146 



VISION 



Table 3. Elasmobranch brain division: body ratios. 



brain divisions do not change at the same rate, the coefficients would differ 

 for different brain divisions (Northcutt et al. 1978). Coefficients for each 

 brain division would provide a more accurate measure of differences in brain 

 development among different species. However, available data are insufficient 

 for these calculations, and the mean coefficient of allometry for elasmo- 

 branchs was used instead. 



When corrections are made for body size, the telencephalon shows a pro- 

 gressive increase in size from Squalus to Sphyrna and Carcharhinus. A 

 similar trend is seen in the batoids from Platyrhinoidis to Dasyatis, which 

 exhibits the most developed telencephalon of the elasmobranchs examined 

 (a 13-fold increase over Platyrhinoidis). Similar increases characterize the 

 other brain divisions in these groups (Table 3), with marked exceptions such 

 as the large olfactory bulbs of Mustelus and the medulla of Rhinobatos. In 

 general, however, Sphyrna (among sharks) and Dasyatis (among batoids) 

 reveal increased size in most brain divisions. 



Sphyrna exhibits the most complex cerebellar foliation and the largest 

 cerebellar increase (fourfold) among sharks. Among the batoids, Dasyatis 

 exhibits a tenfold increase in cerebellar size over Platyrhinoidis. The data 

 also suggest that cerebellar foliation is not merely a simple response to in- 

 crease in cortical volume. Mustelus exhibits very distinct cerebellar convolu- 

 tions (Figures 12, 14), and its cerebellar volume is only 33% larger than that 

 of Squalus while cerebellar volume increases 260% from Platyrhinoidis to 

 Raja, with almost no change in cerebellar foliation (Figure 16). 



Finally, although encephalization quotients exhibit approximately the 

 same range in sharks and batoids (a fivefold to sixfold increase in both 

 groups), the batoids exhibit twice as much actual variation among all brain 

 divisions as the sharks. This is particularly true for mesencephalic and 

 medullar variation, though, again, this may be a function of small sample size 

 and must be determined by further study. 



Central Nervous System Organization 



In chondrichthians and other vertebrates, the greatest evolutionary changes 

 in the brain occur in the roofs of the forebrain, midbrain, and hindbrain. The 



