470 THE EYE IN EVOLUTION 



some Ruminants they are more fully developed, as in the gazelle 

 where they are found both on the upper and lower margins of the 

 pupil, or in the sheep where there are as many as 20, or in the wild 

 goat, Capra dorcas, where the mesodermal portion of the iris, beautifully 

 striped, participates in the projection ; in the Tylopoda (camel, llama) 

 they reach their fullest development, forming a series of ridges and 

 hollows on the upper and lower margins of the pupil which interlock 

 on miosis (Zannini, 1932). 



The second type of structure, called the umbraculum by Lindsay 

 Johnson (1901), is somewhat reminiscent of the operculum of some 

 rays 1 (Fig. 606). In the coneys (hyraxes) it is a flap-like fibro-cellular 

 structure, protruding from the mesodermal portion of the iris 2 mm. 

 from its free edge. It is provided with a fan-like arrangement of 

 (muscular ?) fibres and is remarkably contractile ; apparently without 

 regard to the amount of light and perhaps under voluntary control, it 

 can be retracted out of the pupillary aperture, extended so as to touch 

 the lower margin of the pupil and almost totally occlude it, or protruded, 

 flap-like, to touch the posterior surface of the cornea. An expansile 

 operculum is also seen in some Cetaceans. 



The 2^M^^l ii^ most Placentals is round, both in dilatation and 

 contraction. A slit-shajje on contraction is achieved, however, in 

 some Carnivora either as a protective or an ojjtical device. The slit- 

 or oval-shape is maintained by the arrangement of the fibres of the 

 sphincter, two bundles of which cross above and below the pupil and 

 are continued out to the periphery of the iris, a scissor-like action which 

 compresses the pupillary aperture laterally (Michel, 1881 ; Eversbusch, 

 1885 ; Raselli, 1923 ; Theiler, 1950 ; Rickenbacker, 1953) (Figs. 608- 

 10). In the smaller Felidse and Viverridae and in some Hysenidse and 

 Rodentia, as is well seen in the cat or the chinchilla, the slit-like 

 contracted pupil affords protection to an essentially nocturnal animal 

 against excessive light A\hen basking in the sun. In some of the 

 hyaenas {Hycena striata, H. hrunnea) the contracted slit has a constriction 

 in the middle giving the impression of two pupils (K. M. Schneider, 

 1930). Among the Pinnipedes, in the seals and sea-lions the pupil is 

 dilated and circular under water, but contracts to a vertical slit in the 

 air (except in the bearded seal, Phoca barbata, wherein the slit is 

 horizontal) ; this is almost certainly an adaptation for aerial vision 

 which will be discussed at a later stage (Johnson, 1901).^ The walrus, 

 on the other hand, which feeds on land, has a broad, horizontally oval 

 pupil (Franz, 1934). 



V/hile round i^uj^ils are the rule among Placentals, oval pupils are 

 found in a considerable number of species — usually horizontally oval 

 amori: rbivora and vertically oval among Carnivora, a circumstance 



1 p. 287. 2 p. 641. 



