ELASMOBRANCH BRAIN ORGANIZATION 185 



(Gorbman 1959, Wilson and Dodd, 1973), and inferior lobes (Evan et al. 

 1976, Demski 1977). We have no detailed information on the number of cell 

 groups, their connections, and their integrative and control functions in 

 chondrichthians. Such data are clearly needed if we hope to make any head- 

 way in understanding chondrichthian feeding, reproductive, and homeostatic 

 functions. 



Detailed studies are also needed for the other two diencephalic regions, 

 epithalamus and thalamus. The habenular nuclei form part of a sizable de- 

 scending brain stem pathway, via the fasciculus retroflexus, but little is 

 known about the sources of input to the habenula. The habenular region is 

 likely a focal point of epiphyseal, hypothalamic, telencephalic, and raphe 

 inputs involved in one or more biological rhythms, but the nature of these 

 pathways and their functions are unknown. Spinal, cerebellar, tectal, and 

 telencephalic inputs to the elasmobranch thalamus have been demonstrated 

 experimentally, as have projections to the telencephalon. However, there are 

 no detailed studies on the cytoarchitecture of the thalamus or its variation. 

 Until we have data on the number of thalamic nuclei, their connections, and 

 their physiology, it is impossible to compare chondrichthian thalamic evolu- 

 tion and organization to that of other vertebrates in any detail. 



One of the most striking elasmobranch neural trends is the elaboration of 

 the telencephalic central nucleus (Figures 6, 7, 10, 15). The central nucleus 

 bears certain topographic and embryonic similarities to the expanded pal- 

 lium of actinopterygian fishes (Northcutt and Braford 1977), to the dorsal 

 ventricular ridge of sauropsid reptiles (Cohen and Karten 1974, Northcutt 

 1977c), and to parts of the mammalian isocortex (Karten 1969, Northcutt 

 1969a, 19696). All of these telencephalic structures arise from dorsal and/or 

 lateral telencephalic pallial fields, and all receive various sensory inputs from 

 the thalamus. In land vertebrates, each modality is represented in a restricted 

 portion of the hypertrophied pallium, and extensive efferents project to the 

 striatum. Physiological studies (Cohen et al. 1973, Piatt et al. 1974) indicate 

 that the elasmobranch central nucleus is probably organized in a similar 

 manner. However, anatomical details on the distribution and number of 

 ascending sensory projections to the central nucleus are lacking, as are data 

 on the pattern of efferent projections from this nucleus. 



The elasmobranch central nucleus is a tempting potential homolog to the 

 diverse pallial specializations of other vertebrates. It is far more likely, how- 

 ever, that the elasmobranch pallial condition has evolved independently, in 

 parallel with the pallia of advanced actinopterygian fishes and amniotic ver- 

 tebrates. 



Comparison of elasmobranch brain evolution with that of other vertebrate 

 groups reveals a number of similarities: increase in brain size, restricted 

 olfactory projections to the telencephalon, expansion of the striatum, and 

 expansion and differentiation of the nonolfactory telencephalic pallium. 

 Similar trends of independent origin occur in teleosts (Northcutt and 

 Braford 1977), reptiles (Northcutt 1977c), birds (Stingelin 1958), and mam- 

 mals (Jerison 1973, Welker 1976). Undoubtedly there are inherent con- 

 straints on neuronal evolution, and finite possibilities for change in neural 



