FISHES 



285 



continuation of the retina. Anteriorly its inner surface is broken by 

 low ciliary folds ^ which run in an irregularly radial direction onto the 

 posterior surface of the iris, a formation restricted in some species of 

 rays to the dorsal and ventral quadrants. A gelatinous disc-like zonule 

 runs from the coronal region of the ciliary body to the lens near its 

 equator, augmented in the mid-line dorsally by a firmer suspensory 

 ligament, and ventrally (in most species) by a cushion-like ciliary 

 papilla upon which the lens rests. The zonule and the suspensory 

 ligament are essentially condensations of the anterior part of the 

 vitreous (Teulieres and Beauvieux, 1931). The ciliary papilla, which 

 develops in the lips of the foetal fissure of the invaginating optic vesicle, 

 resembles a hypertrophied ciliary fold, and is continued for some 

 distance onto the back of the iris (Figs. 301-2); it is said to contain 

 smooth muscle fibres, presumably of ectodermal origin, derived from 

 the retinal layer of the ciliary body, so orientated that it acts as a 

 protractor lentis muscle, which on contraction would pull the lens 

 forwards on accommodation (Franz, 1931). It would appear, however, 

 that such fibres are scanty and their presence has been denied (Verrier, 

 1930 ; Rochon-Duvigneaud, 1943). ^ 



The iris is thin but usually extensive, being bowed forwards over 

 the protruding lens. Both ectodermal layers are pigmented near the 

 pupillary margin, but towards the ciliary body the posterior layer 

 usually loses its melanin content ; pigmentation of tliis layer is there- 

 fore more extensive than in the case of Cyclostomes, and in some 

 species the whole of this layer is pigmented (some sharks — Lamna 

 cornuhica — and rays — Trygon, etc.) as is the case in Teleosteans and 

 higher Vertebrates. From the anterior layer are developed the 

 SPHINCTER and dilatator muscles of the pupil which have received 

 considerable study (Franz, 1905 ; Gr\Tifeltt and Demelle, 1908 ; L. 

 Carrere, 1923). They are comprised of long, spindle-shaped ecto- 

 dermal cells which, acting autonomously and directly tlu-ough the 

 stimulus of light, undergo sluggish and delayed contractions (Brown- 

 Sequard, 1847-59 ; Young, 1933) ; they are more primitive than those 

 of higher vertebrate types in that the elongated myo -ectodermal cells 

 never leave their parent epithelial layer. It is interesting that in some 

 sharks and dogfishes prolonged exposure to light may lead to a state 

 of " mydriatic rigor " wherein the pupil remains permanently fixed 

 {Mustelus, Squalus). The mesodermal layer of the iris is thin, contain- 

 ing vessels and chromatophores in its deeper aspects, and in its anterior 

 parts, guanine-laden cells, not, however, arranged in packed parallel 

 layers as is the argentea of Teleosts, but in sufficient numbers to give 

 the iris a distinctly metalHc sheen. In the angle of the anterior 



Trygon 



Mustelus 



Squalus 



^ See footnote, p. 

 » p. 647. 



267. 



