382 



ADAPTATIONS TO MEDIA AND SUBSTRATES 



they overlap the cornea. They consist of basophilic mucous, muco-areolar, 

 or sometimes fibrous or cartiloid connective tissue. In some forms, as 

 Scomber, they are said to become thickened and charged with fat during 

 the breeding season. In some species (salmonoids, particularly) they have 

 been described as anchored to orbital bones by special ligaments, or to be 

 movable by special muscles; but these points are in dispute. 



In various pelagic teleosts, the vertical lids present themselves in essen- 

 tially three conditions. Typically, they consist of a pair of ingrowths 

 (minus an epidermis) of the skin which, in fishes generally, forms the 

 outer lip of the circumocular sulcus (Fig. 151b, w; p. 451) — the line of 



Fig. 131 — Permanent lid-complexes in fishes. 



a, head of a teleost, Scomber scrombrus, showing vertical ('adipose') lids in surface view 



and in seaion. After Hein. b, left eye of a requin shark, Galeorhinus galeus. After Franz. 



i- iris; //- lower lid; n- 'nictitating membrane'; s- sclera; ul- upper lid. 



junction between the conjunctiva and the surface skin of the head. More 

 often than not, the anterior fold smoothly joins the posterior one above 

 the eye, but overlaps the posterior fold inferiorly (Fig. 132b, c, d, f). 

 A series of species could be selected in which, by imperceptible steps, 

 this situation would intergrade with one in which the cornea is sur- 

 rounded and overlapped by a practically circular, continuous fold, of 

 about the same width in all meridians (Fig. 132g). Various conditions in 

 this series may occur in the same genus, as in Mugil, Caranx, Scomber, 

 and others. The anterior and posterior lids may be equally developed, or 

 — much more commonly — the posterior may be the wider of the two. 

 Very rarely {e.g., in Mugil bleekcrii) the anterior is the broader. The 



