1 10 GENERAL CONCEPTS 



special light-sensitive organ show a generalized ability to respond to 

 light. Many higher animals— usually the burrowing ones— have no recog- 

 nizable "eyes" but have a general sensitivity to light over all or a large 

 part of the body. Clams, tor example, respond to sudden changes in 

 light intensity by drawing in their siphon, and earthworms withdraw 

 into their burrows when the light intensity is increased. 



Most animals, even coelenterates, have some sort of specialized 

 structure for the perception of light. A simple invertebrate eye usually 

 consists of a cup-shaped layer of pigment cells which screen the light- 

 sensitive cells from light coming from all directions but one. Light-sensi- 

 tive cells are embedded between these pigment cells. 



The cephalopods— the octopus, squid, and relatives— alone among 

 the invertebrates have well developed camera eyes which are super- 

 ficially similar to vertebrate eyes, with retina, lens, iris, cornea, and a 

 mechanism for focusing for near and far vision. Although it is difficult 

 to determine how well an octopus can see, we can infer from the struc- 

 ture of the eye that it should be the functional equivalent of the verte- 

 brate eye. 



The eyes of arthropods— insects and crabs— are mosaic eyes, com- 

 posed of many, perhaps thousands, of visual units called ommatidia. 

 Each ommatidium has a clear outer cornea, under which is a lens which 

 focuses the light on the end of the light-sensitive element made of eight 

 or so retinal cells. These are believed to respond as a unit. Each om- 

 matidium is separated from the adjacent ones by rings of pigment cells, 

 so that it is a tube with light-sensitive elements at the base which can 

 be reached only by light parallel to the axis of the tube. A mosaic eye 

 presumably forms a very poor image composed of a series of rather large 

 dots like a poor newspaper photograph. But a mosaic eye is particularly 

 sensitive to the motion of objects in its surroundings, for any movement 

 would change the amount of light falling on one or more of the om- 

 matidia. 



Thermoreceptors. Temperature-sensitive cells are found in a wide 

 variety of animals, from the lowest to the highest levels of evolution. 

 Ciliates such as paramecia will avoid warm or cold water and will collect 

 in a region where the temperature is intermediate. Some insects have 

 thermoreceptors, either in the antennae or all over the body. Insects 

 that suck blood from warm-blooded animals are attracted to their prey 

 by the temperature gradients nearby. This has been shown experi- 

 mentally, for blood-sucking bugs are much less able to find their prey 

 after their antennae have been removed. Fish apparently have fairly 

 sensitive thermoreceptors, for a change of only 0.5° C. will change the 

 behavior of sharks and bony fish. 



As far as we know, all nerve impulses are qualitatively similar. The 

 impulse set up by the ringing of a bell is exactly like the impulse 

 initiated by the pressure of a pin against the skin, or the impulse in the 

 optic nerve which results from light falling on the retina. The qualita- 

 tive differentiation of stimuli must depend upon the pattern of connec- 

 tions between sense organ and brain. The ability to distinguish red from 

 green, hot from cold, or red from cold is due to the fact that particular 



