izo VERTEBRATES WITHOUT JAWS iv. 18 



probably for three years and undergo metamorphosis in late summer 

 and autumn. The characteristic of this type of lamprey is that the 

 adults never migrate and never feed. The gonads are already well 

 developed at metamorphosis and ripen during the winter. Spawning 

 takes place in March or April and the animals then die. 



There has been much dispute about the status of these freshwater 

 races. In structure the adult L. planeri is nearly if not quite identical 

 with an adult L.fiuviatilis, except that the latter is much the larger and 

 has sharper teeth. Crossing of the two sorts could presumably never 

 take place in nature, on account of the size difference, but by artificial 

 stripping of the adults cross-fertilization in both directions can easily 

 be achieved. Unfortunately the hybrid larvae have never been reared 

 to maturity; we cannot therefore say whether the small size and 

 failure to migrate of the planeri forms are inherited characters or 

 are produced by the influence of the environment. The effect of the 

 non-migratory condition is to enable the lampreys to colonize very 

 fully rivers that, because of effluents, they would be unable to occupy 

 if a migration to the sea was necessary. By this process of acceleration 

 of the development of the gonads a dangerous stage in the life-history 

 has been avoided. 



Similar pairs of migratory and non-migratory forms of lamprey are 

 found in Japan and in North America. Indeed, the condition appears 

 to be developing independently in several river systems in the United 

 States. Since it may be difficult for the brook lampreys to spread 

 from one river system to another it is possible that many of the 

 planeri forms have evolved separately, perhaps quite recently. If so, 

 this is a remarkable example of a similar response produced in different 

 parts of a population by a similar environmental stimulus, in this 

 case the effluents. This process of alteration in the relative times of 

 metamorphosis and sexual maturity (paedomorphosis) has occurred 

 also in certain amphibians (the axolotl) and in tunicates (Larvacea). 

 Similar changes in rates of development may have been essential 

 factors in the development of the whole chordate phylum (p. 77). 



In one race, found in Italy, ammocoetes with mature gonads have 

 been reported. However, in most of these lampreys the paedomor- 

 phosis is only partial : metamorphosis does take place, but is immedi- 

 ately followed by maturity. Since in mammals injections of anterior 

 pituitary extracts accelerate development of the gonads, it was thought 

 possible that complete neoteny might be produced by making such 

 injections into larvae of L. planeri. No completely sexually mature 

 ammocoetes have yet been produced by this method, but following 



