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.
Look at the trays of insects, Phylum Arthropoda and other specimens provided; identify the three tagmata. Is this homonomous or heteronomous segmentation? What is the nature of the skeleton? How many tagmata does a crawfish or lobster have? Observe other arthropods on display. Fill in the chart. A display of shells on demonstration suggests something of the variety of types to be found in the Phylum Mollusca. Make careful observations concerning the other mollusks available.
Refer back to the information given earlier in this lab concerning skeletons v. Remember, a shell is not a skeleton. Give examples of a few of these shell types and other specimens of interest. A small sea animal called a lancelet, Amphioxus or Brachiostoma , will be our example of the Phylum Chordata. Remember that YOU are an example as well.
Look at the whole specimen mounted on a slide and note the form of the segmentation. A stiff rod runs the length of the body; this is the notochord. Its elasticity opposes the contractions of the muscles. Notice that some of the anterior segments bear gill slits.
These segments become part of the head in the vertebrates, which probably descended from an ancestor much like the lancelet. Review the other chordate specimens on display. Fill in the characteristics on the chart. The last phylum to be examined is also the most peculiar, the Phylum Echinodermata.
A starfish is a typical example. What kind of symmetry does it have? Examine other specimens available in this group. Fill in the chart for this phylum. Your final activity today is to make up a dichotomous key, similar to the one you used several weeks ago, that could be used to identify the eight animal phyla you have just studied. Hand the key in as a part of your evaluation this week. Study for your final practical which includes labs Unfertilized egg.
Radial coelom. Eukaryotic cells 2. Multicellular organization 3. Motility using muscular contraction 4. Feeding by ingestion Although the fossil record of animal life goes back more than half a billion years, early traces are ambiguous and may not be animals at all, or may be animal life of a kind totally unlike what we see today.
Body symmetry: This refers to the general appearance and arrangement of body parts. This is a very early stage of cephalization.
Flatworms: Platyhelminthes flatworms have a more complex nervous system than Acoela and are lightly cephalized, with an eyespot above the brain near the front end, for example. Cephalization provides three benefits to an organism-. For starters, it promotes brain development.
The brain serves as a command and control centre for organising and controlling sensory information. Animals can evolve complex neural systems and higher intelligence over time. The second benefit of cephalization is that sense organs can be concentrated in the front of the body. This allows a forward-facing organism to scan its environment more efficiently, allowing it to find food and shelter while avoiding predators and other dangers.
As the organism moves forward, the front end of the animal senses stimuli first. Third, cephalization moves the mouth closer to the sense organs and brain.
As a result, an animal can quickly analyse food sources. Predators frequently use special sense organs near the oral cavity to gather information about prey when vision and hearing are insufficient. Cats, for example, have vibrissae whiskers that detect prey in the dark and when it is too close to see. Sharks have ampullae of Lorenzini electroreceptors that allow them to map prey location. Vertebrates, arthropods, and cephalopod molluscs are three groups of animals with a high degree of cephalization.
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.
Unlike the animal kingdom in which organisms with radial symmetry developed out of a nascent bilateral structure, the opposite is true for plants. Many plant phyla have gradually evolved from having radial symmetry to having bila teral symmetry.
Much of this is a result of form following function: plants possessing bilateral symmetry are capable of signaling a particular pollinator in the direction of the flowers fertilizing organs.
A good way to judge floral symmetry is to plac e flowers in the following categories: 1 fused petals - radial symmetry, 2 free petals, fully open, most often radial symmetry, 3 free petals, closed, most often bilateral.
Floral Structure Images of Flowers and Fruit the byproduct of angiosperm fertilization through flowers :. Symmetry is a pivotal concept in many other areas of biology, particularly in the study of molecular biology.
Molecular biology is ultimately more complex than organismal biology, but if you want to learn more, here are some excellent links to molecular biology pages regarding symmetry:. For a comprehensive list of links to articles and analyses about macromolecular symmetry primarily proteins click here.
For an interesting article on dihedral symmetry in cancer cells warning, this is very technical , go here.
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