Vertebrate Life, 9th Edition PDF

Figure 2–12 Diagrammatic view of the form and early evolution of the heart and aortic arches of vertebrates. The view is from the ventral side of the animal. In the protovertebrate (a) and the earliest vertebrate condition (b), there were probably six pairs of aortic arches, just as are seen in the...

Figure 2–12 Diagrammatic view of the form and early evolution of the heart and aortic arches of vertebrates. The view is from the ventral side of the animal. In the protovertebrate (a) and the earliest vertebrate condition (b), there were probably six pairs of aortic arches, just as are seen in the embryos of all living vertebrates, although arch 1 is never seen in the adults. Protovertebrates used these arches for feeding, not for respiration, so the blood is not shown as picking up oxygen in this illustration. In lampreys (c), additional aortic arches are added posteriorly to accommodate more gill openings. In gnathostomes (d), the subclavian and iliac arteries are added to the main circulatory system, supplying the forelimbs and hindlimbs, respectively. A fourth chamber is also added to the heart, the conus arteriosus, which damps out the pulsatile component of the blood flow. In jawed vertebrates, elongated portions of the neurons, the axons, are encased in a fatty insulating coat, the myelin sheath, that increases the conduction velocity of the nerve impulse. The axons are generally collected like wires in a cable, forming a nerve. Information enters the neuron via short processes called dendrites. The brain and spinal cord are known as the central nervous system (CNS), and the nerves running between the CNS and the body are known as the peripheral nervous system (PNS). substances associated with the egg, such as the yolk or the shell. The 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. The Spinal Cord The nerves of the PNS are segmentally arranged, exiting from either side of the spinal cord between the vertebrae. The 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. The spinal cord has considerable autonomy in many vertebrates. Even complex movements such as swimming are controlled by the spinal cord rather than the brain, and fishes continue coordinated swimming movements when the brain is severed from the spinal cord. Our familiar knee-jerk Coordination and Integration The nervous and endocrine systems respond to conditions inside and outside an animal, and together they control the actions of organs and muscles, coordinating them so they work in concert. General Features of the Nervous System Individual cells called neurons are the basic units of the nervous system. Nephric ridge Archinephric duct extending by proliferation Archinephric duct derived from tubules Developing lateral plate Pronephric tubules Nephrotomes Developing somites M o Pr ph ne s ro o es s ro ph e n Op a- s et ro M ph ne s ro ph e n ho ist Figure 2–13 Kidney development in a generalized vertebrate embryo, showing the nephrotome regions. Basic Vertebrate Structure 41 reaction is produced by the spinal cord as a reflex arc. Development of more complex connections within the spinal cord and between the spinal cord and the brain has been a trend in vertebrate evolution. The Somatic and Visceral Nervous Systems Vertebrates are unique in having a dual type of nervous system: the somatic nervous system (known as the voluntary nervous system) and the visceral nervous system (called the involuntary nervous system). This dual nervous system mimics the dual pattern of development of the mesoderm. The somatic nervous system innervates the structures derived from the segmented portion (the somites), including the striated muscles that we can move consciously (e.g., the limb muscles), and relays information from sensation that we are usually aware of (e.g., from temperature and pain receptors in the skin). The visceral nervous system innervates the smooth and cardiac muscles that we usually cannot move consciously (e.g., the gut and heart muscles) and relays information from sensations that we are not usually aware of, such as the receptors monitoring the levels of carbon dioxide in the blood. Each spinal nerve complex is made up of four types of fibers: somatic sensory fibers coming from the body wall, somatic motor fibers running to the body, visceral sensory fibers coming from the gut wall and blood vessels, and visceral motor fibers running to the muscles and glands of the gut and to the blood vessels of both the gut and peripheral structures like the skin. The motor portion of the visceral nervous system is known as the autonomic nervous system. In more derived vertebrates, such as mammals, this system becomes divided into two portions: the sympathetic nervous system (usually acting to speed things up) and the parasympathetic nervous system (usually acting to slow things down). Cranial Nerves Vertebrates also have nerves that emerge directly from the brain; these cranial nerves (10 pairs in the ancestral vertebrate condition, 12 in amniotes) are identified by Roman numerals. Some of these nerves, such as the ones supplying the nose (the olfactory nerve, I) or the eyes (the optic nerve, II), are not really nerves at all, but tracts—that is, parts of the CNS. Somatic motor fibers in cranial nerves innervate the muscles that move the eyeballs and the branchiomeric muscles that power the jaws and gill arches, and sensory nerves convey information from the head, including the sense of taste. The special sensory nerves that supply the lateral line in fishes are also derived from the cranial nerves. The vagus nerve (cranial nerve X) ramifies through all but the most posterior part of the trunk, carrying 42 CHAPTER 2 the visceral motor nerve supply to various organs. People who break their necks may be paralyzed from the neck down (i.e., lose the function of their skeletal muscles) but still may retain their visceral functions (workings of the gut, heart, etc.) because the vagus nerve is independent of the spinal cord and exits above the break. Brain Anatomy and Evolution All chordates have some form of a brain, as a thickening of the front end of the notochord. The brain of all vertebrates is a tripartite (three-part) structure (Figure 2–14), and the telencephalon (front part of the forebrain) and the olfactory receptors are probably true new features in vertebrates. In the ancestral condition, the forebrain is associated with the sense of smell, the midbrain with vision, and the hindbrain with balance and detection of vibrations (hearing, in the broad sense). These portions of the brain are associated with the nasal, optic, and otic capsules of the chondrocranium, respectively (see Figure 2–8). The vertebrate brain has three parts: the forebrain, midbrain, and hindbrain. The Forebrain The forebrain has two parts: The anterior region of the adult forebrain, the telencephalon, develops in association with the olfactory capsules and coordinates inputs from other sensory modalities. The telencephalon becomes enlarged in different vertebrate groups and is known as the cerebrum or cerebral hemispheres. Tetrapods developed an area in the cerebrum called the neocortex or neopallium, which is the primary seat of sensory integration and nervous control. Bony fishes also evolved a larger, more complex telencephalon, but by a completely different mechanism. Sharks and, perhaps surprisingly, hagfishes independently evolved relatively large forebrains, although a large cerebrum is primarily a feature of birds and mammals. The posterior region of the forebrain is the diencephalon, which contains structures that act as a major relay station between sensory areas and the higher brain centers. The pituitary gland, an important endocrine organ, is a ventral outgrowth of the diencephalon. The floor of the diencephalon (the hypothalamus) and the pituitary gland form the primary center for neural-hormonal coordination and integration. Another endocrine gland, the pineal organ, is a median dorsal outgrowth of the diencephalon that is a photoreceptor. Many early tetrapods had a hole in the skull over the pineal gland to admit light, and this condition persists in some reptiles (e.g., the tuatara and many lizards). Vertebrate Relationships and Basic Structure MYELENCEPHALON Forebrain (prosencephalon) Midbrain (mesencephalon) Hindbrain (rhombencephalon) METENCEPHALON DIENCEPHALON TELENCEPHALON Lateral view of the exterior of the brain Cerebellum Lateral view of a section cut through the midline of the brain Tectum Pineal gland Cortex Ventricle IV Medulla oblongata Figure 2–14 The generalized vertebrate brain. The Midbrain The midbrain develops in conjunction with the eyes and receives input from the optic nerve, although in mammals the forebrain has taken over much of the task of vision. The Hindbrain The hindbrain has two portions: The posterior portion, the myelencephalon, or medulla oblongata, controls functions such as respiration and acts as a relay station for receptor cells from the inner ear. The anterior portion of the hindbrain, the metencephalon, develops an important dorsal outgrowth, the cerebellum—present as a distinct structure only in jawed vertebrates among living forms. The cerebellum coordinates and regulates motor activities whether they are reflexive (such as maintenance of posture) or directed (such as escape movements). The Sense Organs We think of vertebrates as having five senses—taste, touch, sight, smell, and hearing— but this list does not reflect the ancestral condition, nor does it include all the senses of living vertebrates. Complex, multicellular sense organs that are formed from epidermal placodes and tuned to the sensory worlds of the species that possess them are derived features of many vertebrate lineages. Pituitary gland Hypothalamus Olfactory bulb © 1998 The McGraw-Hill Companies, Inc. Chemosensation: Smell and Taste The senses of smell and taste both involve the detection of dissolved molecules by specialized receptors. We think of these two senses as being closely interlinked; for example, our sense of taste is poorer if our sense of smell is blocked because we have a cold. However, the two senses are actually very different in their innervation. Smell is a somatic sensory system—sensing items at a distance, with the sensations being received in the forebrain. Taste is a visceral sensory system—sensing items on direct contact, with the sensations being received initially in the hindbrain. Vision The receptor field of the vertebrate eye is arrayed in a hemispherical sheet, the retina, which originates as an outgrowth of the brain. The retina contains two types of light-sensitive cells, cones and rods, which are distinguished from each other by morphology, photochemistry, and neural connections. Electroreception The capacity to perceive electrical im- pulses generated by the muscles of other organisms is also a form of distance reception, but one that works only in water. Electroreception was probably an important feature of early vertebrates and is seen today primarily in fishes and monotreme mammals. Many extant fishes produce electrical discharge for communication with other individuals or for protection from predators. Basic Vertebrate Structure 43 Balance and Orientation Originally the structures in the inner ear (the vestibular apparatus) detected an animal’s position in space, and these structures retain that function today in both aquatic and terrestrial vertebrates. The basic sensory cell in the inner ear is the hair cell, which detects the movement of fluid resulting from a change of position or the impact of sound waves. The vestibular apparatus (one on either side of the animal) is enclosed within the otic capsule of the skull and consists of a series of sacs and tubules containing a fluid called endolymph. The lower parts of the vestibular apparatus, the sacculus and utriculus, house sensory organs called maculae, which contain tiny crystals of calcium carbonate resting on hair cells. Sensations from the maculae tell the animal which way is up and detect linear acceleration. The upper part of the vestibular apparatus contains the semicircular canals. Sensory areas located in swellings at the end of each canal (ampullae) detect angular acceleration through cristae, hair cells embedded in a jellylike substance, by monitoring the displacement of endolymph during motion. Jawed vertebrates have three semicircular canals on each side of the head, hagfishes have one, and lampreys and fossil jawless vertebrates have two (Figure 2–15). We often fail to realize the importance of our own vestibular senses because we usually depend on vision to Lamprey (2 semicircular canals) Posterior SSC Posterior ampulla Endolymphatic duct Anterior SSC Anterior ampulla Lagena (a) Sacculus Utriculus Generalized gnathostome (shark) (3 semicircular canals) Posterior SSC Horizontal SSC Endolymphatic duct Anterior SSC Utriculus © 1998 The McGraw-Hill Companies, Inc. Figure 2–15 Anatomy of the vestibular apparatus in fishes. The lamprey (a) has two semicircular canals (SSC), whereas gnathostomes (represented by a shark, b) have three semicircular canals. 44 Detection of Water Vibration Fishes and aquatic amphib- ians have a structure running along the body on either side, called the lateral line. Within this system, hair cells are aggregated into neuromast organs, which detect the movement of water around the body, and information is then fed back to the vestibular apparatus for integration (see Chapter 4). Hearing The inner ear is also used for hearing (recep- tion of sound waves) by tetrapods and by a few derived fishes. In tetrapods only, the inner ear contains the cochlea (organ of hearing, also known as the lagena in nonmammalian tetrapods). The cochlea and vestibular apparatus together are known as the membranous labyrinth. Sound waves are transmitted to the cochlea, where they create waves of compression that pass through the endolymph. These waves stimulate the auditory sensory cells, which are variants of the basic hair cell. The Endocrine System The endocrine system transfers information from one part of the body to another via the release of a chemical messenger (hormone) that produces a response in the target cells. The time required for an endocrine response ranges from seconds to hours. Hormones are produced in discrete endocrine glands, whose primary function is hormone production and excretion (e.g., the pituitary, thyroid, thymus, and adrenals), and by organs with other major bodily functions—such as the gonads, kidneys, and gastrointestinal tract. Endocrine secretions are predominantly involved in controlling and regulating energy use, storage, and release, as well as in allocating energy to special functions at critical times. The trend in the evolution of vertebrate endocrine glands has been consolidation from scattered clusters of cells or small organs in fishes to larger, better-defined organs in amniotes. The Immune System Sacculus (b) determine our position. We can sometimes be fooled, however, as when sitting in a stationary train or car and thinking that we are moving, only to realize from the lack of input from our vestibular system that it is the vehicle next to us that is moving. CHAPTER 2 Vertebrates have adaptive immunity, a type of immune system different from that of invertebrates. While all animals have innate, specifically genetically encoded responses to pathogens that are fixed and unchanging, vertebrates additionally have evolved lymphocytes (a type of white blood cell), which provide a system of adjustable antigen recognition. The adaptive immune systems of jawless and jawed vertebrates are somewhat Vertebrate Relationships and Basic Structure different. While gnathostomes generate lymphocyte receptors via immunoglobulin gene segments, lampreys and hagfishes employ leucine-rich repeat molecules. In addition, lampreys and hagfishes lack a thymus gland, which produces lymphocytes in gnathostomes. The agnathan condition is probably the ancestral condition for vertebrates, as it more closely resembles the mode of antipathogen responses in invertebrates. Summary Vertebrates are large, active members of the phylum Chordata, a group of animals whose other members, tunicates and cephalochordates (amphioxus), are small and sluggish or entirely sessile as adults. Chordates share with many other derived animal phyla the features of being bilaterally symmetrical, with a distinct head and tail end. Both embryological and molecular evidence show that chordates are related to other sessile marine animals, such as echinoderms. Chordates are distinguished from other animals by the presence of a notochord, a dorsal hollow nerve chord, a muscular postanal tail, and an endostyle (which is homologous to the vertebrate thyroid gland). Vertebrates appear to be most closely related to tunicates among nonvertebrate chordates, and most of the differences in structure and physiology between vertebrates and other chordates reflect an evolutionary change to larger body sizes, greater levels of activity, and predation rather than filter feeding. Vertebrates have the unique features of an expanded head with multicellular sense organs and a cranium housing an enlarged, tripartite brain. The features that distinguish vertebrates from other chordates appear to be related to two critical embryonic innovations: a doubling of the Hox gene complex and the development of neural-crest tissue. The diverse activities of vertebrates are supported by a complex morphology. Study of embryology can throw light on the genetic basis and developmental pathways underlying these structures. In particular, neural-crest cells, which are unique to vertebrates, are responsible for many of their derived characters, especially those of the new anterior portion of the head. An adult vertebrate can be viewed as a group of interacting anatomical and physiological systems involved in protection, support and movement, acquisition of energy, excretion, reproduction, coordination, and integration. These systems underwent profound changes in function and structure at several key points in vertebrate evolution. The most important transition was from the prevertebrate condition—as represented today by the cephalochordate amphioxus—to the vertebrate condition, shown by hagfishes and lampreys. Other important transitions, to be considered in later chapters, include the shift from jawless to jawed vertebrates and from fish to tetrapod. Discussion Questions 1. Suppose new molecular data showed that tunicates and vertebrates are sister taxa. What difference would this make to our assumptions about the form of the original chordate animal? What additional features might this animal have possessed? 2. We noted that many features typical of vertebrates, such as a distinct head and limbs of some sort for locomotion, are seen in invertebrates such as insects and crustaceans, and even in cephalopods (squid and octopuses). What characteristic do these animals share with vertebrates that might have led to the independent evolution of such structures? 3. Why would evidence of sense organs in the head of a fossil animal (such as Haikouella) suggest a close relationship with vertebrates—that is, which critical vertebrate feature would have to be present? 4. Vertebrates have been thought of as “dual animals,” consisting of both segmented and unsegmented portions. How is this duality reflected in their embryonic development and the structure of the nervous system? 5. Among the extant vertebrates, only the bony fishes (Osteichthyes) possess bone. (a) Why, then, do we make the assumption that the ancestors of cartilaginous fishes must have had bone that was subsequently lost? (b) Why don’t we think this is true for the lampreys and hagfishes? 6. Amphioxus obtains its oxygen by diffusion over the body surface. Why aren’t vertebrates generally able to do this—that is, why do most aquatic vertebrates rely on gills for gas exchange? Discussion Questions 45