CYCLOSTOMES 265 



of this phylum. Of all vertebrate eyes it is the simplest (Figs. 276 and 

 277). Its characteristic features are : 



an avascular retina wherein the ganglion cell layer 7nerges with the 



inner mcclear layer ; 

 ike embryonic nature of the optic nerve, ivithout septa but ivith an 



ependymal axis, and provided ivith non-myelinated nerve fibres ; 

 the thick epichoroid in certain species ; 

 the large primitive lens lacking sutures ; 

 the absence of intra-ocular musculature ; 

 the separation of the cornea from the surface ectoderm ; 

 the absence, alone among Vertebrates, of a cartilaginous or bony 



orbit ; 

 the blending of some of the extra-ocular muscles ; 

 and the presence of an extra-ocular muscle of accommodation ivhich 



acts by deforming the eyeball from the outside. 



The structure of the eyes of all adult lampreys {Petromyzon 

 marinus, Lampetra fluviatilis, etc.) conforms to the same general plan 

 (W. Miiller, 1875 ; Franz, 1932-34 ; Walls, 1935-42 ; Rochon- 

 Duvigneaud, 1943 ; Henckel. 1944 — Mordacia). 



THE GLOBE, as in most Fishes, is flattened antero-posteriorly, 

 givmg the eye an ellipsoid configuration, the most prominent feature 

 being the large anteriorly-situated lens which makes underwater 

 focusing possible.^ The cornea-sclera is primitive ; the latter is a thin, 

 purely fibrous structure, the former a tenuous lamellated stratum 

 almost reduced to Descemet's membrane together with its endothelium, 

 continuous with the sclera. Superficial to this the skm is transparent 

 and thm, forming a layer in which the dermal glands and vessels are 

 lost and merely the multi-stratified epithelium remains, consistuig of 

 6 or 7 layers of regularly arranged cells. The space between the two 

 structures — the dermal cornea and scleral cornea — is occupied by a 

 delicate mucoid tissue derived from orbital connective tissue, the loose 

 structure of which allows the globe to rotate /reely underneath the 

 skin. 



The composite " cornea " of Cyclostomes thus represents an early stage in 

 the development of the typical vertebrate cornea wherein the superficial layers 

 derived from the surface ectoderm have not yet fused with the deeper layers 

 of mesodermal origin. The eye is thus entirely a subcutaneous organ. To the 

 specialized area of transparent skin constituting the dermal cornea, German 

 authors have given the name of primary spectacle ('-primare Brilla"), the term 

 denoiing a fixed transparent structure separate from the globe underneath irhich the 

 eye is free to rotate (Fig. 278) (Haller, 1768 ; Treviranus, 1820 ; and others ; and 

 Franz, 1934V Such an arrangement is seen in tadpoles and adult aquatic 

 Amphibians as well as in Cyclostomes. A secondary splitting of the cornea into 



1 p. 276. 



