044 RADIATION BIOLOGY 



nections. In many ways these papers supplement the survey of Claus 

 (1891) on median eyes in crustaceans. 



Ocelli. Among crustaceans the median eye is basically a double struc- 

 ture, but fusion may be remarkably complete. In forms that meta- 

 morphose it may disappear. The extreme instance of change is in 

 barnacles: in newly hatched nauplii there is a bilobed median ocellus; 

 at the metanauplian stage a pair of compound eyes are added; at meta- 

 morphosis the compound eyes degenerate or are extruded; but through- 

 out the metanauplian stage the median ocellus remains unchanged; only 

 at metamorphosis does it divide into two and do the separate parts 

 migrate into lateral positions in the mantle between the scutum and the 

 juncture of the rostrum with the lateral plates of the shell (Fales, 1928). 

 Thereafter they are the only photosensitive structures of the adult. 



In the branchiopod Artemia compound eyes are added to the median 

 nauplian eye. But if the compound eyes are blocked from stimulation, 

 essentially all normal adult responses to hght remain (Lochhead, 1939). 

 Exceptions to this are a loss of visual following of females by males and 

 obliteration of a pecuhar convulsive reflex when the animal is illuminated 

 suddenly after a long period in darkness. 



The ventral position of the median eye in Branchipus and Artemia, 

 many copepods, some trilobites, and xiphosuran larvae suggests that 

 inverted swimming may be an ancestral habit. Inverted swimming is 

 characteristic of Limulus, Branchipus, and Artemia, and probably also 

 trilobites. A median ocellus could be of value, whereas the compound 

 eyes were directed toward the bottom rather than the sky. Persistent 

 nauphan eyes have been described in some decapods. 



Among spiders eight ocelli are common. Of these one pair is of far 

 simpler structure, but often each is provided with a cup-shaped retina 

 that can be moved within the body through the contraction of pairs of 

 muscles. These simpler eyes are the "primary" ones, and their optic 

 nerve fibers arise from the proximal ends of the receptor cells; no tapetum. 

 is ever present. The remaining ocelli are "secondary" eyes; they lack 

 muscles and often possess a tapetum. Usually their nerve fibers arise 

 from the distal ends of the receptor cells. The distribution of secondary 

 eyes is subject to enough variation so that the configuration of the ocelli 

 provides a valuable taxonomic character. Morphological studies of 

 spider ocelH (Brants, 1838a,b; Bertkau, 1886; Widmann, 1908) have 

 been extended in an attempt to learn fields of view, possibility of stere- 

 oscopy, and limits of resolution (Petrunkevitch, 1907; Mallock, 1924; 

 Homann, 1928, 1931, 1934, 1947a,b). Unfortunately Mallock's calcu- 

 lations are based upon the outmoded Rayleigh criterion, and his conclu- 

 sions concerning the maximum acuity possible are unreahstic. Whether 

 the muscular movements of the retina permit the spider to keep prey in 

 view without body movements is still an open question (Homann, 1947a). 



