88 VISION 



each but was unable to confirm the presence of ganglion cells. Rudeberg 

 (1968), in a preliminary paper, described the ultrastructure of the pineal 

 receptors in Scyliorhinus. In this first analysis of the shark pineal under 

 electron optics, Rudeberg identified the sensory cells as photoreceptors and 

 more specifically as receptors of the cone type. This criterion for classifica- 

 tion followed that of Nilsson (1964): all the outer segment disks were 

 broadly connected to the plasma membrane. Disks of the outer segment of 

 rods, in contrast, have this connection only at their basal portion (Figure 

 10). In a followup paper (Rudeberg 1969) the pineal of Scyliorhinus was 

 carefully studied under light and electron optics. Rudeberg recognized six 

 cell-types comprising the pineal parenchyma, including photoreceptors and 

 ganglion cells (few). One ganglion cell appeared to project dual tracts to the 

 posterior and habenular commissures of the brain. Evidence of synapses 

 between receptors and ganglion cells was seen in the form of pre- and post- 

 synaptic vesicles and synaptic rods. An improvement in fixation demon- 

 strated unequivocally that these receptors were of the retinal cone-type. Not 

 only was the plasma membrane of the outer segment broadly connected to 

 the saccules, the saccules themselves displayed a conspicuous lumen charac- 

 teristic of retinal cone and lacked the type of fissure which gives the rod 

 saccules their typical bilobed appearance. Thus, Rudeberg concluded, quite 

 correctly, that the pineal of Scyliorhinus is photosensitive. 



While the pineal appears to be light sensitive, Rudeberg (1969) noted that 

 Scyliorhinus ". . . does not have a pineal window or any other adaptation for 

 light reception of the pineal organ" (p. 573). Gruber et al. (1975) took 

 exception to this statement, demonstrating that three elasmobranchs 

 (Negaprion, Carcharhinus, and Mustelus) definitely possess a morphological 

 differentiation of the region directly over the end vesicle of the pineal organ. 

 Photometric measurements indicated that seven times more light impinged 

 on the pineal receptors than on surrounding areas of the brain. It appears 

 that the shark's chondrocranium is modified for transmission of light into 

 the epiphysis. 



Physiology 



The only physiological study of the elasmobranch pineal apparatus appears 

 to be that of Hamasaki and Streck (1971). They recorded from the epiphysis 

 of Scyliorhinus by placing an electrode on the cut end of the epiphyseal stalk 

 transected near the brain. Their objective was to determine whether the 

 pineal was sensitive to light and if so to describe some physiological prop- 

 erties of the pineal system. Stimulation of the epiphysis by a 1-s flash of 

 white light evoked a slow, positive potential lasting up to 15 s. This was 

 accompanied by prompt inhibition of spike activity. The authors investi- 

 gated the effects of stimulus intensity and steady illumination on pineal 

 activity. They also measured the spectral sensitivity of the system, obtaining 

 a curve which was similar to the rhodopsin curve (i.e., peaking at 500 nm) 

 but too narrow to fit precisely. They then presented evidence indicating that 

 the hemoglobin of the blood surrounding the epiphysis acted as a light filter 



