TYPES OF SPECTACLES 449 



(D) The Spectacle 



Injurious Substrates — ^As long as a vertebrate eye is held and pro- 

 pelled in such a way that only clean air or clean water ever ordinarily 

 touch it, it may be adequately protected by the glandular and palpebral 

 devices discussed in Section B, But there is only one ecological type of 

 vertebrate that normally never encounters a substrate which is potenti- 

 ally, at least, injurious to the eye. This is the completely pelagic fish — 

 the free balloon of the vertebrate kingdom. Every other kind of creature 

 must stay on a substrate, either under air or under water, or at least come 

 down to that substrate at more or less frequent intervals. 



Where the animal's size, structure, or feeding habits place the eye in 

 intimate relation to that substrate; and where the latter is sandy, muddy, 

 or beset with protrusions, the Udless eyes of a fish or even the lidded ones 

 of a land animal may be prone to injury. Where vertebrates have found 

 themselves in such predicaments, they have usually gotten out of them 

 by developing protective goggles. 



Types of Spectacles — ^Wherever we find an eye which is free to rotate 

 under a fixed, transparent covering through which it sees unimpeded, we 

 may call that covering a goggle or spectacle. Among spectacles we can 

 distinguish three types: primary, secondary, and tertiary. The first of 

 these is formed by material which, though it ordinarily forms a part of 

 the eyeball itself, has never become attached and permits the eyeball to 

 turn freely underneath it. Secondary spectacles are anatomically prac- 

 tically identical with primary ones; but they represent a secondary split- 

 ting-off of the material of the spectacle from an eyeball to which it had 

 long been joined in the ancestors. Tertiary spectacles represent distinctly 

 extra material overlying a complete eyeball. We may recognize one or 

 two movable coverings as tertiary spectacles, since they seem to have 

 been historically antecedent to the latter; but we shall not include the 

 nictitating membrane even though this is perhaps primarily spectacle-like 

 in usefulness in one or two cases, as in the horse. 



Primary Spectacles and the History of the Cornea and Con- 

 junctiva — The primary spectacle is seen only in lampreys and strictly 

 aquatic adult amphibians, and as a temporary affair in amphibian tad- 

 poles (Table XI, over). It will be recalled that eyelids, where these are 

 present, are lined with a continuation of their outer skin which is called 

 the conjunctiva and which, far back under each lid, turns upon itself to 



