THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 59 



Life History of the Lamprey — not a Degenerate Animal. 



The striking peculiarity of the lamprey is its life-history. It 

 lives in fresh water, spending a large portion of its life in the mud 

 during the period of its larval existence : then comes a somewhat 

 sudden transformation-stage, characterized, as in the lepidopterous 

 larva, by a process of histolysis, by which many of the larval tissues 

 are destroyed and new ones formed, with the result that the larval 

 lamprey, or Ammoccetes, is transformed into the adult lamprey, or 

 Petromyzon. This transformation takes place in August, at all 

 events in the neighbourhood of Cambridge, and later in the year the 

 transformed lamprey migrates to the sea, grows in size and maturity, 

 and returns to the river the following spring up to its spawning beds, 

 where it spawns and forthwith dies. How long it lives in the Ammo- 

 coetes stage is unknown ; I myself have kept some without transfor- 

 mation for four years, and probably they live in the rivers longer 

 than that before they change from their larval state. It is absolutely 

 certain that very much the longest part of the animal's life is spent 

 in the larval stage, and that with the maturity of the sexual organs 

 and the production of the fertilized ova the life of the individual ends. 



Now, the striking point of this transformation is that it produces 

 an animal more nearly comparable with higher vertebrates than is 

 the larval form ; in other words, the transformation from larva to 

 adult is in the direction of upward progress, not of degeneration. 

 It is, therefore, inaccurate to speak of the adult lamprey as 

 degenerate from a higher race of fishes represented by its larval form 

 — Ammoccetes. Its transformation does not resemble that of the 

 tunicates, but rather that of the frog, so that, just as in the case of 

 the tadpole, the peculiarities of its larval form may be expected to 

 afford valuable indications of its immediate ancestry. The very 

 peculiarities to which attention must especially be paid are those 

 discarded at transformation, and, as will be seen, these are essentially 

 characteristic of the invertebrate and are not found in the higher 

 vertebrates. In fact, the transformation of the lamprey from the 

 Ammoccetes to the Petromyzon stage may be described as the casting 

 off of many of its ancestral invertebrate characters and the putting 

 on of the characteristics of the vertebrate type. It is this double 

 individuality of the lamprey, together with its long-continued 

 existence in the larval form, which makes Ammoccetes more 



