TRANSACTIONS OF SECTION D. 673 



for the polyp still proceeds from a planula, •whicli is eminently adapted for loco- 

 motion, though perhaps withm a narrower range. We have two migratory stages 

 in the life-history. Each has its own advantages and disadvantages. The planula, 

 from its small size, is less liable to be devoured, or stranded, or dashed to pieces, 

 but it cannot travel far ; the medusa may cross wide seas, but it is easily captured 

 and is often cast up upon a beach in countless multitudes. 



Adult transformation may be recognised by its occurrence after the normal 

 structure of the group has been acquired, and also by its special motive, which is 

 egg-laying and all that pertains to it ; the special motive of larval transformation 

 is dispersal for food. 



The reproduction of the common Eel has been a mystery ever since the days of 

 Aristotle, though a small part of the story was made out even in ancient times. 

 It was long ago ascertained that the Eel, which seeks its food in rivers, descends 

 to the sea in autumn or early winter, and that it never spawns, nor even becomes 

 mature in fresh waters. The Eels which descend to the sea never return, but 

 young eels or Elvers come up from the sea in spring, millions at a time. The 

 Elvers have been seen to travel along the bank of a river in a continuous band or 

 eel-rope, which has been known to glide upwards for fifteen days together. It 

 was of course concluded that spawning and early development took place in the 

 sea during the interval between the autumn and spring migration, but no certain 

 information came to hand till 1896. Meanwhile this gap in our knowledge was a 

 perplexity, almost a reproach to zoologists. The partially-known migration of the 

 Eel could not be harmonised with the ordinary rule of migratory fishes. We tried 

 to explain the passage of marine fishes into rivers at spawning time by the supposi- 

 tion (a true supposition, as I think) that the river is less crowded than the 

 shallow seas, and therefore a region in which competition is less severe. The river 

 is to some migratory fishes what the tundras of Siberia are to some migratory bird.i, 

 places comparatively free from dangerous enemies, and therefore fit for the rearing &f 

 the helpless young. But the Eel broke the rule, and cast doubt upon the explanation. 

 The Salmon, Sturgeon and Lamprey feed and grow in the sea, and enter rivers to 

 spawn. The Eel feeds and grows in rivers, but enters the sea to spawn. What 

 possible explanation could meet cases thus diametrically opposite ? 



This was the state of matters when Grassi undertook to tell us that part of the 

 history of the Eel which is transacted in the sea. When it leaves the river, it 

 makes its way to very deep water, and there undergoes a change. The eyes 

 enlarge, and become circular instead of elliptical ; the pectoral fins and the border 

 of the gill-cover turn black ; the reproductive organs, only to be discovered by 

 microscopic search before this time, enlarge. The Eels, thus altered in appearance 

 and structure, lay their eggs in water of not less than 250 fathoms' depth. The 

 upper limit of the spawning-ground is nearly three times as far from sea-level as 

 the 100-fathom line which we arbitrarily quote as the point at which the deep sea 

 begins. The eggs, which are large for a fish (2'7 mm. diam.), float but do not 

 rise. The young which issue from them are quite unlike the Eels of our rivers ; 

 they are tape-like, transparent, colourless, devoid of red blood and armed with 

 peculiar teeth. A number of difi'erent kinds of such fishes had been previously 

 known to the naturalist as Leptocephali. Giinther had conjectured that they were 

 abnormal larvie, incapable of further development. Grassi has, however, suc- 

 ceeded in proving that one of these Leptocephali (L. brevirostris) is simply a 

 larval Eel ; others are larvae of Congers and various Mursenoid fishes. He has 

 with infinite pains compared a number of Leptocephali, and co-ordinated their 

 stages, making out some particularly important ones by the direct observation of 

 live specimens. 



You will not unnaturally ask how Grassi or anybody else can tell what goes 

 on in the sea at a depth of over 250 fathoms. His inquiries were carried on at 

 Messina, where the local circumstances are very fortunate. Strong currents now 

 and then boil up in the narrow strait, sweeping to the surface eggs, larvae, and a 

 multitude of other objects which at ordinary seasons lie undisturbed in the tran- 

 quil depths. Further information has been got by dredging, and also by opening 

 the body of a sun-fish (Orthagoriscus mola), which at certain times of the year is 

 1897. X X 



