M. C. Robin on the Sexual Differences of Eels. 387 



with the back black, and the pectoral fins a little larger than 

 in the river-eels, not exceeding 0*38 or 0'40 metre &c., are 

 males. In a lot of Seine eels, having all the ordinary charac- 

 ters, one 0"45 metre long, like most of the others, was a 

 male. I have never found males of greater length, 



Syrski gives 0'43 metre as the greatest length met with in 

 the males that he observed. 



The abundance oiiht pimpeneaux and their strongly marked 

 characters may even lead us to say that there are few species 

 of fishes in which the external sexual characters are so dis- 

 tinctive of the male in comparison with the female as in the 

 Eels. Only the male does not quit the ^lores of the sea, 

 except at the period of reproduction, to go to the bottom ; 

 whilst the female only goes to the sea, quitting the fresh water, 

 temporarily and at the same period. 



The dissection of eels 0"35 metre long, or thereabout, 

 shows at the first glance, in all seasons, whether the animal 

 is male or female. Instead of the well-known characters of 

 the ovary, a continuous semitransparent ribbon, of a yellowish 

 colour, folded like a frill, we see in the same place, with the 

 same relations, the same difierences of length to the right and 

 left, and of diminution of breadth at the posterior extremity, 

 the testis, a delicate narrow ribbon, more or less rose-coloured, 

 or of a semitransparent grey tint, rarely whitish. It is formed 

 of a series of flattened floating lobes, most frequently 

 2 millims. broad and of twice that length, the greatest thick- 

 ness of which does not exceed 1 millim. out of the time of 

 reproduction, with the inner surface convex and the other 

 flat, the outer or free margin thin, rounded into a quadrant, 

 the lobes all united at their base only by the deferent canal 

 &c., and with independent and distinct lobules. 



The peritoneal fold which envelops them, as is also tiie 

 case with the ovaries, attaches them to the sides of the ver- 

 tebral column and of the swim-bladder. In females of the 

 same size it is a continuous ribbon, a centimetre or more in 

 width, of a yellowish white colour, more or less opaque or 

 semitransparent, that we find interposed in the same manner 

 between the abdominal viscera and the corresponding portion 

 of the ventral wall. 



These differences between the male and female eel, percep- 

 tible at the first glance, are sufiicient to enable them to be 

 recognized ; but it is necessary to ascertain them from the 

 moment when there are males different from the pim2)e7ieau, 

 that is to say, having the external characters of the small or 

 middle-sized females. These differences, moreover, are greater 

 than those which exist between the ovary and the testis of the 



