184 VISION 



Preliminary data on spinal projections exist for two shark species— 

 Scyliorhinus caniculus (Hayle 1973a) and Ginglymostoma cirratum 

 (Ebbesson 1972). Similar data are lacking for batoids, and detailed studies are 

 needed to determine the presence of dorsal column nuclei and their projec- 

 tions. At present, we possess no information on the organization of the 

 trigeminal projections and central gustatory pathways. 



Information on acousticolateralis organization is accumulating, but more 

 detailed studies are needed to reveal whether ampullary, auditory, and ordi- 

 nary neuromast systems possess separate pathways throughout the midbrain 

 and forebrain. 



In elasmobranchs, cerebellar size is clearly not correlated with body size in 

 any simple way. It is not true that only small sharks possess smooth cerebella 

 and only large sharks convoluted cerebella. Complex, convoluted cerebella 

 characterize advanced galeomorph sharks and batoids, regardless of size. The 

 cerebellum is traditionally believed to coordinate locomotion, but many 

 vertebrates reveal complex sensory representations in the cerebellar cortex. 

 Almost nothing is known about the sensory inputs or their segregation in 

 elasmobranchs. Such data are needed to elucidate cerebellar development 

 and the role of the cerebellum in integrating sensory modalities related to 

 complex motor behaviors. Studies of sensory input are also most likely to 

 explain why different portions of the cerebellar cortex (Figures 11, 14) have 

 hypertrophied in galeomorph sharks and in advanced batoids. 



Experimental studies have demonstrated that the optic tectum is a major 

 visual center in sharks (Ebbesson and Ramsey 1968, Northcutt 1976), but 

 Graeber and Ebbesson (19726) have also shown that nurse sharks with ex- 

 tensive tectal ablations learn visual discrimination tasks. This result suggests 

 similarities between elasmobranch and mammalian visual organization, and it 

 clearly argues that other elasmobranch brain centers also mediate visual dis- 

 criminations. 



Cohen et al. (1973) revealed that visual information reaches the tel- 

 encephalic pallium in sharks, but we still lack details on the visual pathway 

 (or pathways) to that area. In land vertebrates, two or more visual pathways 

 project upon the telencephalon, after thalamic synapse, forming retino- 

 thalamo-telencephalic and retino-tecto-thalamo-telencephalic channels. We 

 know that in sharks the thalamus receives both retinal and tectal input, but 

 there are no anatomical data to indicate whether two visual pathways project 

 from the thalamus to the telencephalon. 



The vertebrate retina normally projects to several (8-10) primary neural 

 populations, forming a large number of more or less distinct visual circuits 

 with circumscribed functions. At present we possess some information on 

 the anatomy of the chondrichthian retino-tecto-reticular circuit, and frag- 

 mentary information on the retino-thalamo-telencephalic circuit. We have 

 essentially no information on the visual properties of single cells in the 

 chondrichthian central nervous system. Thus, our data on the biology of 

 chondrichthian vision is indeed fragmentary. 



For the most part, information on the chondrichthian hypothalamus is 

 limited to studies on the elasmobranch hypothalamo-pituitary axis 



