Page:Proceedings of the Royal Society of London Vol 69.djvu/513

Rh Considering how much attention has of late years been devoted to- the study of the Lamprey's brain, it seems almost incredible that such definite structures as the cilia ted grooves described above should have hitherto escaped observation. I have, however, been unable to find any reference to them in the literature available. It remains to be seen whether or not they occur in the adult. I am inclined to think that they probably disappear when the development of the first choroid plexus renders their presence no longer necessary, and such circulation of the fluid in the cavity of the brain as is requisite may be safely left to the cilia lining other parts. Ahlborn* figures a transverse sectioir of the brain of Pefoomyzon planeri taken in the region of the posterior commissure, but makes no mention of the structures in question. Gaskellf also figures a transverse section through the corresponding region of the brain of what he terms " an Ammoccetes immediately after its metamorphosis," without giving any indication of the ciliated grooves. It seems highly probable, then, that they are really absent in the adult animal, and that their loss is to be looked upon as another of those striking changes which accompany the transformation of the larval Ammoccetes into the adult Petrvmyzon in Europe, or the closely allied Geotria in New Zealand.

Much more remarkable than the apparent absence of the ciliated grooves in the adult is the fact that they seem, so far as I have been able to discover, to have entirely escaped the notice of those investiga- tors who have so minutely described the development of the Lamprey. Thus in the beautiful memoirs of Shipley} and Scott^ I can find no- reference at all to any such structures.

The general appearance and structure of the ciliated grooves forcibly call to mind the occurrence of more or less similar organs in other divisions of the animal kingdom. We may, for example, com- pare them physiologically with the siphonoglyphs of Alcyonarians, the ambulacral grooves of Echinoderms, and the endostyle of Amphi- oxus and the Tunicates. It is generally admitted that the function of all these organs is to bring about the movements of fluids in definite directions, and by analogy we may, I think, pretty safely conclude that a similar function is performed in the case under discussion, is also tempted to institute a physiological comparison with the ciliated tubes in connection with the brain of the Nemertines, but in the present state of our knowledge such a comparison can be of but little value.

Wiss. Zool.,' vol. 39, p. 191, Plate 14, fig. 26.
 * Ahlborn, " Untersuchungcn iiber das Gehirn cler Petromyzonten," Zeit.

t Gaskell, "On the Origin of Vertebrates from a Crustacean-like Anee*t< ' Quart, Journ. Micros. Sci.,' vol. 31, N.S., p. 379, Plate 25, fig. 3.

t Shipley, " On some Points in the Development of Petromyzon fin 1 Quart. Journ. Micros. Sci.,' vol. 27, N.S., p. 325.

Scott, " Notes on the Development of Petromy/.on," ' Journal of Morplu vol. 1, p. 253.