CYCLOSTOMES 267 



cells and equally large vesicular cells forming a relatively thick cushion 

 between the choroid and the sclera. There is no ciliary body,^ only a 

 flat ciliary zone, and the immobile non-mviscular iris consists merely of 

 the usual two layers of (retinal) epithelium covered anteriorly by a 

 tenuous and lightly pigmented stroma binding together the blood- 

 vessels which are supplied by three anterior ciliary arteries. Contrary 

 to the arrangement in higher ^>rtebrates. the epithelial layers of the 

 iris continue forwards the state of pigmentation of the corresponding 

 retinal layers ; the anterior layer is pigmented, the posterior un- 

 pigmented almost up to the pupillary border. The anterior surface of 

 the iris has a light metallic sheen due to a fine argentea comprised 

 of a layer of closely packed cells containing guanine crystals, a configura- 

 tion which is not continued into the choroid. 



The angle of the anterior chamber is constructed on simple lines. 

 A ring of large endothelial cells encircles the periphery of the cornea 

 as the ANNULAR LIGAMENT, continues anteriorly with the corneal 

 endothelium and sends strands posteriorly to the choroid suggestive 

 of the tensor choroidese of Teleosteans ; while from the region of this 

 ring, fine strands span the angle to reach the anterior surface of the 

 iris, reminiscent of a pectinate ligament. The large and almost 

 circular lens is wedged in the immobile pupil and approximates the 

 cornea, separated from it at most by a capillary space ; it is held in 

 place by the support of the cornea in front and the vitreous behind. 

 The lens is primitive in formation compared with the structures found 

 in other Vertebrates, showing a central zone of polygonal or rounded 

 fibres and a somewhat irregular arrangement in the periphery without 



1 The origin of the aqueous humour of Cyclostomes and Fishes is obscure, but it 

 is possible that the ocular fluids are maintained directly by osmosis through the cornea, 

 the pressure being ccjuilibrated through the blood-stream. In fresh-water fish and the 

 lamprey the blood is hypertonic to the mediimi so that the body fluids are constantly 

 replenished by the absorption of water through the skin by osmosis, the fish excreting 

 the large cjuantities of fluid thus absorbed by producing immense quantities of urine. 

 In marine teleosts, on the other hand, the blood is hypotonic to the sea-water ; dehydra- 

 tion is avoided only by the copious drinking of the latter. This is actively absorbed 

 in the gut against tlie osmotic gradient while the excess of salts is excreted differentially 

 in the gut, kidneys and gills. Selachian fishes maintain a high level of urea in the 

 blood (some 2%) thus keeping it at a higher osmotic level than sea-water ; the latter 

 is thus absorbed osmotically while the excess of salts is excreted through the gills. 

 It would appear that Myxine has a salt concentration in the blood approaching that of 

 sea-water and thus higher than that of any other vertebrate (Robertson, 1957). It 

 is probable that the fluid-exchange and the pressure ecjuilibrium in the eye is main- 

 tained in much the same manner. There would seem to be no anatomical basis in 

 any cyclostome or fish for an elaborate secretorj' mechanism for the intra-ocular 

 fluid ; the only types which possess ciliary processes are the Selachians and these, in 

 Franz's view (1934), probably serve merely as a mechanism for supporting the lens. 

 All land animals, on the other hand, secrete the aqueous humour. It should not be 

 considered strange that the tissues of the earliest Vertebrates (fresh-water agnathous 

 fishes) were hj'pertonic to the medium in which they lived ; the same relationship is 

 seen in the tissue-cells of inan which are maintained in a state of hypertonicity in com- 

 parison with the surrounding tissue-fluid by an osmo- regulation depending on respira- 

 tory activity (see Bartley, Davies ami Krebs, Proc. n-y. Soc. B., 142, 187, 1954). 



