Why is cephalization associated with bilateral symmetry

They have often fused with the head; most arthropods have a head that includes at least 6 segments. Your head includes 10 segments, marked in your nervous system by 10 pairs of cranial head nerves. For example in Chordates, heteronomous segmentation can be seen in the spinal column and ribcage. Arthropods have armored their segments with exoskeletal plates.

The insects show the greatest level of heteronomous segmentation and tagmatization; each insect body consists of three multisegmented units. The head collects information and feeds, the thorax is concerned with movement, walking and flying, and the abdomen contains most of the internal organs, including those involved in reproduction.

From this listing of body plan characteristics, you can see that the simplest animals have the smallest number of these features, and the most complex the largest number. Since the characteristics were probably acquired by animals in the order in which they are listed, they can also be used to show the course of evolution, as in the phylogenetic tree attached to this lab.

In lab today you will examine a variety of animals to determine their body plan features, and learn the names and characteristics of some of the major animal phyla. The similarities in early development among all animals can be seen by examining the slide of starfish embryos going through the early stages of this process. Observing these early stages will help you to understand several of the features of the body plan that have been presented—bilateral symmetry, cephalization, gut, and coelom development.

Obtain a slide of starfish development. Locate the cells with the clear nucleus , which contains another structure called a nucleolus. This is the unfertilized egg of the starfish. Now find single cells that are exactly the same size but that have no visible nucleus. You are likely to see the fertilization membrane around such cells. The fertilization membrane looks like a cellophane wrap around the cell.

This is the fertilized egg or zygote. If you cannot find a fertilization membrane here, you might see it more clearly as you move to the next stage of development. Draw and label the items in bold the unfertilized egg and the zygote on the sheet provided.

Estimate the size of the zygote in micrometers. Keep your measurement in mind as you observe later stages of starfish development. Do the embryos get larger, smaller, or stay the same size as they continue to develop? Be prepared to explain your observations. The zygote divides to form the two-cell stage. Cell division in embryos is referred to as cleavage.

The cells are called blastomeres. Draw the 2-cell stage, labeling the fertilization membrane. Find an eight-cell embryo and draw that. Notice that even though the embryo contains more cells, blastomeres, that the overall size of the embryo is essentially the same as the zygote.

The next stage to look for is the morula , 16 to 32 cells in a tight ball. Draw an embryo at this stage. Find an embryo that appears as a hollow ball of cells. The space in the center of the embryo is the blastocoel. An embryo at this stage is referred to as a blastula. Sketch the blastula. After the formation of the blastula, one surface of the hollow ball begins to move into the space. With this inward sweep of cells, two tissue layers are established: the outer surface layer called the ectoderm and the inner layer called the endoderm.

The opening is called the blastopore and leads to the primitive gut tube, or the archenteron. Find an embryo at this stage, called the gastrula , and draw and label the structures: blastocoel, gut or archenteron also called the gastrocoel , endoderm, ectoderm, blastopore, and mesoderm. In Chordates and Echinoderms, like you and this starfish, respectively, the blastopore becomes the anus of the primitive gut tube, and this tube touches the other surface of the embryo and forms the mouth.

Organisms in this line of descent, with the two openings formed in this order, are called the deuterostomes. The other phyla are called protostomes ; in protostomes the blastopore becomes the mouth. In both of these lines of descent, a space, called a coelom , may form within a third layer of tissue called the mesoderm which arises between the outer surface of the body, the ectoderm, and the gut, composed of endoderm.

The three primary tissue layers, endoderm, ectoderm, and mesoderm, differentiate into the following structures in the mature organism:. The next developmental stages occur relatively rapidly. You should be able to find a multicellular larval form, that is no longer spherical, that possesses the third tissue layer, the mesoderm , which cannot be specifically detected but makes up some of the internal structures of the organism. Draw the larva. This background on development, the diagram provided in this lab, your textbook remember to use its index!

Examine preserved specimens of jellyfish and sea anemones, both members of the Phylum Cnidaria. What kind of symmetry do they have? Is a complete gut present? Do they have a skeleton? Name several characteristic specimens and fill in the column under Cnidaria in the chart. Obtain a live flatworm, or planaria of the Phylum Platyhelminthes , in a depression slide and watch it under a microscope.

How does it move? Is it segmented? Draw a sketch of the specimen. Observe the other platyhelminthes on display—flukes and tapeworms. Fill in the proper column in the chart. Make a wet-mount slide from the culture of vinegar eels, a minute nematode worm from the Phylum Nematoda. How do the worms move? This worm has a pseudocoelom that helps it move in this fashion. Observe the other "roundworms" on display. Fill in the proper column on the chart. Observe a live earthworm from the P hylum Annelida in the fingerbowl.

Notice how the shapes of the segments change as the worm crawls across the towels. Does this worm show homonomous or heteronomous segmentation? Observe the segmented worms on display. These worms have a true coelom. Echinoderms starfish, sea urchins, and sea cucumbers and cnidarians are two examples corals, anemones, jellyfish. Animals that can't move or are affected by currents must be able to find food and defend themselves against threats coming from all directions.

The majority of introductory textbooks classify these animals as acephalic or lacking cephalization. While none of these creatures has a brain or central nervous system, their neural tissue is organised in such a way that they can experience rapid muscular excitation and sensory processing.

Nerve nets have been discovered in these creatures by modern invertebrate zoologists. Cephalization progressed in arthropods with increasing incorporation of trunk segments into the head region. This was beneficial because it allowed for the evolution of more efficient mouth-parts for capturing and processing food.

Insect brains are strongly cephalized, with three fused ganglia attached to the ventral nerve cord, which has a pair of ganglia in each segment of the thorax and abdomen. The insect head is a complex structure composed of several segments that are rigidly fused together and equipped with both simple and compound eyes, as well as multiple appendages such as sensory antennae and complex mouthparts maxillae and mandibles.

Planarians are free-living flatworms that are completely harmless. They live in water freshwater or saltwater or on moist soil. Tapeworms and flukes are both internal parasites that live in the tissues, cavities in body organs, or blood vessels of their hosts. Animals in the Phylum Platyhelminthes have bilateral symmetry, as opposed to those in the Phylum Cnidaria, which have radial symmetry. This implies that there is only one plane of symmetry one way you can slice the animal in half and produce two pieces that are mirror images of one another.

It also means that you can tell the difference between the animal's anterior and posterior, right and left, and dorsal and ventral halves. A bilaterally symmetrical animal moves forward with its anterior and crawls on its ventral surface with its dorsal surface upward. Members of the Phylum Platyhelminthes particularly planarians, Class Turbellaria have brain and sense organs in front of the animal.

This is known as cephalization. The sense organs are the first to make contact with the environment in cephalized animals. Molluscs are another group that has lost and regained cephalization. The voltage-gated channels necessary for generation and propagation of action potentials have homologues in prokaryotes, indicating that their evolution predates the origins of nervous systems.

In addition, many molecular components of synapses evolved well before the origin of nervous systems. Genomic studies also suggest that the nervous system of ctenophores evolved independently of nervous systems of other metazoa. The phylum Ctenophora comb jellies had previously been linked with Cnideria jellyfish, sea anemones, Hydra as the Coelenterata, but they are clearly distinct and probably a sister group to all other animals.

Ctenophores therefore separated from both Cnideria and Bilateria before sponges, which lack neurons and nervous systems. It is likely that ctenophore nervous systems evolved independently, rather than sponges having lost a nervous system.

Genomic studies strongly support this conclusion: A recently sequenced ctenophore genome lacks genes for most small-molecule neurotransmitter pathways and many other neuron-specific genes of other animals; instead it contains many ctenophore-specific genes that suggest an independent evolutionary path.

Flatworms phylum Platyhelminthes 3 , squids phylum Mollusca 4 , earthworms phylum Annelida 5 , and humans phylum Chordata 6 all display CNSs that feature brains. Cnidarian nervous systems appear to be not very centralized, with fibers running in all directions and little apparent organization into central integrating areas see Part 1 of the figure. Cnidarians have radial symmetry , a body form with no front or back and with apparently limited potential for the evolution of nervous system centralization.

Echinoderms, evolutionarily closer to vertebrates but having secondarily evolved radial symmetry, also have relatively simple and uncentralized nervous systems Part 2 of the figure. In contrast, all groups with bilateral symmetry Parts 3—6 of the figure show evolutionary trends of increasing centralization and complexity of nervous system organization.

Two major trends characterize the evolution of nervous systems in the bilaterally symmetrical phyla of animals: centralization and cephalization. Centralization of nervous systems refers to a structural organization in which integrating neurons are collected into central integrating areas rather than being randomly dispersed. Cephalization is the concentration of nervous structures and functions at one end of the body, in the head.

Both trends can be seen even in flatworms, which belong to the phylum Platyhelminthes, considered the most ancient phylum to have bilateral symmetry see Part 3 of the figure.

Apparently the presence of a distinct anterior end and the development of a preferred direction of locomotion in bilateral animals have been important in the evolution of centralized, cephalized nervous systems. In flatworms and animals of more complex bilaterally symmetrical phyla, centralization is anatomically evident by the presence of longitudinal nerve cords , discrete aggregations of neurons into longitudinally arranged clusters and tracts to constitute a distinct CNS.

Motor neurons extend out from the CNS to effectors, and sensory neurons extend from the periphery of the body into the CNS. Increasing numbers of interneurons — neurons that are neither sensory nor motor and are confined to the CNS—make their appearance as nervous systems become more complex. The interneurons enhance capacities for centralized integrative processing in the nervous system.

The peripheral nervous system PNS also is increasingly consolidated in bilaterally symmetrical animals. Instead of a random meshwork of processes running in all directions in an unpolarized nerve net, the peripheral sensory and motor processes are coalesced into nerves, discrete bundles of nerve axons running between the CNS and the periphery see Parts 3—6 of the figure.

Cephalization, the other general evolutionary trend in nervous system organization, involves varying degrees of anterior concentration of nervous system organization. In the most primitive of centralized nervous systems, each region of the CNS largely controls just its own zone or segment of the body see Parts 3 and 4 of the figure ; indeed, elements of such segmental or regional organization persist in all phyla, including vertebrate chordates.

In most bilaterally symmetrical animals, however, the most anterior part of the CNS exerts a considerable degree of domination and control over other regions.