ORIGIN OF INTRAOCULAR FLUIDS 371 



eyes first evolved by the crustaceans, the trick was never hit upon. Such 

 eyes being soUd, there is no possibility of rapid, gross, internal move- 

 ments for accommodation, regulation of incoming light, and so forth. 

 The vertebrate eye at the very outset received a tremendous boost 

 toward its eventual superiority, when it luckily developed a vesicular 

 plan of organization. 



Origin of Intra-Ocular Fluids — In the eyes of the higher vertebrates, 

 we can put our finger upon the immediate source of the internal fluid. 

 It is certainly the ciliary epithelium, covering the ciliary folds and proc- 

 esses (Fig. 3, p. 7). But in the lowest vertebrates we see no such secretory 

 structures, and a fertile field awaits the investigator of the sources of 

 their aqueous humors. 



Comparative physiology indicates that until there was need of an 

 intra-ocular secretory epithelium, there was no ciliary body. And, until 

 there was a ciliary body there could be no ligamentary anchorage of the 

 lens, and no lens-squeezing methods of accommodation. We do not 

 know for certain that the modern fish eye gets its water by osmosis 

 through the cornea; but in the absence of any experimental work on this 

 whole question, the assumption of such a process would explain much 

 of the anatomical simplicity of the piscine anterior segment. The absence, 

 from fish eyes, of those structures which terrestrial eyes have had to 

 produce or have found it possible to produce, in consequence of their 

 removal from water, is not adaptation to environment — unless one 

 extends the term to include refraining from producing anything which 

 is not needed. But these simplicities of the fish eye and complexities of 

 the terrestrial eye are certainly related to environmental differences, 

 rather than primarily to taxonomic ones. 



All lampreys begin their lives in fresh water, and thus have the oppor- 

 tunity to fill their eyeballs by osmosis via the cornea, maintaining the 

 desired intra-ocular pressure by controlled drainage through the ocular 

 blood vessels. The several large, parasitic species which make their way 

 to the sea must surrender any such ability unless they are able to raise 

 the osmotic pressure of their intra-ocular fluids by excreting salts, glu- 

 cose, or other substances into them. Failing this, they must somehow be 

 able to secrete more fluid inside the eye from some of its tissues, even 

 though we can see no special anatomical provision for such secretion. 

 Moreover, since the marine lampreys are anadromous, they must shut 

 off these compensatory intra-ocular secretions when they return to fresh 

 water at the end of their lives to breed. 



