Page:Natural History Review (1861).djvu/329

Rh imperfections in the vision of minute objects, cannot hold as true for all eyes, now that the existence of bloodless retinas is an established fact.

The retina then, of four classes of vertebrate animals is not nourished by the direct intervention of the circulatory system, and can be preserved in health and vigour, only by some endosmotic process.

And here I may mention, that this endosmotic action is limited in birds to the choroid surface alone. In the other three classes, Reptiles, Amphibia and Fish, absorption, on the contrary, may take place both from it and from the hyaloid membrane. This latter membrane, as I was the first to show, many years ago, is in some reptilia and amphibia a highly vascular one, and late investigations of mine have made it evident, that the hyaloidea of all fishes perfectly resembles that of the reptiles alluded to, in the richness of its supply of blood-vessels. The result of these investigations I reserve for a special treatise on the vascularity of the hyaloidea of fish. This subject is one well worth further investigation.

I have had an opportunity of investigating the anatomy of this very rare and most valuable fish, and have discovered the following modifications to exist in its respiratory apparatus, which though partially found in some other clupeid and salmonoid fish, yet are most fully developed only in this genus.

Attached to the gills there is an accessory respiratory organ, presenting the form of a tube, partly membranous, partly cartilaginous; this tube is twisted upon itself like a helix, one and a half-times, and is of equal calibre throughout: its length is one inch and three quarters and its diameter is two lines. It is situated above the fourth