634 



RADIATION BIOLOGY 



perception. Careful investigation of how far these worms use their 

 remarkable visual apparatus is still to come. 



Nereis, Eunice (= Leodice), and others with known habits of swarm- 

 ing in definite relation to light have been examined in terms of visual 

 organs— Nereis by Andrews (1892), Langdon (1900), and Mosella (1927) 

 and Eunice by Schroeder (1905) and Hess (1931). Mosella reported a 



RANZANIA 



SYLLIS 



NEREIS 



PHYLLODOCE 



HA LI OTIS 



SCUTUM 



MUREX 



PATELLA 



SECRETION 



LIMULUS 



SCOLOPENDRA 



DYTISCUS LARVA 



SENSORY 

 CELLS 



— CUTICLE 



SAGITTAL 



AND 



TRANSVERSE 



SECTIONS 



Fig. 14-6. Analogous series of photoreceptor specializations can be traced in annelids 

 (top), mollusks (middle), and arthropods (bottom). From a simple cup lined with 

 sensory cells, various provisions for lens action are found. Separation of the sensory 

 layer from the lens may permit formation of an image and potential increase in resolv- 

 ing power. (Limulus ajler Demoll, 1914; Scolopendra after Heymons, 1901; Dytiscus 

 after Guenther, 1912; others after Hesse, 1899 and 1901.) 



structural asymmetry of the photosensory epithelium which would adapt 

 the anterior pair of Nereis ocelli to lateral and forward vision and the 

 posterior ocelh to upward vision. Response to Hght has been investi- 

 gated in Nereis. The effect of each ocellus was checked, and alterations 

 in responses were studied when nerve connections were severed in various 

 combinations. Herter (1926) concluded that the direction of movement 

 of the animal had no relation to the direction of illumination, but that 

 the anterior ocelli mediated negative responses to light, and the posterior 

 ocelli, positive responses. Brand (1933) found that, with any one ocellus 

 intact on either side, the animal behaved as though unilaterally blinded 

 and that removal of half the supra-esophageal ganglion without destruc- 



