14 



be divided into groups; they form on the contrary one large group, obvioiisly 

 iucluding several yeargroups which must be separated by other methods, if 

 such can be obtained. 



There is fortuuately aiiother method, wliose usefulness for the determina- 

 tion of the age of the eel will, it is hoped, be clearly shown by the data given 

 in the present paper. 



This is based ou using the concentric zones or rings in the scales of the 

 eel for determining the age; because these are annual rings and by coun- 

 ting their number and then adding 2 to the number found we ascer- 

 tain how many years the specimen investigated has lived in our waters. 

 — As O-point I take not the date of hatching but the date of immigration, that 

 is, the time when it reached our coasts as elver (niontée), and thus do not include 

 the period passed as I^eptocephalus out in the Atlantic. As the date of immigra- 

 tion falls in the spring and may with approximate accuracy be placed at May 

 Ist., it is naturally only for this date quite correct to add the number 2 — other- 

 wise we have to remember to return to the O-point. The number 2 must be 

 added, because the eel lives 2 years in our waters before it takes on scales. 



To be able to appreciate the grouuds ou which the hypothesis framed 

 rests, an exact knowledge of the structure of the scales of the eel, their anatomy, 

 is necessary; and as the scales are not formed in the eel all at the same time, 

 it is advisable likewise to kuow something of the scaly covering of the fish as a 

 whole. The two following sections are devoted to these questions. 



Structure of the scales of the eel. 



lu the Archives de Zoologie expérimentale et générale. Tome deuxiéme, 

 Paris 1873, a treatrise is published by M. E. Baudelot: Recherches sur la structure 

 et le développement des écailles des poissons osseux, in which a description based 

 on careful investigation is given of the scales of several fishes, amongst them of 

 the eel. As Baudelot shows himself in this to be a specially fine observer and 

 presents his observations in a clear and well-arranged manner, I may give cita- 

 tions from his work, which so far as my own investigations go I am able to 

 fully substantiate. 



First of all, Baudelot describes briefly the situation of the scales in the 

 skin of the eel and their differeut size in difterent specimeus, on which heremarks: 

 »The dimensions of the scales show variations, which — with all due reservation — 

 are so distinct, that they might seen to be dependent upon the age of the fish.« 

 In au eel of 60— 70 cm. in length a certain number of scales, 2 to 3 mm. in 

 length and about 1 mm. in breadth, were measured. 



Then he passes to an investigation and description of the scales of the 

 eel. »With a magnificatiou of about 30 diameters, an isolated scale appears as a 

 small transparent plate, the whole surface of which is divided into a number of 

 small round or oval piates, which are very sharply defined, juxtaposed and arranged 

 in rows more or less parallel to the outer contour of the scale. The piates men- 

 tioned, which appear light against the darker ground of the scale, have been cou- 

 sidered by some naturalists, e. g. Owen, as hollows or holes in the structure 



