519 



Gaskell^ also figures a transverse section through the corresponding 

 region of the brain of what he terms "an Ammocoetes 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 Ammocoetes into the adult Pctromyzon in Europe, or the closely 

 allied Geoiria 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 Scotti*^ 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 ambulacra! 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. One 

 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. 

 Probably, however, the closest physiological parallel is really to be 

 found in the ciliated groove which, according to Beard, is to be found 

 lying "at the base" of the neural tube in the embryos of nearly all 

 vertebrates i', and which also doubtless serves to promote the circula- 

 tion of the fluid in the cavity of the central nervous system. 



s G as kell, On the Origin of Vertebrates from a Crustacean-like Ancestor. 

 Quart. Journ. Micr. Sc., Vol. 31. N.S. p. 379. Plate 2ô. iig. 3. 



'^ Shipley, On some Points in the Development of Petroiiii/zon Jlaoiaiilis. 

 Quart. Journ. Micr. Sc, Vol. 27. N.S. p. 325. 



'"Scott, Notes on the Development oi reiroim/zon. .Journal of Morphology, 

 Vol. 1. p. 2.53. 



11 Anatomischer Anzeiger. Vol. 3. ]). 902, 9U3. 



