FISHES 



287 



horizontal)^ (Figs. 303 to 312). Amongst fishes this shape of pupil is 

 characteristic only of Selachians. An expansible opeeculum, a 

 structure described by Cuvier (1805) and subsequently by Leuckart 

 (1875), is a feature of the flattened Batoidei with their upward-looking 

 eyes ^ ; it is a structure on the upper part of the pupillary margin 

 which expands downwards in bright light to block the aperture so that 

 the eyes appear to " close." The mechanism whereby this non-muscular 

 structure contracts and exjDands is unknow^l. These opercula are of 

 varying shapes : thus the contracted pupil of the electric ray, Torpedo, 

 or the spotted dogfish, ScyJliorhinus, is a horizontal slit divided in the 

 middle by a tiny operculum (Fig. 313) ; the 

 operculum may be provided with a smooth edge, 

 as in the sting-ray, Trygon, and Torpedo, or the 

 margin may be serrated as in other members of 

 the ray family {Raja clavata, R. bat is, Trygono- 

 rkina and others), so that on full expansion it 

 reduces the pupil to a crescent of stenopoeic 

 apertures (Fig. 312). 



The voluminous leyis is never completely 

 spherical as in Teleosteans, but is always lenti- 

 cular in shape with the transverse diameter 

 slightly greater than the antero-posterior. Un- 

 like the cyclostome lens and as occurs in all other 

 Vertebrates except lizards, a system of sutures 



is present ; it is, however, very simple consisting merely of a single line- 

 suture rumiing vertically in the anterior part and horizontally in the 

 posterior 3 (Rabl, 1898) (Fig. 314). The epithelium clothing the 

 anterior surface is continued beliind the equator, whereafter, as m 

 other ^>rtebrates, the cells are prolonged into fibres, the nuclei of 

 which lie in the posterior cortex. The vitreous is of a dense consistency 

 particularly in its anterior parts where it forms the susjDensory apparatus 

 of the lens ; it has little adherence to the retina posteriorly whence it 

 is readily detached. 



The retina has received a considerable amount of study. "^ In the 

 embryo, blood vessels lie in the foetal fissure (de Waele, 1900) but these 

 disappear and in the adult the retina is quite avascular and shows no 

 trace of the foetal fissure except a tiny wliite area on the summit of the 

 ciliary papilla (Fig. 301). The retinal epithelium is comprised, as is 



1 The basking shark, Selache ; the spiny dogfish, Squalus ; the porbeagle shark, 

 Latnna ; and so on. 



2 Thus it is absent in the devil-fisli rays, Mobuhda?, wliich have lateral eyes and 

 also in the dorso-lateral eyes of Myliobatis. 



3 A single line-suture is found also in the lenses of most Teleosts, Anurans, Reptiles, 

 some Birds and the rabbit. 



* Krause, 1886-89; Xeumayer. 1897; Schaper, 1899; Greeff, 1899; Addario, 

 1903 ; Retzius, 1905 ; Schnaudigel, 1905 ; Franz, 1905 ; Verrier, 1930 ; and others. 



Fig. 314. — Lenticular 

 Sutures of Selachians. 



Showing the vertical 

 anterior suture. Pos- 



teriorly there is a short 

 horizontal suture. 



Sci/lliorhinus 



Raj.t 



