168 VISION 



Mustelus and the other galeomorphs. However, if the upper half of the dense 

 cellular band in Squalus is layer 5, then optic fibers terminate dorsal to the 

 cell bodies in Squalus, whereas in Mustelus they terminate partly among the 

 cell bodies of layer 5. 



Traditionally, the ventral border of the superficial tectal zone is said to 

 coincide with the ventral extent of the optic terminals (except in actino- 

 opterygian fishes, where optic fibers enter the periventricular gray zone). I 

 have retained this definition in recognizing the tectal zones in Squalus and 

 Mustelus, though layer 5 is assigned to different zones on this basis. I believe 

 this approach is preferable to redefining the boundaries of the zones; if zones 

 are to be recognized at all, they should denote differences in tectal func- 

 tions. However, this problem could be avoided by abandoning the concept 

 of tectal zones and recognizing only individual tectal layers among 

 chondrichthians. 



Layer 5 consists of pyramidal and bipolar cells according to Leghissa 

 (1962). Both cell types possess superficial dendrites that branch repeatedly 

 in layer 6 and receive retinal input, as well as deeper dendrites that extend 

 into layers 4 and/or 3. Many of the bipolar cells are likely to be multimodal, 

 as they receive optic terminals superficially and other sensory input via their 

 periventricularly directed dendrites. 



Layer 6 consists of horizontal cells and scattered marginal cells (Leghissa 

 1962). The horizontal cells are bipolar neurons with horizontally radiating 

 dendrites and an axon that terminates among the cells of layer 5 and pos- 

 sibly layer 4. These cells are integrative in function, and their axons do not 

 leave the tectum. The marginal cells are neurons with two to three dorsally 

 directed dendrites that reach the tectal surface and axons that descend into 

 the deeper tectal layers. 



Leghissa (Table 5) has recognized more tectal layers than other workers 

 (seven, excluding stratum 1, which denotes the ependymal layer). I have 

 followed Gerlach's nomenclature here. However, a seventh layer can be 

 recognized, as the optic terminals do not reach the surface (Figures 2-5), 

 which is capped by a thin layer of unmyelinated fibers of unknown origin 

 and could be termed a marginal layer. 



Considerable variation exists among the tecta of chondrichthians, but too 

 few genera have been examined to identify^ trends, particularly for the 

 batoids. The tectum of the chimaerid Hydrolagus is distinctly different from 

 that of elasmobranchs. Eighty to ninety percent of the tectal neurons are 

 located periventricularly, and the neurons are in contact with the ventricular 

 ependyma so that a deep fiber layer (tectal layer 1) does not exist as it does 

 in elasmobranchs. The most ventral periventricular layer consists of neurons, 

 seven to eight cells thick, capped by three additional periventricular layers 

 (one to two cells thick) separated by thin fiber layers. The central tectal 

 zone in chimaeras consists of heavily myelinated fibers (homologous to the 

 elasmobranch layer 3) arranged in vertical columns with a few cells scattered 

 between the columns. The superficial tectal zone accounts for approximately 

 half the thickness of the tectum and consists of four distinct layers: margi- 

 nal, superficial optic fibers, neuropil and deep optic fibers. 



