Monday, April 4, 2011
Different Eyes
On page 150, Shubin discusses the different types of eyes in different animals. Every creature with a skull has camera-like eyes while other animals have eyes "ranging from simple patches of cells specialized to detect light, to eyes with compound lenses..., to primordial versions of our own eye" (150). Explain the differences between these types of eyes. Give examples of animals that have each type of eye. Also, what are some advantages and disadvantages to having each type of eye? Which type of eye would be the most useful for animals? Danielle Webb (dwebb456@gmail.com)
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Most animals have some sort of light-detecting organ, which, as Shubin had said, can range from a few cells made to detect light all the way to eyes like ours that consist of different types of specialized cells and lenses. One of the simplest eyes is found in planarians and is a pair of ocelli, normally called eyespots that are located in the head region. On three sides they are surrounded by cells that block light. “Light shining on the animal stimulates light sensitive cells called photoreceptors in each ocellus only through the opening where there are no pigmented cells” (Campbell 1099). The light shining on one side stimulates the ocellus on which the light shines. The brain then compares the rate of action potentials coming from both ocelli and directs the body’s movement where there is less stimulation of the ocelli (and the animal moves away from the light).
ReplyDeleteAside from planarians, in invertebrates, there are two major image forming eyes that have evolved: the compound eyes and the single lens eyes. Compound eyes, normally found in insects and crustaceans, consist of thousands of individual photoreceptor units or ommatidia. They have a very large view angle and can detect movement very quickly. These insects have color vision and also can see into the UV spectrum (something humans cannot do) (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CompoundEye.html). Also among invertebrates, single-lens eyes are found in jellies and polychaetes, as well as spiders and many mollusks. Light enters through the pupil and the diameter of it is affected by the iris. Muscles in single-lens eyes move the lens forward and backward to focus on objects at different distances (Campbell 1100).
In vertebrates the eye forms by invagination of the optic vesicle, and the inner layer of the double walled optic cup becomes the retina. “Therefore, the retina is ‘reversed’ with the light-sensitive portion (the rods and cones) on the outside. This means that incoming light has to pass through all of the layers before it reaches the rods and cones” (http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab11/Eye/NOTES/EYENOTE.HTM).
Each species can have a different sort of eye that is adaptive to their environment and body. Planarians don’t need complex eyes like humans do, nor do they need to discriminate between colors because this is unimportant to how they survive. All they prefer is darker areas, and eyespots are sufficient enough to do this task with less energy expense. And finally, in regards to the question of which eye is better for animals, this is impossible to answer. Each animal has different needs, and from an evolutionary standpoint, the more complex an animal is, the more complex their eye needs to be, however the simpler invertebrates are successful enough with their simpler eyes.
-Michelle Layvant, michellel94@hotmail.com
Michelle had discussed the eye structures of planarians and other invertebrates. However, to expand on this topic, there is really a wide range of animals with a variety of eye structures, similar to the ones that she had discussed, aside from ones that simply belong to invertebrates. To begin, our eye, the human eye, is “camera-like”, and this is common to all animals with a “skull and backbone” (Shubin, 149). Light travels through a pathway by targeting a screen at the back of the eye. Light penetrates the first level of the eye known as the cornea, which is the thin, protective layer over the lens of the eye. The “diaphragm” or iris of the eye controls the amount of light allowed into eye. The next layer that light passes through is the lens, and the surrounding muscles contract the lens allowing it to focus the image. Finally, the retina, the screen at the back of the eye, has numerous blood vessels and light receptors, which sends signals to the brain and allows the brain to decipher the signals as an image (Shubin, 150).
ReplyDeleteAs Michelle had stated in her response, simple patches of cells specialized in detecting light may include organisms such as planarians or other flatworms. The planarian, like other invertebrates of the same structure, has “a pair of light-sensitive eyespots” (Campbell, 674). These eyespots or “ocelli” detect light intensity by acting as photoreceptors and aid in moving away from light sources (“Characteristics”, New World Encyclopedia). This type of eye is more of an eye slit, lacking in the complexity that is known to belong to the eyes of all animals from fish to mammals. Planarians are simply able to detect light intensity rather than understand or interpret an image with full clarity and depth perception.
The compound lens eye is common to the fly (Shubin, 150). In general though, all arthropods (insects and crustaceans) have these compound eyes. They are made up of ommatidia, which are repeating units that act as separate visual receptors. In specific, each ommatidium contains a lens, a transparent crystalline cone, light-sensitive visual cells, and pigment cells (“The Compound Eye”, Biology Pages). This type of eye may be necessary in animals as small as flies because this allows them to visualize their surroundings in a more complete way because the rest of their bodily structures are not as developed yet. The eyes of most insects are the largest features on their bodies apart from their wings because it is their most developed sense for their size.
Primordial versions of our own eyes may have most likely been less developed versions of the human eye. The layers of their eyes may not have been as healthy or up to par to carry out their function completely. These primordial versions may have evolved over time with their respective organisms.
The most advanced or advantageous eye would be the human, camera-like eye, but this may not be ideal for all animals. In agreement with Michelle, there is no one advantageous eye for all animals. While the human eye is the most helpful by allowing a complete interpretation and understanding of an image, the compound eye for the insect provides the most advantageous eye for the insects’ circumstances. Therefore, it truly depends on the animal’s hierarchy in the food chain, and their ability to survive. It would not be a selective advantage for all animals to have human eyes although they may be the most advanced structure out of the other versions of the eye.
Sonia Doshi (soniadoshi7@gmail.com)