ELASMOBRANCH BRAIN ORGANIZATION 163 



years ago, but Knudsen (1977) has demonstrated such segregation at mid- 

 brain levels in teleosts. 



Cerebellum— The cerebellum of chondrichthians consists of a central 

 unpaired corpus and laterally situated auricles (Figures 8, 9, 11-14, 16, 17, 

 26). Each auricle is divided into a dorsomedial upper leaf and a ventrolateral 

 lower leaf continuous with the acousticolateralis region of the medulla 

 (Figures 21-23, 26). 



Earlier studies have emphasized that the size of the corpus is correlated 

 with body size and is thus related to somatic musculature (Kappers et al. 

 1936, Aronson 1963), but my own observations do not support this con- 

 tention. Animals of the same size (e.g., Squalus acanthias, Mustelus canis, 

 and Sphyrna tiburo) exhibit the total range of cerebellar complexity seen 

 among sharks. Shark cerebellar: body ratios (Table 3) also indicate that 

 cerebellar size is not proportional to body size. Mustelus (6 kg) possesses a 

 relatively larger cerebellum than Carcharhinus (36 kg). 



The distribution of complex cerebellar foliation among elasmobranchs 

 indicates that this condition has evolved a number of times independently. 

 Chimaerids, squatinomorphs, and all squalomorph sharks examined possess a 

 smooth corpus divided into anterior and posterior lobes (Figures 8, 9, 11 A, 

 26 A), suggesting that this condition is ancestral for cartilaginous fishes. A 

 similar pattern is seen in heterodontids and scyliorhinids. However, lamni- 

 form and advanced carcharhiniform sharks possess a complexly convoluted 

 corpus divided into three lobes (Figures 12-14, 26B). Batoids have evolved 

 independently a complex corpus. The rajoids (Figure 16 and some torpedini- 

 forms possess a nonconvoluted corpus, while the rhinobatoid corpus is 

 divided into three lobes (Figure 11B), as in many advanced sharks. The 

 myliobatiforms (Figures 11C, 17) and some torpediniforms extend this trend 

 and possess complex foliation, as do the carcharhinid sharks. However, dif- 

 ferent parts of the cerebellar cortex undergo hypertrophy in galeomorph 

 sharks and myliobatiforms. The posterior cerebellar lobe in galeomorphs 

 accounts for approximately half the cerebellar cortex (Figure 14C), 

 while the anterior lobe accounts for half of the cortex in myliobatiforms 

 (Figure 11C). 



The functional significance of increased cerebellar volume in elasmo- 

 branchs is unknown, but it is very likely related to an increase in sensory 

 inputs involved in motor control. Unfortunately there is little information 

 on sensory pathways to the cerebellum. Ascending spino-cerebellar tracts to 

 the corpus have been demonstrated (Hayle 1973b), but direct lateralis pro- 

 jections to the corpus, described by earlier studies (Kappers et al. 1936), 

 have not been confirmed experimentally (Boord and Campbell 1977). 

 Boord and Campbell demonstrated that the lateral-line nerves project 

 directly to the auricle of the cerebellum, as do vestibular fibers. 



Lesions of the cerebellar corpus do not result in locomotor impairment in 

 Squalus, but lesioned animals do exhibit a decrease in general motor activity 

 (Karamyan 1962). However, if lesions involve the cerebellar nucleus, the 

 animals can not swim in a straight line and lack coordination of the various 



