THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 59 
Lire 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 
—Ammoceetes. 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 Ammocoetes more 
