FISHES 317 



North America and Asia. They are the largest fish inhabiting fresh water and 

 are the most primitive of the bony fishes,^ showing many selachian charac- 

 teristics. 



In its general shape the globe of the sturgeon is flattened as is usual 

 in fishes (Figs. 3(59-70). The cornea has the standard layering and 

 Descemefs endothelium is piled up at the angle of the anterior chamber 

 to form an annular ligament which fills the angle with loose tissue 

 refiected onto the iris (Baecker. 1931). The sclera is usually tliick and 

 its inner half is occupied by an immensely thick scleral cartilage which 

 forms a feature of the eye ; and in some species two crescentic bony 

 plaques lie. one superiorly and one inferiorly, athwart the hmbus 



Fig. 368. — The Sterlet, AciPEysER rctbesvs (Zool. Soc, London). 



external to the scleral cartilage, extending onto the cornea where they 

 lie under the epithelium in tlie periphery (Soemmerring, 1818 ; the 

 CONJUNCTIVAL EOXE of H. Muller, 1872; Edinger, 1928). 



The choroid is heavily pigmented and richly vascular, being lined 

 externall}' with an argentea as in Teleosteans, while its inner 2/5 just 

 external to the choriocajjillaris is occupied by a tapetum lucidum ^ 

 comj^rised of some 12 laj^ers of cells packed with guanine crystals 

 intersj^ersed with occasional pigment cells, the dense structure being 

 pierced at intervals by vessels supj)lying the capillary layer for the 

 vascular layer of the choroid (Fig. 371) (Briicke, 1845 ; Miirr, 1927). 



The amuscular ciliary body may hardly be said to exist 

 (Fig. 372) ; sui^eriorly it gives rise to a suspensory ligament of the lens 

 resembling that of Teleosts. and inferiorly to a papilla resembling that 

 of Selachians which ajDj^arently does not contain muscle fibres (v. Hess, 

 1912). The iris also is devoid of muscles and like that of the lamprey 

 is immobile, while the stroma contains a thick argentea. a continuation 

 of the corresjDonding layer in the choroid. The immobile puj^il is of 

 the form of a vertical ellipse {AcijJenser) (Fig. 368) or a square with 

 rounded corners {Scaphirhynchus). 



1 p. 234. 2 p. 609. 



