrate cartilage Mandibular cartilage

(articular)

Dermal gular bones Dermal lower jaw bones

Splanchnocranium Chondrocranium Dermatocranium Nasal capsule

Optic capsule Otic capsule

Position of first gill pouch

Arcualia (vertebral rudiment)

Gill slit Notochord

Pericardial cartilage Horizontal bar

Branchial arch Hypobranchial rod

Lingual cartilage Cartilages around

buccal funnel

Chondrocranium and splanchnocranium of a lamprey (a)

(b)

(c)

Chondrocranium and splanchnocranium of a shark

Dermatocranium of a primitive generalized (basal) bony fish

© 1994 Cengage Learning, Inc.

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 fi sh.

combination of large body size and high levels of activity make specialized gas-exchange structures essential for most vertebrates. Gills are eff ective in water, whereas lungs work better in air. Both gills and lungs have large surface areas that allow oxygen to diff use 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 car-bon 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 fl uid tissue composed of liquid plasma, red blood cells (erythrocytes) that contain the iron-rich protein hemoglobin, and several diff erent types of white blood cells (leukocytes) that are part of the into the gut. Th e pancreas also secretes the hormones

insulin and glucagon, which are involved in the regula-tion 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. Th e intestine empties to the cloaca , which is the shared exit for the urinary, reproductive, and digestive systems in all ver-tebrates except therian mammals.

Respiration and Ventilation Ancestral chordates prob-ably relied on oxygen absorption and carbon dioxide loss by diff usion across a thin skin (cutaneous respi-ration). Th is is the mode of respiration of amphioxus, which is small and sluggish.

Cutaneous respiration is important for many ver-tebrates (especially modern amphibians), but the

Amphioxus (a)

(b)

(c)

(d)

Lamprey

Shark (dogfish)

Bony fish (perch)

Myomeres V-shaped

Myomeres W-shaped

Myomeres more complexly

folded Horizontal

septum

Red muscle Epaxial muscle

Hypaxial muscle

Figure 2–10 Chordate body muscles (myomeres). (a) amphioxus (nonvertebrate chordate), (b) lamprey (jawless vertebrate), and jawed vertebrates, (c) shark, and (d) bony fi sh.

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 di-rectly 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 ).

Th e renal portal system is not well developed in jaw-less vertebrates and has been lost in mammals.

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Th e heart. Th e vertebrate heart is a muscular tube folded on itself and is constricted into three sequen-tial 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.

Th e sinus venosus is a thin-walled sac with few cardiac muscle fi bers. Suction produced by muscular contraction draws blood anteriorly into the atrium, which has valves at each end that prevent back-fl ow. Th e ventricle is thick-walled, and the muscular walls have an intrinsic pulsatile rhythm, which can be speeded up or slowed down by the nervous sys-tem. Contraction of the ventricle forces the blood into the ventral aorta. Mammals no longer have a distinct structure identifi able as the sinus venosus;

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 sys-tem, and the walls of arteries have a layer of smooth muscle that is absent from veins. Th e following fea-tures are typical of vertebrate circulatory systems:

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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. Th eir walls are only one cell layer thick; so diff usion 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. Arteriove-nous 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 fl owing through them.

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Portal vessels. Blood vessels that lie between two capillary beds are called portal vessels . Th e hepatic

Carotid

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, fl anking the carotid arteries (ante-rior cardinals) or the dorsal aorta (poste(ante-rior cardinals).

is known as an opisthonephric kidney . Th e compact bean-shaped kidney seen in adult amniotes (the meta-nephric kidney ) includes only the metanephros, drained by a new tube, the ureter , derived from the basal portion of the archinephric duct.

Th e basic units of the kidney are microscopic struc-tures called nephrons . Vertebrate kidneys work by ul-trafi ltration: 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). Th e effl uent is dis-charged to the outside via the atriopore. Th e 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 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. Th e 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 fi shes, amphibians, and lizards.

Th e gonads are paired in jawed vertebrates but are single in the jawless ones: it is not clear which represents the ancestral vertebrate condition. Th e 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. Th e gonads (ovaries in females, testes in males) also produce hormones, such as estrogen and testosterone.

In living jawless vertebrates, which probably repre-sent the ancestral vertebrate condition, there is no spe-cial 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, how-ever, the gametes are always transported to the clo-aca via specialized paired ducts (one for each gonad).

In males, sperm are released directly into the archi-nephric 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 . Th e oviducts produce rather, it is incorporated into the wall of the right

atrium as the sinoatrial node, which controls the basic pulse of the heartbeat.

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Th e aorta. Th e 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 fi shes, the aortic arches lead to the gills, where the blood is oxygenated and returns to the dorsal aorta.

Th e 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 re-gion, the two vessels unite into a single dorsal aorta that carries blood posteriorly.

Th e dorsal aorta is fl anked 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 lungfi shes 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.

Excretory and Reproductive Systems Although the func-tions of the excretory and reproductive systems are entirely diff erent, both systems are formed from the nephrotome or intermediate mesoderm, which forms the embryonic nephric ridge ( Figure 2–13 on page 41).

Th e kidneys are segmental, whereas the gonads ( ovaries in females and testes in males) are unsegmented.

The Kidneys Th e 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, bicar-bonate, and phosphate. In tetrapods the kidneys are re-sponsible for almost all these functions, but in fi shes and amphibians the gills and skin also play important roles (see Chapter 4 ).

Th e kidney of fi shes 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 ). Th e pronephros is functional only in the embryos of living vertebrates and possibly in adult hagfi shes. Th e kidney of adult fi shes and amphibians includes the mesonephric and metanephric portions and

Conus arteriosus blood from body and head Paired dorsal aortae (above gills, blood runs back toward body)

In jawed vertebrates, elongated portions of the neurons, the axons, are encased in a fatty insulating coat, the my-elin sheath, that increases the conduction velocity of the nerve impulse. Th e axons are generally collected like wires in a cable, forming a nerve. Information enters the neuron via short processes called dendrites. Th e brain and spinal cord are known as the central nervous sys-tem (CNS), and the nerves running between the CNS and the body are known as the peripheral nervous sys-tem (PNS).

The Spinal Cord Th e nerves of the PNS are segmentally arranged, exiting from either side of the spinal cord be-tween the vertebrae. Th e spinal cord receives sensory inputs, integrates them with other portions of the CNS, and sends impulses that cause muscles to contract and glands to alter their secretion. Th e spinal cord has con-siderable autonomy in many vertebrates. Even complex movements such as swimming are controlled by the spinal cord rather than the brain, and fi shes continue coordinated swimming movements when the brain is severed from the spinal cord. Our familiar knee-jerk substances associated with the egg, such as the yolk or

the shell. Th e oviducts can become enlarged and fused in various ways to form a single uterus or paired uteri in which eggs are stored and young develop.

Vertebrates may deposit eggs that develop outside the body or retain the eggs within the mother’s body until embryonic development is complete. Shelled eggs must be fertilized in the oviduct before the shell and albumen are deposited. Many viviparous vertebrates and vertebrates that lay shelled eggs have some sort of intromittent organ—such as the pelvic claspers of sharks and the penis of amniotes—by which sperm are inserted into the female’s reproductive tract.

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