656 RADIATION BIOLOGY 



tAvo are indistinguishably fused on the mid-line. The fused median com- 

 pound eye of Daplmia consists of about 20 ommatidia and is somewhat 

 unusual in that it can be rotated several degrees within the body through 

 the action of a series of oculomotor muscles. Anostracan branchiopods 

 such as Artemia and Triops, which swim inverted, depend enough upon 

 visual cues so that behavior patterns change when compound eyes 

 are covered or act without the supplemental ocelli (Lockhead, 1939). 

 Hanstroem (1934c) summarized compound-eye development among 

 branchiopods and was able to show a logical series from a stalked eye 

 (in anostracan genera) through incomplete fusion (in Lepidurus and 

 Triops) to complete fusion. This sequence matches habits closely; both 

 anostraeans and notostracans habitually swim ventral surface uppermost, 

 whereas the rest of the branchiopods maintain the more usual orientation. 

 Copepod compound eyes range from the median fused structure of Cyclops 

 and Calamis through the four eye types on each individual of Argulus 

 (various stages of degeneration) to the extremely aberrant single omma- 

 tidia of the corycaeids Copilia, Corycaeus, Lahidocera, Phyllosoma, Poniia, 

 PonteUa, and Sapphirina. In the corycaeids each of the two forward- 

 looking ommatidia includes a large protruding lens capable of forming 

 an image on a small internal structure consisting of an additional lens 

 (perhaps a crystalline cone), a short rhabdom, and a group of receptors. 

 The whole mechanism is more of a sighting device than an eye; unfortu- 

 nately nothing is known of its function. Ostracod compound eyes are 

 commonly separate if a median ocellus is present, but fused if the ocellus 

 is lacking. Some ostracods lack compound eyes entirely. The lumi- 

 nescent Cypridina has fully developed eyes. 



Among trilobites compound-eye development ranged all the way from 

 complete absence to rather large eyes comparable to those of the xipho- 

 suran Limidus. In any single species a gradual increase in the number 

 of ommatidia appears to be correlated with increasing size (Reed, 1898; 

 Richter, 1922). 



The compound eyes of Limidus seem uniciue among living arthropods 

 in that no ganglionic tissue is adjacent to the eye, and it is possible to 

 dissect out individual nerve fibers from individual ommatidia and on 

 them to study the passage of nerve-impulse trains in response to stimu- 

 lation of single units of the compound eye (Hartline, 1930). 



Among chilopods only Scutigera and some related genera possess com- 

 pound eyes. In Scutigera each eye consists of 100-200 ommatidia with 

 crystalline cones and 10-12 receptors apiece (Hanstroem, 1934b) arranged 

 in a double circle, as in the thysanuran insect Lepisma. 



The structure of insect compound eyes has been the subject of detailed 

 study. The most satisfactory accounts for adult insects are those by 

 Eltringham (1919), van der Horst (1933), von Buddenbrock (1935c), and 

 Tischler (1936). Dethier (1942, 1943) reported on the ommatidium-Hke 



