628 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



more complex than those of other genera in which 

 the structure of photoreceptors is simpler (267). 



The other group of invertebrates with compound 

 eyespots conceals its photoreceptors so well that they 

 were unknown before 1946. Most maggots (larval 

 flies, insect order Diptera) give strong negative re- 

 sponses to any but very dim illumination, and many 

 can orient themselves with remarkable accuracy. 

 Yet it remained for Bolwig (22) to locate the photo- 

 sensory structures using microdissection. He found a 

 small group of rounded cells somewhat anterodorsal 

 to the supraesophageal ganglion. In early first-instar 

 lar\ae these cells are not fully developed; neither is 

 photosen,sitivity. By the second instar, the cell group 

 is well organized but not yet surrounded i^y opaque 

 tissues; these larvae orient well, apparently bv dis- 

 criminating the faint shadow of their own translucent 

 bodies. In the third instar, the growth of the pharyn- 

 geal .skeleton provides an opaque cup around the 

 compound eyespot without blocking the light from 

 anterior directions; these larvae will follow the vec- 

 torial resultant path between two light sources. With 

 later development, both overgrowth of the pharyn- 

 geal skeleton and increased opacity of the body re- 

 duce the accuracy of orientation and raise the thresh- 

 old for response. 



Eyes of types other than compound eyespots agree 

 in having a layer of photosensory cells, i.e. a retina. 

 It may line a pit (fig. 3, left") or lie below a lens (fig. 

 3, right). Where the retina consists of many cells so 

 close to the dioptric elements that no clear image 

 seems possible, the term ocellus (simple eye) may be 

 applied. If a similar retina is remote enough from 

 the dioptric system that a reasonable image is cast 

 upon it, the phrase camera-style eye seems preferable; 

 camera-style eyes usually have an accessory mecha- 

 nism permitting accommodation. If the retina con- 

 sists of only a ring of receptor cells, clustered around 

 the pro.ximal end of the dioptric system like sections 

 of a citrus fruit, the structure is an ommatidium. 

 Ordinarily ommatidia are grouped into a compound 

 eye with the optic axes of the individual units diverg- 

 ing from one another on a quasiradial plan. Ocelli 

 may also be grouped as compound ocelli, or 'aggre- 

 gate eyes.' A puzzling intermediate between an ocel- 

 lus and an ommatidium is found in the larvae of 

 some holometabolous insects; for this structure the 

 word stemma is useful. 



OCELLI OR SIMPLE EYES. OcelH with large lenses are 

 located around the rim of many coelenterate medusae, 

 but the degree to which their photosensiti\ity is used 



— PIGMENT CELLS 



FIG. 3. A pigment-surrounded cup lined with photoreceptor 

 cells (/(>//), a cuticular lens above the retina (jrighl'), or a com- 

 bination of the two, are characteristic of true eyes. .\ narrowing 

 of the cup's aperture improves the ability of an eye to dis- 

 criminate between events at A and B but reduces the amount 

 of light admitted. A lens provides an aid to discrimination and 

 also can collect more light, hence increasing the organ's sensi- 

 tivity. [After Nagel; from Milne & Milne (193).] 



to modify responses arising from a general responsive- 

 ness to light has never been established (e.g. 200). 

 Nerve fibers from these eyes communicate with the 

 diffuse nerve net and may be part of a much more 

 direct .sensory-motor mechanism than is found among 

 animals with a highly developed nervous system. 



Shallow and deep pigment cups without lenses are 

 the characteristic ocelli in turbellarians. Their num- 

 ber ranges from one pair to many and their size from 

 minute to relatively large. No phylogenetic pattern 

 is discernible, and no correlation has been made with 

 habits or habitat (133). Best known are the con- 

 spicuous ocelli of Planaria in which a pigment cup 

 open lateralis' conceals the distal ends of the receptor 

 cells (fig. 4). Hesse (115) related the visual field of 

 each ocellus to the behaxior of the intact flatworm. 

 Taliaferro (249) found in addition that receptors in 

 the posterior and ventral portions of each pigment 

 cup arc invoked in responses wherein the animal 

 turns toward the eye of that side, whereas stimulation 

 of the remaining receptor cells is followed by a turn 

 in the opposite direction. 



Amono nemertineans (123) and rotifers (263, 264) 

 the presence of ocelli rather than eyespots has been 

 noted in se\eral genera. But no special significance 

 has been sttributed to the more complex photosensory 

 mechanism. 



So wide a variation in ocellar structure is present 



