UNLIMITED Audiobooks and eBooks

Over 40,000 books & works on all major devices

Get ALL YOU CAN for FREE for 30 days!

The Origin of Vertebrates

Walter Holbrook Gaskell

How does All You Can Books work?

All You Can Books gives you UNLIMITED access to over 40,000 Audiobooks, eBooks, and Foreign Language courses. Download as many audiobooks, ebooks, language audio courses, and language e-workbooks as you want during the FREE trial and it's all yours to keep even if you cancel during the FREE trial. The service works on any major device including computers, smartphones, music players, e-readers, and tablets. You can try the service for FREE for 30 days then it's just $19.99 per month after that. So for the price everyone else charges for just 1 book, we offer you UNLIMITED audio books, e-books and language courses to download and enjoy as you please. No restrictions.

Book Excerpt: 
. . . style="width:100%" src="fig037.png" alt="" title="">

Fig. 37.—Pineal Eye of Ammocœtes, with its Ganglion Habenulæ.

In order to complete the dioptric apparatus a lens is necessary. Where, then, is the lens in these pineal eyes? In all the arachnid eyes, whether median or lateral, the lens is a single corneal lens composed of the external cuticle, which is thickened over the corneagen cells. This thickened cuticle is composed of chitin, and is not cellular, but is dead material formed out of the living underlying corneagen cells. Such a lens is in marked contrast to the lens of the lateral vertebrate eye, which is formed by living cells themselves. This {86}thickening of the cuticular layer to form a lens could only exist as long as that layer is absolutely external, so that the light strikes immediately upon it; for, if from any cause the eye became situated internally, the place of such a le. . . Read More