iv. 18 NOMENCLATURE FOR LAMPREYS 121 



the injections the larvae assumed the secondary sexual characters, 

 which are normally shown only at maturity, namely, swelling of the 

 cloaca, opening of the pore from coelom to exterior, and the changes 

 in body form. No signs of metamorphosis were produced by these 

 injections and we are left without information as to the cause of that 

 change in the lamprey. In Amphibia even very young larvae undergo 

 metamorphosis when treated with thyroid extracts, but similar treat- 

 ment of ammocoete larvae has failed to produce any change. Further 

 investigation of the problem should be very interesting, since it seems 

 likely that the differences between the fluviatilis and planeri forms are 

 the result of an endocrine factor accelerating the onset of sexual 

 maturity in the latter. The fact that the change is occurring in various 

 parts of the world adds further interest to this example of evolution 

 in progress. 



Besides all these relatively small lampreys, there is a much larger 

 form, the sea lamprey, Petromyzon marinns Linn., reaching to over a 

 metre in length. This animal differs from Lampetra in body form, 

 structure of sucker, and other features, as well as in size. Like most 

 other groups of animals lampreys therefore present several problems 

 of nomenclature. Linnaeus included the three types that occur in 

 Europe in the one genus Petromyzon ; since they are all rather alike in 

 shape this is in some ways a reasonable procedure. But are we then 

 also to include in the same genus forms that differ more widely, such 

 as those occurring in the southern hemisphere ? As so often happens, 

 systematists have chosen the course of splitting up the Linnaean 

 genus, even though several of the resulting genera have only one 

 species. Thus Gray suggested the genus Lampetra for the brook and 

 river lampreys, keeping Petromyzon for the larger species of sea 

 lamprey. Other genera have been added, such as Entosphenus Gill for 

 some of the North American forms and Mordacia Gray and Geotria 

 Gray for the forms from the southern hemisphere (Chile, Australia, 

 and New Zealand). Such distinctions, though they may seem irritating 

 at first sight, are an advantage in that they call attention to the differ- 

 ences which exist. For instance, it is a striking fact that lampreys are 

 found in temperate waters of both hemispheres, but not in the tropics, 

 and it is interesting to learn that the forms from New Zealand, 

 Australia, and South America (there are none in South Africa) show 

 distinct peculiarities. Thus Geotria possesses a large sac behind the 

 sucker. 



A special problem of nomenclature arises from the fact that the 

 river and brook lampreys are almost identical in structure and differ 



