104 LOWER VERTEBRATES. 



plant Vdlisneria was named) in 1710, again byMondini in 1777, and almost contempo- 

 raneously by O. J. Muller, of Denmark. Later, the illustrious Rathke (in 1824, 1838, and 

 1850) and also Hornbaum-Hornscliuch published the results of special investigations, and 

 figured the eggs. But it was only in 1873 (after several futile endeavors by others) 

 that the male organ of the eel was recognized, also by an Italian naturalist. Dr. Syrski, 

 in small individuals of the species, and a previous idea that the eel was hermaphroditic 

 thereby dispelled. The sexual differences are correlated with external ones, and gen- 

 erally the males and females, when adult, can be told apart. Jacoby testifies that 

 he examined large numbers with a view to solve this question. The most important 

 differences relate to (1) size, (2) form of the snout, (3) color, (4) dorsal fin, and (5) 

 size of the eyes. (1) The males rarely attain a length of more than seventeen to nine- 

 teen inches, while adult females are generally much larger; (2) the snout in the male 

 is attenuated and rather jjointed, while in the female it is comparatively broad and 

 blunt ; (3) the male is a deep darkish green, or often a deep black with a shining lustre, 

 and a whitish belly, while the female has a clearer color, usually of a greenish hue on 

 the back and yellowish on the belly ; (4) the dorsal fin is lower and less developed in 

 the male than in the female ; and (5) the eye of the male is large, and that of the female, 

 as a rule, comparatively small. These characters, however, do not always hold good. 

 Jacoby remarked that " special reference having been jiaid to the height and narrow- 

 ness of the dorsal fin, much success has been met with in picking out, in the fish-market 

 of Trieste, the eels which possessed the organ of Syrski [that is, the male organ] ; ab- 

 solute certainty in recognizing them, however, cannot be guaranteed. If one is search- 

 ing among living eels with no characters in mind, — with the exception of the first, 

 that of length, — he will find in every ten eels, on an average, eight females and two 

 with the supposed male organ ; but, if the selection is made with a careful reference 

 to all these marks of difference, the proportion changes, and out of every ten examples 

 about eight will be found with the supposed male organ." 



According to Herr Benecke, " it may be assumed with the greatest safety that the 

 eel lays its eggs like most other fish, and that, like the lamprey, it only spawns once 

 and then dies. All the eggs of a female eel show the same degree of maturity, while 

 in the fish which spawn every year, besides the large eggs which are ready to be de- 

 posited at the next spawning period, there exist very many of much smaller size, which 

 are destined to mature hereafter and be deposited in other years. It is vei-y hard to 

 understand how young eels could find room in the body of their mother if they were 

 retained until they had gained any considerable size. The eel embryo can live and 

 grow for a long time supported by the little yolk, but, when this is done, it can only 

 obtain food outside of the body of its mother. The following circumstances lead us to 

 believe that the spawning of the eel takes place only in the sea : (1) that the male eel is 

 found only in the sea or brackish water, while female eels yearly undertake a pilgrimage 

 from the inland waters to the sea, a circumstance which has been known since the 

 time of Aristotle, and upon the knowledge of which the principal capture of eels by 

 the use of fixed apparatus is dependent ; (2) that the young eels, with the greatest 

 regularity, ascend from the sea into the rivers and lakes. 



"All statements in opposition to this theory are untenable, since the young eels 

 never find their way into land-locked ponds in the course of their wanderings, while 

 eels planted in such isolated bodies of water thrive and grow rapidly, but never in- 

 crease in numbers. Another still more convincing argument is the fact that in lakes 

 which formerly contained many eels, but which, by the erection of impassable weirs, 



