ELASMOBRANCH BRAIN ORGANIZATION 171 



telencephalon may modulate motor movements, in part, via a similar cortico- 

 rubrospinal pathway. 



In batoids, the rostrolateral tegmentum is dominated by a large spherical 

 nucleus that receives lateralis input via the lateral lemniscus; it is termed the 

 mesencephalic lateralis nucleus (mln, Figures 20D, 27). This nucleus consists 

 of medium-sized bipolar and triangular cells scattered among ascending 

 fascicles of the lateral lemniscus. The cell density is greatest along the lateral 

 half of the nucleus. Its efferents are unknown, but they likely project 

 to the telencephalon via thalamic relay. A comparable nucleus has not been 

 identified in sharks, but the midbrain cell group, nucleus profundus mesen- 

 cephali (np, Figures 2D, 3 A, 5A), in Squalus and Mustelus may be homolo- 

 gous to the mesencephalic lateralis nucleus of batoids. 



More ventrally, a basal optic nucleus occupies the ventrolateral tegmental 

 floor (bon, Figures 3B, 5B, 20C). This nucleus receives optic fibers from the 

 contralateral eye, and its efferents are unknown in elasmobranchs. In other 

 vertebrates, the basal optic nucleus projects to the vestibulo-cerebellum, 

 mediating optokinetic nystagmus (Lazar 1973, Brauth and Karten 1977, 

 Karten et al. 1977). 



Diencephalon— The diencephalon of chondrichthians consists of 

 three divisions: epithalamus, thalamus, and hypothalamus. The epithalamus 

 is formed by the habenular nuclei, the habenular commissure, and a complex 

 series of afferent and efferent pathways (stria medullaris complex) related to 

 the habenular nuclei. Little variation is discernible among the various species. 

 The habenular nuclei consist of dorsal and ventral divisions (Figures 2, 4, 

 27). The ventral habenular nucleus fuses across the midline below a much 

 shortened habenular commissure. The main efferent pathway of the habe- 

 nular nuclei (fasciculus retro flexus) terminates caudally in the interpeduncu- 

 lar nucleus (Figures 3, 5, 20). No experimental studies exist on the afferent 

 pathways to the habenula of cartilaginous fishes. 



The epithalamus of many vertebrates is characterized by two dorsal evagi- 

 nations, the epiphysis (pineal organ) and paraphysis (parietal organ), in addi- 

 tion to the habenular nuclei and related tracts. The chondrichthian epiphysis is 

 well developed and runs rostrally from the habenular area to end beneath the 

 roof of the skull over the telencephalon. The epiphysis of sharks consists of 

 both ganglion and sensory cells (Studnicka 1905), and electron microscopic 

 studies have shown that the sensory cells possess outer segments similar to 

 retinal cone photoreceptors (Rudeberg 1969). Hamasaki and Streck (1971) 

 demonstrated that the epiphysis of Scyliorhinus is photosensitive to very low 

 levels of illumination (theoretically, levels as low as those associated with a full 

 moon) and that such illumination elicits both slow potentials and inhibition of 

 spike activity. Similar results are known for a wide variety of vertebrates, and 

 the epiphysis is believed to function in control of skin color and circadian 

 rhythms. 



The thalamus of chondrichthians is divided into dorsal (dt) and ventral (vt) 

 divisions (Figures 2-5, 27). The thalamus of chimaeras and squalomorph sharks 



