Photoreception 387 



increased random activity and ultimately aggregate in the darkest region 

 of their environment. ^^^ 



A similar type of behavior was obtained in another planarian, Dendro- 

 caelum, by Ullyott.^^^ Under uniform conditions of illumination without 

 directional components (i.e., from above), the animal lacks a directional 

 response. The perception of light merely increases its random angular mo- 

 tion, which may be expressed as rate of change of direction. Since this 

 rate of change of direction increases with light intensity, it follows that 

 these animals will usually be found in the least intensely illuminated 

 part of their environment, simply because their activity is lower here 

 than elsewhere. This type of orientation is called klinokinesis.'*-* 



Localized Photoreceptors and Pattern Vision 



Light-sensitive organs capable of pattern vision have evolved several times. 

 The eyes of Nautilus, Helix, and Pecten may possibly function in pattern 

 vision; the complex eye of cephalopods, illustrated by Sepia (Fig. 103, C), 

 resembles in structural and functional complexity the eye of vertebrates and 

 certainly is capable of pattern vision and form perception. In the arthropods 

 the compound eye serves in pattern vision, and in the vertebrates a distinctlv 

 different photoreceptor has evolved. 



The Compound Eyes of Arthropods. The eyes of arthropods are of two 

 general types: compound eyes and ocelli. All compound eyes are constructed 

 so that an image is formed; some ocelli (e.g., the lateral ocelli or stemmata of 

 insect larvae and pupae) are capable of forming images, but others (e.g., 

 ocelli of adult insects) are primarily simple organs of light detection. 



The compound eye is an effective organ of vision which has developed along 

 quite different morphological and optical lines from the eye of vertebrates. 

 The focusing mechanism works on a different optical principle, and the focus 

 is always fixed. In many compound eyes at least, the light-gathering power 

 is far inferior to that of the vertebrate eye. However; the compound eve also 

 has advantages, especially in that the field of view is very great and may 

 encompass well over 200 degrees, as in the stalked eyes of the crayfish. 



Structure of the Compound Eye. The compound eye is made of a number 

 of transparent facets in the cuticle of the head or eyestalk, and beneath each 

 facet is an elongated light-sensitive structure, which is connected to the cen- 

 tral nervous system. Each facet, together with the underlying sensitive struc- 

 ture, is called an ommatidium, and the entire group of receptors is called the 

 retina. The distal dioptric part of each ommatidium consists of a facet of the 

 cuticle; all such facets, together, form the cornea. Beneath each facet there is 

 usuallv a crvstalline cone which serves as a lens. Below this is the retinula 

 which usually consists of seven elongated sensory cells, arranged around a 

 central retractile rod, the rhabdom. Each retinula cell is a primary neurone 

 and is continuous with a post-retinal axon. The rhabdom is the collective 

 secretion of the retinula cells, and its function is apparently to conduct light 

 along the length of the retinula cells, which presumably contain the photo- 

 sensitive pigment. 



In some insects the rhabdom is optically continuous with the crystalline 

 cone, and the two form a light-collecting and distributing structure which 

 transmits light entering the facet over the length of the photosensitive retin- 



