134 



VISION 



x on 



A BC D 



Figure 9 Dorsal view of the brain of an hexanchiform shark, Notorynchus maculatus. 

 alln, anterior lateral line nerve; en, central nucleus; lob, lateral division of olfactory bulb; 

 mob, medial division of olfactory bulb; lp, lateral pallium; o, terminal nerve; on, occipital 

 (hypoglossal) nerve; op, olfactory peduncle (tract); ot, optic tectum ;plln, posterior lateral 

 line nerve; sn, spinal nerve; II, optic nerve; III, oculomotor nerve; IV, trochlear nerve; 

 V, trigeminal nerve; VIII, statoacoustic nerve; IX, glossopharyngeal nerve; X, vagal nerve. 



The myliobatiforms and carcharhiniforms are characterized by the most 

 complex neural development among living elasmobranchs. At present, all 

 measures of neural complexity fail to distinguish which of these two groups 

 is more advanced. Clearly each has independently reached a complex level of 

 neural organization, in a beautiful example of convergent or parallel evolu- 

 tion. 



Quantitative Brain Data 



The difference in brain volume or weight among different radiations is one 

 important measure of brain evolution. Direct comparisons are obviously 

 complicated by the wide range of body sizes encountered among vertebrates. 

 However, data for such comparisons can be obtained by making use of the 

 allometric relation between brain weights and body weights expressed by the 

 equation E = kP® , where E and P are brain and body weights, respectively, 

 and k and o: are constants. 



The logarithmic transformation of the above equation becomes log E = 

 log/e + a logP. This is a linear equation in log E and log P with a slope a 

 termed the coefficient of allometry. This coefficient is a measure of the 

 rate of change in brain weight or volume for a given change in body weight 

 or volume. Log/z is the intercept, often termed the index of cephalization. 



Gould (1966, 1971) and Jerison (1973) have discussed a number of 

 problems associated with the concept of such an index, one of the most 



