Invertebrates: Development of Centralized Nervous Systems
Evolution of
Nervous Systems
Invertebrates: Development of Centralized Nervous Systems
Various metazoan phyla reveal a progressive increase in complexity of nervous systems that probably reflects in a general way the stages in evolution of nervous systems. The simplest pattern of invertebrate nervous systems is the nerve net of radiate animals, such as sea anemones, jellyfishes, hydras, and comb jellies (Figure 35-9A). A nerve net is a quantum leap in complexity beyond sensory systems of the unicellular forms, which lack nerves. A nerve net forms an extensive network in and under the epidermis over all the body. An impulse starting in one part of this net spreads in all directions, since synapses in most radiates do not restrict transmission to one-way movement, as occurs in more complex animals. There are no differentiated sensory, motor, or connector components in the strict meaning of those terms. Branches of a nerve net connect to receptors in the epidermis and to epithelial cells that have contractile properties, and there is evidence of organization into reflex arcs . Although most responses tend to be generalized, many are astonishingly complex for so simple a nervous system. This type of nervous system is found among vertebrates in nerve plexuses located, for example, in the intestinal wall; such nerve plexuses govern generalized intestinal movements such as peristalsis and segmentation.
Bilateral nervous systems, the simplest of which occur in flatworms, represent a distinct increase in complexity over the nerve net of radiate animals. Flatworms have two anterior ganglia, composed of groups of nerve cell bodies from which two main nerve trunks run posteriorly, with lateral branches extending throughout the body (Figure 35-9B). This is the simplest nervous system showing differentiation into a peripheral nervous system (a communication network extending to all parts of the body) and a central nervous system (a concentration of nerve cell bodies), which coordinates everything. More complex invertebrates exhibit a more centralized nervous system (brain), with two longitudinal fused nerve cords and many ganglia. The elaborate nervous systems of annelids contain a bilobed brain, a double nerve cord with segmental ganglia, and distinctive afferent (sensory) and efferent (motor) neurons (Figure 35-9C). Segmental ganglia are relay stations for coordinating regional activity.
The basic plan of molluscan nervous systems is a series of three pairs of well-defined ganglia, but in cephalopods (such as octopus and squid), the ganglia have burgeoned into textured nervous centers of great complexity; those of the octopus contain more than 160 million cells. Sense organs, too, are highly developed. Consequently, cephalopod behavior far outstrips that of any other invertebrate.
The basic plan of arthropod nervous systems (Figure 35-9D) resembles that of annelids, but ganglia are larger and sense organs are much better developed. Social behavior is often elaborate, particularly in hymenopteran insects (bees, wasps, and ants), and most arthropods are capable of considerable manipulation of their environment. Despite the complexity of much insect behavior, insects are nevertheless reflexbound animals incapable of involved learned behavior principally because of their small size.
Invertebrates: Development of Centralized Nervous Systems
Various metazoan phyla reveal a progressive increase in complexity of nervous systems that probably reflects in a general way the stages in evolution of nervous systems. The simplest pattern of invertebrate nervous systems is the nerve net of radiate animals, such as sea anemones, jellyfishes, hydras, and comb jellies (Figure 35-9A). A nerve net is a quantum leap in complexity beyond sensory systems of the unicellular forms, which lack nerves. A nerve net forms an extensive network in and under the epidermis over all the body. An impulse starting in one part of this net spreads in all directions, since synapses in most radiates do not restrict transmission to one-way movement, as occurs in more complex animals. There are no differentiated sensory, motor, or connector components in the strict meaning of those terms. Branches of a nerve net connect to receptors in the epidermis and to epithelial cells that have contractile properties, and there is evidence of organization into reflex arcs . Although most responses tend to be generalized, many are astonishingly complex for so simple a nervous system. This type of nervous system is found among vertebrates in nerve plexuses located, for example, in the intestinal wall; such nerve plexuses govern generalized intestinal movements such as peristalsis and segmentation.
Bilateral nervous systems, the simplest of which occur in flatworms, represent a distinct increase in complexity over the nerve net of radiate animals. Flatworms have two anterior ganglia, composed of groups of nerve cell bodies from which two main nerve trunks run posteriorly, with lateral branches extending throughout the body (Figure 35-9B). This is the simplest nervous system showing differentiation into a peripheral nervous system (a communication network extending to all parts of the body) and a central nervous system (a concentration of nerve cell bodies), which coordinates everything. More complex invertebrates exhibit a more centralized nervous system (brain), with two longitudinal fused nerve cords and many ganglia. The elaborate nervous systems of annelids contain a bilobed brain, a double nerve cord with segmental ganglia, and distinctive afferent (sensory) and efferent (motor) neurons (Figure 35-9C). Segmental ganglia are relay stations for coordinating regional activity.
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| Figure 35-9 Invertebrate nervous systems. A, Nerve net of radiates, the simplest neural organization. B, Flatworm system, the simplest linear-type nervous system of two nerves connected to a complex neuronal network. C, Annelid nervous system, organized into a bilobed brain and ventral cord with segmental ganglia. D, Arthropod nervous system with large ganglia and more elaborate sense organs. |
The basic plan of molluscan nervous systems is a series of three pairs of well-defined ganglia, but in cephalopods (such as octopus and squid), the ganglia have burgeoned into textured nervous centers of great complexity; those of the octopus contain more than 160 million cells. Sense organs, too, are highly developed. Consequently, cephalopod behavior far outstrips that of any other invertebrate.
The basic plan of arthropod nervous systems (Figure 35-9D) resembles that of annelids, but ganglia are larger and sense organs are much better developed. Social behavior is often elaborate, particularly in hymenopteran insects (bees, wasps, and ants), and most arthropods are capable of considerable manipulation of their environment. Despite the complexity of much insect behavior, insects are nevertheless reflexbound animals incapable of involved learned behavior principally because of their small size.
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