44 THE BIOLOGY OF THE SEASONS 



the gradual transformation of a Leptocephalus into a young 

 conger. 



Let us linger for a little over the Leptocephali. They 

 are about 3 in. long, almost as thin as knife-blades, and 

 as clear as glass, except that the iris of the eye is silvery. 

 In this transparency they resemble many of the open-sea 

 animals " sea-butterflies," " salps," and the like with 

 which they consort, and it may be that the character is of 

 protective value, for they are almost invisible in the water. 

 They are found in greatest abundance at a depth of about 

 fifty fathoms, but are often got close to the surface. They 

 seem to have a diurnal change in their level in the sea, 

 coming nearer the surface at night. They are said to 

 swim with beautiful undulating movements, more leisurely 

 than rapid, and they often remain quite still at rest in 

 the water. So far as is understood, they do not feed and 

 therefore do not grow. They may be spoken of as the 

 second great chapter in the life-history of the eel. 



At this point it may be explained that besides Lepto- 

 cephalus brevirostris, which is the larva of the common eel, 

 there are two other kinds of Leptocephali, which are the 

 larva of the conger-eel and of the deep-sea eel, and four 

 other kinds which have not yet been linked to their re- 

 spective adults. One of them, called Leptocephalus 

 hyoproroi'des, is of particular interest, because it is as yet 

 the only form in which it has been possible to trace back 

 the Leptocephalus stage to an earlier, not fully grown, 

 pre-Leptocephalus stage. In other words, the end of 

 Chapter I. is in this case known, and connected with 

 Chapter II. It is noteworthy that the pre-Leptocephalus 

 is also pelagic. 



To return to the life-history. In the late autumn, so 

 the story goes, the knife-blade-like larvae of the eel begin to 

 undergo a slow metamorphosis, as the outcome of which 



