418 ADAPTATIONS TO MEDIA AND SUBSTRATES 



New Extra-Ocular Structures — Associated with the typical fish eye 

 there are only the standard oculomotor muscles and a circular lid-fold 

 rimming the orbit. The immobile lid-fold is simply the margin of a 

 circumocular sulcus, which is lined with a conjunctiva containing mucous 

 goblet-cells (which persist into the mammals), but without massive 

 glands of any kind. On taking to the land, the vertebrates were able to 

 develop sharper vision by reason of the greater amount of light available, 

 making possible an increase in the relative number of retinal cones. But 

 while vision through air meant better vision, it also meant exposing the 

 eye to desiccation which would ruin it for optical purposes and leave it 

 an easy prey to infection. On land, too, there was a new danger of injury 

 to the eye from dry, hard, windblown particles and from sharper col- 

 lisions of all sorts than are possible in the cushioning medium of water. 

 The vertebrates' first solution to these new problems was the production 

 of fluid-secreting structures (the ciliary processes) inside the eyeball, and 

 of two or more lids, new glands and new muscles outside it. In some 

 animals this artificial aquatic environment of the cornea proved inade- 

 quate for its protection, and there was manufactured a still more perfect 

 shielding device, the tertiary spectacle (section D) . 



Adnexa in Amphibia — It was pointed out in the preceding section 

 that the great majority of Amphibia are not amphibious, but are aquatic 

 as larvae and terrestrial as adults. The adult amphibian, then, has need 

 of about as complete a set of protective adnexa as does any land animal 

 which never goes near water. If the terrestrial Amphibia fail to show 

 such an elaborate array of ocular glands as higher forms possess, one 

 should not be too ready to attribute this solely to primitiveness; for, 

 after all, they mostly remain in damp situations even though on land. 

 Salamanders are to be found in such situations as on the moist, cool soil 

 under fallen logs. Frogs sit at the water's edge or move about in the 

 humid air at the grass-roots of waterside meadows. Spadefoots may be 

 very numerous in a locality, yet never seen until a prolonged rainstorm 

 brings them out — the basis of one of the several legends of 'rains of 

 frogs'. Other toads, and tree-frogs, may be found in the driest of places 

 — but they are careful to keep out of the sun. Toads often burrow 

 through dry soil into earth which is not so dry. If the skins of amphibians 

 had permitted them to adopt drier environments, we may be quite sure 

 that the ocular adnexa would quickly have gained the complexity shown 

 later by the scaly sauropsidans. 



